SOIL DEGRADATION OF RAISED BEDS
ON ORCHARDS IN THE MEKONG DELTA
FIELD AND LABORATORY METHODS
Pham Van Quang
September 2013
TRITA-LWR PhD Thesis 1073
ISSN 1650-8602
ISRN KTH/LWR/ PhD 1073-SE
ISBN 978-91-7501-857-7
Pham Van Quang
TRITA LWR PhD Thesis 1073
© Pham Van Quang 2013
PhD Thesis
Land and Water Resources Engineering
Department of Sustainable development, Environmental science and Engineering
Royal Institute of Technology (KTH)
SE-100 44 STOCKHOLM, Sweden
Reference should be written as: Van Quang, P. (2013) “Soil degradation of raised-beds on
orchards in the Mekong delta - field and laboratory methods” TRITA LWR PhD thesis
1073.
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Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods
D EDICATION
To my family with respectful gratitude,
My wife Nguyen Thi Thanh Xuan,
My daughter Pham Xuan Huong, and
My son Pham Quang Duy
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Pham Van Quang
TRITA LWR PhD Thesis 1073
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Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods
S UMMA RY IN V IETNAMESE (T OÙM LÖÔÏC )
Suy thoái ÿҩt là mӝt tiӃn trình phӭc tҥp xuҩt hiӋn ӣ mӑi nѫi, mӑi lúc làm tác ÿӝng trӵc tiӃp
ÿӃn các quá trình lý, hóa và sinh hӑc trong phүu diӋn ÿҩt. Nó có thӇ là kӃt quҧ cӫa các hoҥt
ÿӝng do tӵ nhiên hoһc do con ngѭӡi nhѭ sӱ dөng sai hoһc thӵc hành quҧn lý ÿҩt ÿai bҩt hӧp
lý. Cho dù nguyên nhân thӃ nào chăng nӳa, suy thoái ÿҩt cNJng gây ra các ҧnh hѭӣng bҩt lӧi
nһng nӅ lên cây trӗng và sӭc sҧn suҩt cӫa ÿҩt. Suy thoái ÿҩt có thӇ thúc ÿҭy hàng loҥt các
quá trình nhѭ là xói mòn, nén dӁ, mҩt vұt liӋu hӳu cѫ và sinh vұt ÿҩt, ÿóng váng bӅ mһt và
ô nhiӉm. Luұn văn này trình bày sӵ ÿánh giá vӅ các ÿһc tính cӫa ÿҩt ÿӇ mӣ mang sӵ hiӇu
biӃt vӅ suy thoái ÿҩt trên các vѭӡn cây ăn trái ӣ ÿӗng bҵng song Cӱu Long. Thí nghiӋm
thӵc hiӋn trên 10 vѭӡn cam quít vӟi khoҧng thӡi gian thành lұp vѭӡn tӯ 1970 ÿӃn 1998 tҥi
tӍnh Hұu Giang. Mүu ÿҩt ÿѭӧc lҩy vào mùa khô năm 2010 ӣ hai ÿӝ sâu cho mӛi vѭӡn ÿӇ
phân tích các chӍ tiêu lý hóa ÿҩt. Sӭc kháng xuyên cӫa ÿҩt ÿѭӧc ÿo ÿӏnh kǤ mӛi tuҫn kӃt
hӧp vӟi lҩy mүu ÿӇ xác ÿӏnh ҭm ÿӝ ÿҩt trong suӕt khoҧng thӡi gian 5 tháng. KӃt quҧ phân
tích cho thҩy pH ÿҩt có khuynh hѭӟng giҧm, sӵ thiӃu và mҩt cân bҵng dinh dѭӥng ÿҩt ngày
càng lӝ rõ, và cҩu trúc ÿҩt ÿang xҩu ÿi theo ÿӝ tuәi cӫa vѭӡn. Các biӋn pháp phòng ngӯa và
phөc hӗi cҫn ÿѭӧc quan tâm ÿӕi vӟi viӋc phөc hӗi và duy trì chҩt lѭӧng cӫa ÿҩt và nѭӟc
ngҫm. Các biӋn pháp nên bao gӗm (1) trung hòa ÿӝ chua ÿҩt, (2) cân bҵng dinh dѭӥng, (3)
duy trì vұt liӋu hӳu cѫ trong ÿҩt, và (4) áp dөng chӃ ÿӝ tѭӟi thích hӧp.
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Pham Van Quang
TRITA LWR PhD Thesis 1073
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Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods
A CKNOWLEDGEMENTS
I would like to express my sincere gratitude to the Department of Land and Water
Resources Engineering, Royal Institute of Technology (KTH), Stockholm, Sweden
and the many people who have helped and supported me in various ways, during my
field survey in Vietnam as well as data analysis in Sweden. This statement is to thank
them for their tremendous enthusiastic help.
I greatly appreciate Mr. Ngo Xuan Hien, section of agriculture and rural development,
Chau Thanh district, Hau Giang province in Vietnam for help and encouragement
during the fieldwork activities. I thank all the local farmers for their kind support at
the study locations. College of Agriculture and Applied Biology at Can Tho University
in Vietnam are gratefully acknowledged with the facility support.
I would like to express my deepest gratitude to my main supervisor Per-Erik Jansson
for his great guidance, understanding, encouragements, advice and support for my
research works. I would also like to thank my co-supervisor Dr. Le Van Khoa for his
kind advice and comments during the study.
I would like to thank Britt Chow and Aira Saarelainen for their efficient and kind help
with all administrative matters. Joanne Fernlund was able to share her precious time to
help me to meet all the format regulations of my thesis by KTH.
Many thanks go to Dr. Ewa Wredle, department of Animal Nutrition and
Management, the Swedish University of Agricultural Sciences for arranging the
comfortable accommodation during the time I stayed in Uppsala, Sweden.
I also thank Mr. Huynh Ngoc Duc and Mr. Pham Xuan Phu for their assistance
during data collection. Miss Ly Ngoc Thanh Xuan, Mr. Nguyen Van Chuong and
analysis laboratory staff, Faculty of Agriculture and Natural Resources, An Giang
University, Vietnam for their help with soil chemical testing.
I thank my close friends from Vietnam for sharing many funny stories and parties
during my stay in Uppsala, Sweden. I also thank my colleagues in Faculty of
Agriculture and Natural Resources, An Giang University: Pham Huynh Thanh Van,
Thai Huynh Phuong Lan, Ly Ngoc Thanh Xuan, Huynh Ngoc Duc, Pham Xuan Phu,
Pham Duy Tien, Nguyen Van Kien, Tran Van Hieu. Many thanks go to Mr. Nguyen
Thanh Trieu and Nguyen Thanh Tinh for many kinds of help.
Many thanks go to Dr. Charles Howie for checking the English language.
I would also like to express my deepest gratitude to my parents-in-law for their
continuous supports, taking care of my children and encouragements during my study.
I wish to express my heartfelt gratitude to my parents for their holy motivation, which
led to successful completion of this thesis.
Last, but not least, I want to thank to my wife Nguyen Thi Thanh Xuan and my
children Pham Xuan Huong and Pham Quang Duy for their understanding, supports
and encouragements, which inspired me to accomplish this work.
Financial support from the Vietnamese Government, partly by KTH and the
utilization of enormous facilities at the Royal Institute of Technology (KTH),
Stockholm, Sweden, is gratefully acknowledged.
Pham Van Quang
Stockholm, September 2013
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Pham Van Quang
TRITA LWR PhD Thesis 1073
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Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods
T ABLE OF CONTENT
Dedication .................................................................................................................... iii
Summary in Vietnamese (Toùm löôïc) .............................................................................. v
Acknowledgements ..................................................................................................... vii
Table of content ........................................................................................................... ix List of tables ................................................................................................................. xi
List of figures................................................................................................................ xi
Abbreviations and symbols ........................................................................................ xiii
List of papers ............................................................................................................... xv
Abstract.......................................................................................................................... 1
Introduction................................................................................................................... 1
Study objectives ........................................................................................................ 3
Scope of the study..................................................................................................... 4
Soil compaction formation concept ......................................................................... 4
Methods ......................................................................................................................... 5
Field experiment performance ................................................................................. 5
Sites descriptions .............................................................................................................................................. 5
Sampling and analysis ..................................................................................................................................... 6
Soil penetration resistance measurement ........................................................................................................... 7
Results ........................................................................................................................... 8
Chemical properties on raised-bed soil (Paper II) ................................................... 8
Physical properties on raised-bed soil (Paper III) ................................................... 8
Soil penetration resistance (Paper IV)...................................................................... 8
Discussion ................................................................................................................... 12
Soil properties assessment...................................................................................... 12
Chemical properties........................................................................................................................................ 12
Physical properties ......................................................................................................................................... 13
Soil amendments .................................................................................................... 16
Essential Plant Nutrients - their functions, and role of fertilizers ................................................................... 16
General soil amendments to raised-bed orchards in the Mekong delta ............................................................. 17
Conclusions and recommendations ............................................................................ 23
Future work ................................................................................................................. 24
References ................................................................................................................... 25
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Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods
L IST OF TABLES
Table 1. Major soil groups in the Mekong Delta. ................................................................. 2
Table 2. 10 study locations selected in Hau Giang province, Mekong delta,
Vietnam ........................................................................................................................ 6
Table 3. Ratings of chemical properties on observed raised-beds ..................................... 8
Table 4. Fertilizer recommendation for citrus trees (adapted from Le Thanh
Phong et al., 1996) .................................................................................................... 18
Table 5. Different methods of irrigation scheduling.......................................................... 20
L IST OF FIG URES
Figure 1. Cross section of raised-bed construction – reverse order compared to
original soil. .................................................................................................................. 3
Figure 2. Cross section of a raised-bed construction – the same order as in the
original soil. .................................................................................................................. 3
Figure 3. Conceptual diagram of soil formation processes in raised-bed soils ................ 5
Figure 4. (a) Provincial Administrative Boundary Map of the Mekong Delta, (b)
Map of Hau Giang Province, (c) Studied Sites Map – Extracted from
Google Earth. .............................................................................................................. 6
Figure 5. Average penetration resistance at the different age of the raised-beds
and soil depths on all the observed period of 22-01-2010 to 02-06-2010.
Red dashed line: critical PR of 2.5 MPa. ................................................................. 9
Figure 6. Soil penetration resistance recalculated by PR = a1ƨ + a0 (ƨ estimated
from water retention curves at pF value of 2, 2.5, 3 and 4.2) for soil
depths. ......................................................................................................................... 10
Figure 7. Slope coefficient plotted versus age of raised-beds on different soil
layers............................................................................................................................ 11
Figure 8. Slope coefficient plotted versus age of raised-beds on different soil
layers............................................................................................................................ 12
Figure 9. Soil test interpretation categories redrawn from Havlin et al. (1999) ............. 19
Figure 10. Fertilizer recommendation scheme published in Vitosh et al. (1995). ......... 20
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TRITA LWR PhD Thesis 1073
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Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods
A BBREVIATIONS AND SYMBOLS
ANOVA
AWC
CWR
FBMP
FC
4Rs
LSD
MD
mEq
MPa
PCA
PR
SOM
SPSS
SWRC
WP
ANalysis Of VAriance
Available Water Content
Crop Water Requirements
Fertilizer Best Management Practices
Field Capacity
Right source/Product, Right rates, Right time, and Right place
Least Significance Difference
The Vietnamese Mekong delta
Milliequivalents
Megapascal
Principal Component Analysis
Soil Penetration Resistance
Soil Organic Matter
Statistical Package for the Social Sciences
Soil Water Retention Curves
Wilting Point
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TRITA LWR PhD Thesis 1073
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Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods
L IST OF PA PERS
I.
Van Quang P., Jansson P-E., 2008. Development and description of soil
compaction on orchard soils in the Mekong Delta (Vietnam). Scientific Research
and Essays 3 p:500-504.
II. Van Quang P., Guong V.T., 2011. Chemical properties during different
development stages of fruit orchards in the Mekong delta (Vietnam). Agricultural
Sciences Vol.2, No.3, p:375-381.
III. Van Quang, P., Jansson, P-E., and Guong, V. T., 2012. Soil physical properties
during different development stage of fruit orchards. Journal of Soil Science and
Environmental Management Vol. 3(12), p: 308-319.
IV. Van Quang, P., Jansson, P-E., and Le Van Khoa, 2012. Measurements of soil
penetration resistance to investigate tendencies and explanations of compaction
for orchard soil. International Journal of Engineering Research and Development,
Volume 4, Issue 8 (November 2012), p: 87-96.
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Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods
A BSTRACT
Soil degradation is a complex process which may occur anywhere and at any time. It
directly affects the physical, chemical and biological processes within the soil profile.
Soil degradation can either be as a result of natural hazards or due to manmade
actions, such as mismanagement on cropping patterns, soil preparation and cultivation
practices. Regardless of how it is caused, soil degradation has strong negative effects
on plant and soil productivity. Soil degradation can accelerate a series of processes
such as erosion, compaction, loss of organic matter, loss of whole soil biota, surface
sealing and contamination. This thesis presents the assessment of soil properties to
improve our understanding of soil degradation on raised-bed orchards in the
Vietnamese Mekong delta (MD). Measurements were made on 10 citrus plantations
which had been established during a range of years from 1970 to 1998 at Hau Giang
province. Soil sampling was made in the dry season of 2010 at two soil depths for
each raised-bed to determine soil chemical and physical properties. The soil
penetration resistance (PR) was periodically measured once a week together with soil
sampling for moisture measurements during a period of 5 months. Analysis indicated
the pH value of the soil was tending to decrease, nutrient imbalance and deficiency
was developing, and the soil structure was deteriorating during the age since the
raised-beds were originally constructed. Preventive and restorative measures need to
be considered for restoring and retaining the quality of the soil and the ground water.
These measures should consist of (1) neutralizing of excess acidity, (2) balancing of
nutrients, (3) maintaining of soil organic matter, and (4) application of appropriate
irrigation schedules.
Key words: citrus orchards; nutrient; soil fertility; soil strength; soil
degradation; alluvial soil; Mekong delta
I NTRODUCTION
The Vietnamese Mekong Delta (MD) covers an area of 3.9 million ha
located in the southern territory of Vietnam. It extends from latitudes
104°30’E to 107°E and longitudes 8°30’N to 11°N, bordered by the
Gulf of Thailand to the west, the Eastern Sea to the east and to the
south, Ho Chi Minh City to the north, and Cambodia to the north-west.
There are 13 provinces located in the MD with a population estimated to
be 17.3 million people (20 % of the total of the Vietnamese population
in Vietnam). The average population density is 426 persons per square
kilometer over the whole delta (General Office of Statistics, 2010).
The MD has a monsoon tropical climate, characterised by two distinct
seasons - the dry season and the wet season. The dry season runs from
December to April and the wet season from May to November. The
average rainfall ranges from less than 1500 mm to more than 2500 mm,
90 % of which occurs during the rainy season. The mean temperature
ranges from 23 to 25 °C during the coldest months and from 32 to 33
°C during the warmest months. The humidity is high in the rainy season,
the highest in September (91%), and the lowest in the dry season (79 –
82%).
The MD is a young land mass formed and developed during the
Holocene period by transgression and regression of the sea (Nguyen
Huu Chiem, 1993). The soils are mainly formed by the deposition of
sediment from the rivers (Mekong and Bassac rivers) and the sea. The
sediment is carried by flood water deposited along the banks of the
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Pham Van Quang
TRITA LWR PhD Thesis 1073
Table 1. Major soil groups1 in the Mekong Delta.
2
Soil type
Area (km )
Alluvial
Acid sulphate
Saline
Saline acid sulphate
Old alluvial
Peat
Mountainous
10943
10543
8903
6214
1090
341
347
Mekong1 and Bassac rivers. It tends to form ridge-shaped natural levees
with relatively large particles of sediment parallel to the riverbanks
(Funabiki et al., 2007; Hori, 2000; Tamura et al., 2007). This effect has
given rise to different distributions of soil texture over the different
areas: the further from the riverbanks, the finer the soil texture.
There are seven major soil types in the MD, as summarised in Table 1
(Le Van Khoa, 2002; Nguyen Bao Ve and Vo-Tong Anh, 1990).
Agriculture in the MD is based on land that has been reclaimed and used
since the 17th century (Huynh Lua, 1987). Many management methods
have been used to grow paddy rice, such as burning, ploughing, making
dikes, etc. Traditionally, a single rice crop was cultivated but due to food
security issue and the rapid population growth, a double rice crop system
was initiated in the 1930s, and a triple rice crop system was introduced
around 1980. In addition to rice cultivation, large areas are used for
upland crop and fruit trees.
The topography of the MD is generally flat and low. The elevations
range from 0 to 4 m above sea level, with the exception of some hills and
mountains (Mount Cam in An Giang Province 716 m, Mount Co To in
Kien Giang Province 258 m). The MD receives annual flooding. It
usually happens in August - November, with an average flooding depth
from 0.8 to 1.5 metre. Consequently, farmers have had to build so-called
‘raised-beds’ to avoid submergence of upland crops, most of which are
grown in lowland areas with alluvial soils, to keep the trees above the
flood level.
Excavating and heaping of soil materials have pushed the raised-beds up
from the adjacent lateral ditches. The raised-beds are the long soil strips
that are higher than the original ground surface as illustrated in Figs. 1-2,
which show cross-sections of stages in raised-bed constructions. Soil
layers in the raised-beds can be arranged in reverse order (Fig. 1) or in
the same order as the original soil (Fig. 2).
Efforts have been made to improve soil quality/primary production,
plant varieties and management methods. Research on plant varieties and
farming systems has resulted in positive effects on crop yield, disease
tolerance, better practical techniques and a more suitable cropping
calendar (e.g. Buu and Lang, 2004; Dang Quang Tinh and Pham Thanh
Hang, 2003; Hien and Thi, 2001; Lang et al., 2007; Lang et al., 2001; Thi
Lang and Chi Buu, 2003; Thi Lang et al., 2001; Tu et al., 2003; Vo-Tong
Xuan, 1991).
1
Based on USDA/Soil Taxonomy, 1996 and grouped into major soils
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Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods
Figure 1. Cross section of raised-bed construction – reverse order compared to
original soil.
Figure 2. Cross section of a raised-bed construction – the same order as in the original
soil.
Soil chemical processes, soil nutrients and soil biology have also been
taken into account, e.g. in studies conducted by Vo Thi Guong et al.
(1995), Tran Kim Tinh (1999), and Tran Kim Tinh et al. (2001), Berglöf
et al. (2002), Nguyen My Hoa (2003), Tan et al. (2003), Chau Minh Khoi
et al. (2006) and Chau Minh Khoi et al. (2008).
Process-orientated models which describe the behaviour of the entire
soil-plant-atmosphere system are important tools in understanding water
management and the extent to which soil physical conditions impact on
plant growth (Jansson, 1996). Previously, Van Quang (1998; 2009) and
Vo Khac Tri (1998) applied a soil physical model to the conditions in
the MD. Experimental studies on soil physics in the MD have also been
reported by Le Quang Minh (1996), Uppenberg et al. (1997) and Le Van
Khoa (2002).
There have been warnings of soil degradation in the MD during recent
years, not only in rice farming areas but also in fruit-growing areas
especially in intensive cropping. Some studies of soil physics indicated
that soil compaction has occurred between 20-40 cm soil depth and its
thickness varies from 35 to 50 cm in the intensive rice cultivation areas;
this compaction was apparent at higher soil density, lower soil porosity
(less than 50 vol-%) and lower saturated hydraulic conductivity (Le Van
Khoa, 2002). Chemical soil degradation has also occurred, as evidenced
by nutrient depletion, acidification and sodification (Le Van Khoa,
2002). Vo Thi Guong et al. (2005) reported soil physical and chemical
degradation in citrus plantations in the MD and identified trends of soil
compaction during the ageing of the raised-beds. The cross-sectional
area and length of soil pores decreases during the ageing of the raisedbeds, resulting in a decrease in bypass flow rates (Le Quang Minh, 1996).
Study objectives
Research aims to improve the knowledge on soil degradation of the
raised-beds on fruit orchards in the Mekong delta. Specific objectives
were:
•
To review the available literature from the MD and the impacts of
agricultural activity on soil degradation on fruit orchard soils.
•
To assess soil fertility based on soil chemical properties on raisedbeds of different age.
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Pham Van Quang
TRITA LWR PhD Thesis 1073
•
•
•
To assess the soil compaction on soil physical properties during
different development stages of fruit orchards.
To examine soil penetration resistance and its dependence on soil
moisture and age of the raised-beds in the MD.
To provide general guidelines regarding what soil
amendments/conditioners should be used to increase the fertility
and adjust the pH as well as improve the water management
practices.
Scope of the study
Paper I reviews the previous available studies with respect to soil
degradation from the MD on both rice farming systems and upland
crops. The results in Papers II-IV are further steps performed at the field
scale on 10 selected citrus plantations in the Hau Giang province in the
MD. This information is based on Paper I and a previous study on soil
formation and soil moisture dynamics in agricultural fields in the MD
(see Van Quang, 2009). In Papers II and III, an examination of soil
chemical and physical properties was made in order to evaluate soil
fertility and soil compaction during different development stages of fruit
orchards by analyzing soil samples in the laboratory. Paper IV dealt with
soil penetration resistance measured by using a portable electronic
penetrometer. The relationship between PR and soil moisture was
analyzed by using linear regression methods based on the penetration
resistance data series and soil moisture data.
Soil compaction formation concept
Due to raised-beds’ construction, the soil rearranges itself to get a stable
state under natural conditions and human activities by time. The
reconstruction processes of soil physical characteristics in the raised-beds
can briefly be described as follows:
• The alternate drying and wetting cycles of the raised-bed soils in the
MD are in combination with the fine soil texture.
• Cracks on the raised-beds are normally formed during the dry season
or during drought periods in the rainy season.
• The soils in the raised-beds have been subject to overtopping and
internal erosion processes.
• When the bank is built for the first time, the space distribution in situ
is very high.
• Larger soil blocks break down into smaller blocks through weathering
processes and the mechanical process of preparing the soil surface
for planting. The soil particles are gradually rearranged until they
become stable.
• Fine-grained soil particles move with the flow of rain or irrigation
water and gradually fill up the soil gaps. If the clay content of the soil
is high, this can cause an increase in soil resistance.
• Soil aeration exposes soil organic matter, and speeds up the
breakdown of organic matter by oxidation processes. This is harmful
to soil structure and leads to a rapid increase in soil density.
• The rise in water level due to the rainy season or irrigation and the
fall in water level due to the dry season or drainage will lead to
changes in soil volume.
• Cultivation activities, such as irrigation, drainage and weed removal
from the raised-beds can accelerate the soil compaction rate.
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Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods
Wet and dry circle
Human’s activities
Breakdown of soil blocks
Forming cracks
Time
Modify the effects of texture on other properties (cementing agents)
Erosion
All soil particle size
Suffusion
Smaller soil particle sizes
Soil loss/bulk density
Bulk density
Aggregate/structure
Conductivity
Finer soil particle sizes
Clogging
Compaction
Figure 3. Conceptual diagram of soil formation processes in raised-bed soils
Therefore, the soil in the raised-beds is affected by a complex process
comprising of a core component of physical, chemical and biological
factors, together with factors relating to climate and agricultural
practices. The physical process can be summarized as a diagram as
shown in Fig 3.
M ETHODS
Field experiment performance
Sites descriptions
The study was carried out on 10 randomly selected raised-beds at
different age in the Hau Giang province, MD (Table 2, Fig. 4). The
selection of raised-beds was based on the investigation of 60 currently
used raised-beds – a previously unpublished report from the Can Tho
University, Vietnam. The selected sites were then reinvestigated to
exactly record the year of construction. Citrus trees have been grown
since the onset of raised-bed construction. The soil is classified as an
alluvial soil (Soil Science Department, CTU 1985-1996; Soil Survey Staff,
1996). The soil texture is classified as silty clay. The raised-beds were
constructed by excavating and heaping up soil materials from adjacent
lateral ditches to form the long raised strips that are higher than the
original ground surface (Fig. 1). Soil layers on the raised-beds were
commonly arranged in the reverse order compared to the sequence of
soil master horizons in natural soils.
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Pham Van Quang
TRITA LWR PhD Thesis 1073
Figure 4. (a) Provincial Administrative Boundary Map of the Mekong Delta, (b) Map
of Hau Giang Province, (c) Studied Sites Map – Extracted from Google Earth.
Table 2. 10 study locations selected in Hau Giang province,
Mekong delta, Vietnam
Locations
Latitude
1
2
3
4
5
6
7
8
9
10
9 53' 44.38''
o
9 53' 28''
o
9 53' 35.74''
o
9 53' 31.7''
o
9 53' 28''
o
9 53' 35.7''
o
9 53' 29.18''
o
9 53' 38.47''
o
9 54' 2.7''
o
9 53' 29.22''
Longitude
o
o
105 43' 41.09''
o
105 43' 57.14''
o
105 43' 47.75''
o
105 43' 42.78''
o
105 43' 29.46''
o
105 43' 41.27''
o
105 43' 50.7''
o
105 43' 45.34''
o
105 43' 49.4''
o
105 43' 41.16''
Age (years)
15
17
19
28
30
31
32
33
35
37
Sampling and analysis
Data collection started at the beginning of the dry season (January 2010)
after the flooding level had receded and the ground water was below the
surface of the raised-beds. Soil samples were randomly taken on each of
the selected raised-beds at 0-20 cm (topsoil) and 20-50 cm (subsoil)
depths with four replicates including undisturbed core samples (5 cm
long and 5 cm in diameter - approximately 100 cm3 in volume) and
disturbed soil samples of about 4 kg for each layer. All undisturbed core
samples were taken by augering to a desired depth using a bulk density
auger. Disturbed samples were collected by digging using a shovel.
Disturbed soil samples were air-dried, homogenized and sieved through
a 2 mm mesh screen to determine soil chemical properties including pH,
organic matter, CEC, total nitrogen, NH4+, NO3– and exchangeable
Ca2+, Mg2+, K+ as well as available phosphorous. Statistical analyses were
performed by using the SPSS statistical package (version 16).
Soil physical analysis included bulk density, saturated conductivity, water
retention, and soil texture. Data were analyzed by using the SPSS
statistical package (version 16). A two-way ANOVA with the two factors
(age of raised-beds and soil depths) was performed to evaluate the soil
physical properties. In case no interaction effect between age and soil
depths on soil properties was detected, a one-way ANOVA with an
6
Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods
“age” factor was applied for each of the depths individually. Differences
in the mean soil physical properties were determined using the least
significant difference (LSD) at the 0.05 significant levels. A Pearson
correlation analysis was applied to estimate the relationships among soil
properties and age of the raised-beds. A principal component analysis
(PCA) was used to elucidate the main relationships between variables
based on the correlation matrix. The final component structure was
included only when principal components with eigenvalues greater than
unity were found. The purpose of the PCA was to discriminate between
clay content and age from the impact of soil ageing on compaction.
Soil penetration resistance measurement
This study used a portable electronic penetrometer, with a built-in data
logger for storage and processing of a great number of measuring data
(500 measurements), developed by Eijkelkamp Agrisearch Equipment,
the Netherlands. The penetrologger method had a measuring range of
1000 N (10 MPa). The PR was measured using a load cell connected to a
cone screwed onto the bottom end of a bipartite probing rod. The cone
used in this study had a 60° angle and the base area of 1 cm2. An internal
ultrasonic sensor accurately registered the vertical distance above the soil
surface, and the load cell was used to calculate the PR at each cm and the
device stored data up to the depth of 80 cm in the profile. Speed
penetration was set at 1 cm·s-1. Data were periodically collected from 10
raised-beds with once a week, started from 22-01-2010 to 02-06-2010 (a
data series of ten points). On each raised-bed, penetrometer
measurements were designed into three plots and three ‘penetrations’
were carried out to obtain a representative average result per plot
automatically. At the same time as PR measurements, soil samples were
also taken from a distance within 0.1 m from the PR measurement
points. Sampling depths were selected in the following intervals, 0 to 10
cm; 10 to 20 cm; and so on down to water level. Soil samples were then
collected to determine the soil moisture by the dried method at 105 °C in
the laboratory. The measured soil water retention characteristic curves
were used to convert the volumetric moisture content data to water
potential (pF). The volumetric moisture content was calculated from the
mass moisture content and bulk density. A subset of PR data series was
extracted from the PR measurements. The depths of 5, 15, and 25 cm
from the soil surface were considered as representatives for the layers of
0-10, 10-20, and 20-30 cm respectively.
The relationship between the PR and soil moisture (and pF values as
well) was calculated by a linearly regressive technique for the observed
period (from 22-01-2010 to 02-06-2010) on 10 raised-beds. The general
equations are as following:
PR = a1ƨ + a0 (for soil moisture)
and PR = a1pF + a0 (for pF values)
(1)
(2)
Where PR is soil penetration resistance; a1 is slope coefficient; a0 is
intercept; ƨ is soil moisture; and pF is the base 10-logarithm function of
the soil pressure head expressed in cm of water.
Comparisons of the differences between the raised-beds were
determined by using Student’s t-test with 0.05 significant levels.
7
Pham Van Quang
TRITA LWR PhD Thesis 1073
R ESULTS
The results obtained from Paper I played a role as a groundwork for
further steps in this thesis. There was a tendency of gradual decrease of
soil pH, organic matter, total nitrogen and available nitrogen according
to the ages of the raised-beds (Table 3. in Paper I). Higher soil
penetration resistance (Table 4. in Paper I) and bulk density was
identified through statistical analysis in respect of older raised-beds.
Remarkable points were that the trends of soil degradation during the
ageing of the raised-beds on both chemical and physical views.
Chemical properties on raised-bed soil (Paper II)
The results of soil chemical analysis on the raised-beds are summarised
in Table 3. The ratings were according to the criteria reported from
Abbott (1985), Bruce and Rayment (1982), Marx et al. (1996) and
Metson (1961).
Physical properties on raised-bed soil (Paper III)
Soil texture can be classified as clay to silty-clay soil. The bulk density of
topsoils ranged from 0.76 to 1.24 g·cm-3. Saturated hydraulic
conductivity spanned the range from 2.04-5.5 m·day-1. Organic matter
was in the range of 3.0-12.2% (Table 4 and 5 in Paper III).
Soil water retention curves (SWRC) had rather gentle shape as the
volumetric water content gradually changed with soil pressure heads but
different shapes among the sites as well as the soil depths within each
site were found (Fig. 2 in Paper III).
The results of correlation analysis and Principal component analysis
(PCA) are shown in Table 9 - 12 and Fig. 4 in Paper III. The PCA result
was consistent with the correlation analysis.
Soil penetration resistance (Paper IV)
Soil penetration resistance (PR) plotted against soil depth under the
different raised-beds was shown in Fig. 5 for the observed period of 2201 to 2-06-2010 (average values). The PR values changed in the range of
0.13 - 3.05 MPa with confidence intervals from 0.03 to 0.52 MPa in a soil
profile of 60 cm depth (Fig. 3 in Paper IV).
Soil bulk density data and SWRC were reused from Paper III to calculate
the volumetric moisture content and to convert volumetric moisture
content data to water potential expressed as pF. The results of soil water
content (vol-%) and PR with time were shown in Fig. 4 (in Paper IV) for
three soil depths (0-10 cm, 10-20 cm and 20-30 cm). Volumetric soil
water content and pF values at different raised-beds were shown in Fig.
6 and 7 (in Paper IV) for 3 soil layers.
Table 3. Ratings of chemical properties on observed raised-beds
Very low
pH-water (1:2.5)
–1
CEC (cmol·kg )
OM (%)
+
–1
K (cmol·kg )
2+
–1
Ca (cmol·kg )
2+
–1
Mg (cmol·kg )
Ntotal (%)
–1
+
NH4 (cmol·kg )
–
–1
NO3 (cmol·kg )
–1
Available P (mg·kg )
Low
Moderate
High
Very high
3.93 - 5.11
14.18 - 22.07
2.99 - 12.19
0.22 - 0.81
4.41 - 9.85
1.45 - 6.43
0.10 - 0.27
14.31 - 37.44
9.37 - 64.83
14.64 - 43.21
8
Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods
Figure 5. Average penetration resistance at the different age of the
raised-beds and soil depths on all the observed period of 22-01-2010
to 02-06-2010. Red dashed line: critical PR of 2.5 MPa.
9
Pham Van Quang
TRITA LWR PhD Thesis 1073
Figure 6. Soil penetration resistance recalculated by PR = a1ƨ + a0
(ƨ estimated from water retention curves at pF value of 2, 2.5, 3 and
4.2) for soil depths.
10
Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods
The average PR at the soil layers of 0-10 cm (layer 1), 10-20 cm (layer 2)
and 20-30 cm (layer 3) for the period of 22-01 to 02-06-2010 were
shown in Figure 8. Linear relationships between the PR against age of
the raised-beds (R2 > 0.7) were obtained for three soil layers. The
relationships represented the increasing trend of PR versus the aging of
the raised-bed. The highest PR values obtained at all three layers were
measured on the 35-year-old raised-bed and the lowest PR were found
on 15-year-old raised-bed, except for 28-year-old raised-bed. The PR
values, which were greater than 2.5 MPa, were found at the 30, 31, 32,
33, 35 and 37-year-old raised-beds for layer 1, at the 31, 32, 33, 35 and
37-year-old raised-beds for layer 2, and at the 33, 35 and 37-year-old
raised-beds for layer 3 (Fig. 5).
The recalculations of PR based on regression equations (PR = a1ƨ + a0)
correspondingly using soil moisture values at pF of 2, 2.5, 3 and 4.2 were
presented in Fig. 6 for layer 1, 2, and 3. The results showed the
increasing trend of PR with age of the raised-beds on whole soil profile.
PR values were higher corresponding to the lower in soil moisture.
Similarly, the results of PR by using equation 2 with pF values of 2, 2.5, 3
and 4.2 were not significant differences compared to equation 2
The slope coefficients increased with the soil depth as well as with age of
the raised-beds (Fig.7). The relationship between a1 (from eq. 1) and age
of the raised-beds was significantly consistent with linearity (R2 > 0.7).
The plot of a1 versus age becomes more scattered for deeper layers, this
was reflected by the decrease in R2 (Fig. 7). In contrast to soil moisture,
the slope coefficients (from eq. 2) decreased with soil depth as well as
with age of the raised-beds (Fig. 8); however, the plot of a1 versus age of
the raised-beds was more dispersed.
Figure 7. Slope coefficient plotted versus age of raised-beds on
different soil layers.
11
Pham Van Quang
TRITA LWR PhD Thesis 1073
Figure 8. Slope coefficient plotted versus age of raised-beds on
different soil layers.
D ISCUSSION
Soil properties assessment
Chemical properties
Soil plays a vital role in the growing of crops and it is also a major source
of plant nutrients. The constituents of soil include living organisms,
residual mineral matter, organic matter so-called “humus”, air and water.
Each of these components is not only highly important in soil genesis
but it also plays a major role in determining the suitability of the soil for
crop production (Magdoff and Van Es, 2009). In the nature, they may be
able to adjust to reach a dynamic equilibrium state among themselves.
However, the activities of humans may directly or indirectly disrupt this
state. For example, natural vegetation is removed from the ground
surface. This may benefit on one side, i.e., less nutrient competition, but
the soil will face to a problem of water erosion. In the long term, soil
organic matter is depleted and it becomes a source of carbon dioxide for
the atmosphere (a greenhouse gas associated with global warming).
Hence, there could be potentially harmful effects all the way back to soil
organisms and plants. Imbalances of soil nutrients due to fertilizer
addition are another example.
It is obvious that one cannot get healthy plants from unhealthy soils.
Generally speaking, soil quality involves the sufficient quality of physical,
chemical, and biological properties. Chemical properties (or soil fertility)
depend on a favourable interaction between soil components and phases
12
Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods
that optimize the soil physical quality (Lal and Shukla, 2004).
Consequently, they successively affect the biological soil properties.
In this study, the results obtained from the analyses of soil chemical
properties in the raised-beds (Paper II), indicate that there were two
main issues including a low soil pH and a nutrient deficiency or nutrient
imbalance. A low pH may lead to a deficiency of major plant nutrients
such as Ca2+ and Mg2+, as well as Nitrogen and Phosphorus. Adsorbed
H+ and Al3+ ions may predominate over the cation exchange capacity. At
soil pH (<5), aluminium and manganese could become toxic to plants. A
decrease in organic matter with the aging of raised-beds may contribute
to the decrease in negatively charge components and this may then
reduce nutrient holding capacity of the soil. Restrictions of soil nutrients
to plants was reflected through a moderate cation exchange capacity
(CEC) and an imbalance of cations in the observed raised-bed soils,
which was clarified by the concentrations of exchangeable cations K+,
Mg2+ and Ca2+ and a low base saturation percentage of calcium,
magnesium, and potassium in the soil. In addition, the soil contained too
much fertilizer residues which was illustrated by high soil NH4+-N and
NO3–-N levels as well as high levels of available phosphorus (Table 3).
In summary, the soils in observed raised-beds have resulted in several
adverse soil chemical characteristics.
Physical properties
Soil physical properties during different development stages of fruit
orchards were discussed in Paper III. The results of a correlation analysis
and PCA calculations were enough to discriminate between clay content
and age from the impact of soil ageing on compaction. The clay content
mostly showed the impact on the range of small pore sizes, while the
aging of the raised-beds exhibited the influence on the range of larger
pore sizes. The clay content showed a covariance with age that may have
acted upon the results of the other soil properties, and it seems to
counteract on the compaction processes. The clay content was correlated
with the age; although, the correlation was weakly negative, suggesting
that the clay content was decreased with the increase of the aging of the
raised-bed. This may be true, if the losses of clay content greater than the
additions derived from the soil formation processes.
However, the results of soil texture indicated a high variability within
sites and relatively low between sites for all the master soil horizons in
the soil profile, suggesting that there could be a high degree of
uncertainty and the errors could be raised from data collection. On the
other hand, the soil texture is mainly inherited from the characteristics of
parent materials; the clay content has not been expected to change with
time. From that fact, the slight differences in soil texture of the raisedbeds could be seen as an inherent distribution, i.e., a common
consequence has been derived from the soil formation processes of the
MD.
From another point of view, soil texture is a basic physical property
including the components of soil particle distribution as clay, silt and
sand. These primary particles are bound together into larger masses by
the cementing agents that consist of microbial gums, iron oxides, and
organic matter (known as soil structure/soil aggregates). Although
management practices do not change the soil texture, they may modify
the effects of texture on other properties. Pores between the particles
and between aggregates are much more important than sizes of the
particles themselves, because the total amount of pore spaces governs
13
Pham Van Quang
TRITA LWR PhD Thesis 1073
water movement and availability for sustaining soil organisms and plants
(Magdoff and Van Es, 2009).
Therefore, the next discussion will be of more concern with the
alteration of soil structure and aggregates under conditions of natural
and management practices in terms of the particles’ rearrangement and
their consequences. For the soil surface of raised-beds in the MD, it is
easy to recognize that heavy traffic/equipment can be ignored; the soil is,
therefore, mainly subjected to the dry and wet cycle as well as the
handwork management practices.
Due to the effect of construction, the soil in raised-beds was reorganized by the excavation processes, where the soil mass was
randomly piled up from the original soil. These actions initiate the
dominance of macro-pores in the soil. By this case, a phenomenon of
rearranging the aggregates and soil particles has been occurred with time.
The larger aggregates have been broken down into smaller ones and the
dispersion of aggregates and soil particles under the impacts of internal
and external forces. These processes may have drastically occurred in a
few years after constructing. The soil evolution will be slowing down
when the soil has relatively stabilized. The stability of raised-beds’
surface and reduction in soil bulk density may occur at the beginning
period and be combined with the decomposition of the original root
plants.
Repeated cycles of drying and wetting play a major role in aggregation
through shrinking and swelling, which leads to formation of aggregates
in the raised-beds’ soil. Swelling leads to reorientation of particles; and
shrinking leads to formation of cracks. The cracks on the raised-beds are
normally formed during the dry season or during the drought period on
the rainy season. These cracks not only significantly facilitates infiltration
of storm-water into the soil, but also speed up the progress of dispersive
processes of the soil; therefore, the soil particles may be detached from a
soil mass and then carried off by flowing water. It is evident that the soil
in the raised-beds has been subject to overtopping and internal erosion
processes. Of which, internal erosion occurs when water flows through
the cavities, cracks, and/or other continuous void within the
embankment (Fell et al., 2003; Jantzer, 2009); these openings may be a
result of soil block arrangement during construction, differential
settlement, desiccation, and/or decay of woody vegetation roots.
According to Packer et al. (1992), the failure of aggregates and the
elimination of smaller particles or entire layers of soil or organic matter
can weaken the structure and even change the texture.
Due to the undergoing complex processes of the nature and
management practices, the continuous rearrangement of soil
particles/aggregates leads to changes of soil structure year to year.
Aggregation derives from the rearrangement of dispersive
aggregates/particles, flocculation and cementation (Duiker et al., 2003).
Aggregation is mediated by soil organic carbon, biota, ionic bridging and
clay. Since the more intensively soils are dried, as well as the more often
they are wetted, the smaller the aggregate diameter then becomes.
Smaller aggregates/particles can be lodged in between the larger
aggregates/particles, clogging the pore spaces and increasing soil bulk
density. Continuation of the processes indefinitely, without any
biological activity, could also result in a structure of smaller and dense
aggregates. Reduction of structure to smaller aggregates will cause the
loss of the coarse pores that is important for soil drainage and aeration
(Gardner et al., 1999). Consequently, the decline in soil structure arises,
14
Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods
which is increasingly seen as a form of soil degradation (Chan et al.,
2003) and is often related to land use and soil/crop management
practices. Soil structure influences soil water movement and retention,
erosion, crusting, nutrient recycling, root penetration and plant yield. Soil
tillage at inappropriate water moisture as well as the loss of organic
matter due to oxidation may cause structural degradation; continued
cultivation without organic additions can result in loss of microaggregation leaving a soil very vulnerable to compaction and erosion
(Gardner et al., 1999).
The evidence from the summary statistics for the observed raised-beds
in the correlation analysis (Paper III) showed that soil structure gradually
has declined with time. It was illustrated by the aging of raised-bed was
significantly correlated with bulk density (positive), saturated hydraulic
conductivity, wet range suctions (pF 0.4 to 3.0) and organic matter
(Tables 9 and 10 in Paper III). It was also demonstrated through a
decreased tendency with age of the fast and slow drainage pores (Figure
3 in Paper III), indicating a decreased-level of soil macroporosity; and
the increasing in micro-pore proportion with age was identified by soil
water retention curves (Table 6 and Figure 2 in Paper III). According to
Assouline et al. (1998), an increased proportion of microporosity can be
as an indicator of soil compaction.
Next discussion accentuates in soil penetration resistance (soil strength)
and its relation to soil moisture and water potential (pF) to understand
how soil degradation will develop during cultivations in the raised-beds
(Paper IV). These analyses clarify the results of physical properties
discussed in previous section.
The penetration resistance (PR) of soils is an important parameter that
influences to nutrient uptake, water infiltration and redistribution,
aeration, seedling emergence and root growth, resulting in decrease of
plant yields, increased erosion and difficulty in soil cultivation (Bengough
et al., 2011.; Taylor and Brar, 1991). The PR is attributed to forces of
cohesion and adhesion and varies with soil moisture (Lal and Shukla,
2004). Since the plant root system is mostly growing in porous media, it
must therefore overcome mechanical soil resistance. The PR has limited
to effective rooting depth; if the soil status is too weak, anchor capacity
of plant roots will not be adequate to withstand the forces of wind and
water; on the other hand, if it is too strong, the plant roots will not have
required strength to penetrate to the soil matrix. Plant roots will be
dramatically affected if the soil strength exceeds the capacity of root
penetrability (Jarmillo-C et al., 1992). As the results showed in Fig. 6
indicated that, the PR has reached to 2 MPa on over the top 30 cm
depth for starting from 30-year-old raised-beds at the field capacity
condition and PR is still able to be higher when the soil becomes drier.
This value was higher than the value reported by Lutz et al. (1986) - the
study on relationship between citrus root development and soil physical
conditions, of which documented that 1.5 MPa is the maximum soil
strength restricting root growth. Studies conducted by Kees (2005) and
Raper et al. (2005) showed that roots of most plants are inhibited at PR
of 1.5 MPa and roots of many plants stop to grow at PR of 2.5 MPa. A
penetrometer measurement of 2 MPa generally concerned as sufficient
to impede the growth and development of plants (Taylor and Gardner,
1963). At PR larger than 2.5 MPa, root elongation is significantly
restricted (Whalley et al., 2007). However, some studies showed
significantly higher PR values due to the influence of the soil moisture
15
Pham Van Quang
TRITA LWR PhD Thesis 1073
content at the time of penetrometer readings with visually healthy plants
(Smith et al., 1997; Sojka et al., 2001; Whalley et al., 2007).
The relationship between the PR and soil moisture (and pF values as
well) was shown through the regressive equation (1) and (2). The results
demonstrated that the less change in PR associated with a change in ƨ (or
pF value) when the soil of raised-beds has been more exposed to wet
and dry cycle and cultivating since construction. This indicated that the
soil in the young raised-beds has been more macropores than that in the
old ones, and it has been easier to disturb. While the soil in the old ones
are in a mode of more inert system with smaller sensitivity. In other
words, the soil structure has become smaller and dense aggregates with
time, resulting in higher PR values. It can see that both of predictors (ƨ
and pF values) showed the same phenomena but in the different ways.
Because the comparisons of PR, predicted by ƨ and pF values, were not
significantly different, so the pF value and either ƨ can be used to predict
the PR. However, it will be easier to obtain the ƨ than pF values. In
addition, according to the slope coefficients plotted against age of the
raised-beds on the different soil layers, drawn from regression equations
(1) and (2); the results showed that the explanations based on ƨ were
better than pF values (Fig. 7). This suggested that the equation (1) could
predict the PR more credible than using (2).
In summary, based on above discussions, the soil in the observed raisedbeds has been exhibited a tendency of degradation in term of soil
structure decline. Management of soil physical conditions to improve the
constraints for plant growth will not only conserve the soil functions for
the future but also contribute to the minimization of soil degradation.
Favourable soil structure and high aggregate stability are important role
to enhancing porosity and resistance to erosion susceptibility, improving
soil fertility, and hence increasing agronomical productivity. Soil
structural and aggregate stability are affected by many interactional
factors including the environment, soil management, plant influences
and soil properties. The soil properties consist of such as mineral
composition, texture, soil organic carbon (SOC) concentration,
pedogenic processes, microbial activities, exchangeable ions, nutrient
reserves, and moisture availability (Lal and Shukla, 2004). The rate and
stability of aggregation generally increases with SOC, clay surface area
and CEC. The loss of organic matter and consequently soil fertility is
often caused by unsustainable cultivation practices e.g. unsuitably
commercial fertilizer supply, working at an inappropriate soil moisture
and/or continuous removal of vegetation without returning biomass
residues into the soil.
Soil amendments
Essential Plant Nutrients - their functions, and role of fertilizers
Plants require 16 essential chemical elements for normal functioning,
growth and completion of their life cycle (Maheshwari, 2012; Roy, 2006).
Out of these elements carbon, hydrogen, and oxygen, are usually not
considered as nutrient elements because they are supplied by air and
water, and the other 13 elements are primarily derived from the soil and
are generally managed by the farmers. The thirteen classified as
macronutrients and micronutrients based on their plant requirements
including nitrogen (N), phosphorus (P), potassium (K), calcium (Ca),
magnesium (Mg), sulphur (S), boron (B), chlorine (Cl), copper (Cu), iron
(Fe), manganese (Mn), molybdenum (Mo) and zinc (Zn). Plants obtain
nutrient elements through root uptake from the soil solution; nutrient
16
Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods
uptake depends on nutrient absorbability of the plant, its growth stage
and the nutrient level at the root zone (Maheshwari, 2012; Roy, 2006).
The sources of these soluble nutrients in a soil can be natural, synthetic
or recycled wastes. Although most soils can physically sustain more
nutrients than a plant needs in a growing season, yet little of these
nutrients are available in the soil solution. Moreover, plants remove a
large amount of nutrients from the soil during the growing season
(especially N, P, K), leading to deficiencies of one or more essential
nutrients; therefore, application of fertilizers is necessary to compensate.
Yet, an integrated plant nutrient management needs to be carefully
considered to optimize the condition of the soil with regard to its
physical, chemical, biological and hydrological properties, for enhancing
crop productivity, while minimizing soil degradation. Balanced
application of appropriate fertilizers is a major component of integrated
nutrient management (Maheshwari, 2012). Fertilizers need to be applied
at the level required for optimal plant growth based on plant
requirements and agro climatic considerations (Maheshwari, 2012).
According to Roy (2006), an optimal nutrient supply requires: (1)
sufficient available nutrients in the root zone of the soil; (2) rapid
transport of nutrients in the soil solution towards the root surface; (3)
satisfactory root growth to access available nutrients; (4) unimpeded
nutrient uptake, especially with sufficient oxygen present; (5) satisfactory
mobility and activity of nutrients within the plant. While deficiency of
any essential nutrient will become create a limiting factor for plant
production, and could have also affected access of other elements,
conversely too much of any nutrient could be toxic to plants and reduce
the disease resistance of the crop.
General soil amendments to raised-bed orchards in the Mekong delta
Soils in the MD mostly contain kaolinite, illite and montmorillonite in
the clay fraction which have been inherited from parent materials (Le
Van Khoa, 2002; Nguyen Huu Chiem, 1993). Growers often use a single
rather than compound fertilizers, which stresses the use of nitrogen over
phosphorus, potassium, lime and micronutrients which are less
appropriately considered (Siem, 1997). As a result the soils become
acidic, their fertility is lowered, and their phosphorous fixation is high.
The reasons mentioned above could partly explain the nutrient
imbalance as described in Paper II. Therefore, it is necessary that soil
acidity be corrected before fertilizers are applied because without
neutralizing excess acidity, balanced nutrient application could not bring
into play effectively (Maheshwari, 2012; Roy, 2006).
Target pH and Critical pH
Target pH is defined as the soil pH range at which plant nutrients are
optimally available for plant uptake and growth. Critical pH refers to the
maximum pH value at which liming increases plant yield (Adams, 1984).
Because the characteristics of growth vary with plant type, target pH also
varies with the type of plant to be grown. Critical pH is dependent on
plant type and reflects the management practices as well as an economic
consideration.
The first step in soil pH management is to identify the suitable target
pH. For citrus trees, the best range is between 5.5 and 6.5 (Calabrese,
2002). Liming practices are then used to ensure that soil pH maintains
close to the target pH and does not fall above or below the critical pH
values. For this study, critical pH values are about pH 5.5 - 6.5. Hence,
lime should be applied whenever the soil pH approaches 6.5 because
lower pH values will adversely affect plants.
17
Pham Van Quang
TRITA LWR PhD Thesis 1073
Lime Requirement
A lime recommendation is based on the suitable target pH, the
exchangeable acidity level in the soil, and the efficiency of the liming
reaction. The lime requirement of a soil is the amount of calcium
carbonate (in tonnes) calculated to raise the pH of a hectare of soil 20
cm deep, under field conditions, to, and maintain at, 6.5 (Faithfull, 2002).
The lime requirement cannot be calculated directly from the pH value
because of the need to also neutralize reserve acidity, which is not
reflected in the pH value. However, pH and soil texture can be used to
estimate the amount of lime approximately.
There are several methods for determining the lime requirement,
including Adams-Evans buffer (Adams and Evans, 1962), Mehlich
buffer (Mehlich, 1976), SMS buffer (Sims, 1996), and Lime Buffer
Capacity (Kissell and Sonon, 2008). All these methods for estimating
lime requirement need to be based on the soil samples being
representative of the area sampled.
Fertilizer Recommendations
Unfortunately, there is not much specialised literature that would offer
detailed approaches on rates of application of fertilizer for citrus trees in
the MD. One of such was written by Phong et al. (1996), the fertilizer
recommendations are presented in Table 4 for citrus plantations. This
promotion was mainly based on the experiences of farmers in the MD
and reference materials related to the recommended fertilizer for citrus
trees in the world.
In fact, soil fertility fluctuates throughout the growing season each year
because of fertilizer addition, plant growth and development, plant
harvest, and nutrient losses. Therefore, regular soil testing and leaf
analysis are crucially necessary to assess the current status of nutrients
for both soils and plants. These tests provide the necessary information
needed to maintain suitable fertility year after year, and help to adjust and
make appropriate fertilizer program decision. In addition to yield
response functions, fertilizer recommendations should be considered in
relation to sustainability, thus, nutrient management should be based on
the Fertilizer Best Management Practices (FBMPs), which address on the
right source/product, right rate, right time, and right place (4Rs). More
detailed discussions on the 4Rs are available in Roberts (2007), Force
(2009), and Mikkelsen (2011). Fertilizer recommendation is a component
within the FBMPs because understanding the details of fertilizer
recommendation philosophy may contribute to sustainable nutrient and
soil management and hence toward sustainability for agriculture.
In general, there are some common methods used to make fertilizer
recommendations. These methods consist of the following (1) sufficient
level of available nutrients, (2) build and maintenance, and (3) base
cation saturation ratios. All are based on either the plant or the soil.
Table 4. Fertilizer recommendation for citrus trees (adapted from
Le Thanh Phong et al., 1996)
Age of tree
1-3
4-6
7-9
> 10
N
P2O5
(g/tree)
K2O
50-150
200-250
300-400
400- 800
50-100
150-200
250-300
350-400
60
120
180
240
18
Soil Degradatio
on of Raised-b
beds on Orchaards in the Mekkong Delta - Field
F
and Labo
oratory Methodds
Figuree 9. Soil test interpretatio
ion categoriees redrawn from
f
Havlin
n et
al. (19999)
1. Sufficciency level of avvailable nutriennts (SLAN): the
t focus of this
t approach
h is
fertilizin
ng the plantt. To providde the best response,
r
thiis provides the
t
greatestt economic benefits to the plant producer.
p
T
The
amount of
fertilizeer recommen
nded dependss on the soill test level; fertilizer
f
is on
nly
applied when the soil
s test leveel drops belo
ow some criitical ranges to
achievee optimum reesponse of th
he plants, as shown
s
in Figg. 9. The SLA
AN
methodd requires reegular and accurate
a
soill testing to determine the
t
nutrient requiremen
nt for the cuurrent status of the plan
nts, and preccise
knowledge of optim
mum application rates for particular
p
plants.
2. Buildd and maintenaance fertility: Fiigure 10 reprresents a moddel for fertilizzer
recomm
mendation suuggested by Ohio
O
State University.
U
In
n this approach,
build-up
p range is co
onsidered as an indication
n of nutrientt deficiency for
f
plant growth.
g
Ferrtilizer additiion provides for increasing availab
ble
nutrients to the criitical test levvel. Plant ressponse to ferrtilizer additiion
graduallly decreasess as soil test
t
level reaches
r
criticcal level. For
F
mainten
nance range, nutrients aree added based
d on the estim
mation of plaant
removaal or losses for
f maintainin
ng the soil nutrient
n
levell. In this ran
nge,
plant yield
y
does not
n respond to nutrientt application
n. Soils in the
t
mainten
nance range are usually not
n expected to improve yield
y
but in the
t
purposee of maintain
ning soil test levels over time.
t
In draw
wdown rangee is
at the soil
s test levells above the maintenancee limit. Fertillizer addition
n is
made to
o slow the drraw of nutrieent levels dow
wn and quickkly become 0 as
soil tesst levels incrrease. This method
m
is on
nly suitable for
f less mob
bile
nutrients such as P and
a K.
3. Base cation saturatio
ion ratio (BCSR
R): fertilizer recommenda
r
ations are bassed
on the concept that maximum plant responsee is only attaiined by creatiing
the ideaal ratios of Ca,
C Mg and K.
K If the ratio
o is out of baalance, then the
t
applicattion of fertiliizer is recom
mmended. Thee ideal ratios of Ca, Mg and
a
K weree proposed by
b Bear et al.
a (1945), Bear
B
and Totth (1948). T
This
approacch often reesults in thee greatest amount
a
of fertilizer beiing
recomm
mended.
These three
t
method
ds may apply as general guuidelines for better
b
decisio
onmakingg in the application of fertilizer
f
undder condition
ns of soils aand
farmingg system in the MD. To
T bring thiss about, it is
i necessary to
calibratte soil tests to
o adapt to thee specific loccal conditionss of the MD. In
addition
n, different methods
m
are based on th
heir own assuumptions abo
out
what different
d
plan
nts need, andd hence diffeerence of ferrtilizer quanttity
recomm
mended. Thee choice of an
a appropriatte method selection shouuld
also con
nsider econom
mic, social an
nd environmeental targets.
19
Pham Van Quang
TRITA LWR PhD Thesis 1073
Figure 10. Fertilizer recommendation scheme published in Vitosh
et al. (1995).
Table 5. Different methods of irrigation scheduling
Method
Soil water managements
- Observe and feel
- Soil moisture sample
- Neutron probes
- Time Domain
Reflectrometry (TDR)
- Tensiometers
- Electrical resistance
blocks
Soil water balance
- Water budget approach
- Atmometer
Measured parameter
Equipment needed
Soil moisture content by
feeling
Soil moisture content by
taking samples
Soil moisture content
Soil moisture content
Hand probe
Soil moisture content
Auger, caps, oven
Soil moisture content
Neutron moisture meter
TDR meter
Soil moisture content
Soil moisture content
Tensiometers including
vacuum gauge
Electric resistance of soil Resistance blocks AC
moisture
bridge (meter)
Soil moisture tension
Soil moisture tension
Climatic parameters:
temperature, radiation,
wind, humidity and
expected rainfall,
depending on model
used to predict ET
Reference ET
Irrigation criterion
Soil moisture tension
Weather station or
available weather
information
Estimation of
moisture content
Atmometer gauge
Estimate of moisture
content
Pepista system
Estimate of moisture
content
Estimate of moisture
content
Estimate of moisture
content
crop water stress
index
Use of plant water stress criteria
- Trunk or branch diameter Measuring of stem,
change
branch or fruit diameter
- Leaf water potential
Leaf cell water potential
Pressure chambers
- Sap Flow
Reference ET
Sap flow sensors
- Canopy measurements
Surface temperature
Infrared radiometers
Soil moisture content
Software
Soil moisture content
Growth and yield
Software
Water status of the
plants at different
stages
Use of models
- Models based on soil
water balance
- Mechanistic models
Source: ICID/FAO (1996) and FAO-56 (Allen et al., 1998).
20
Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods
Soil organic matter management
Although soil organic matter (SOM) constitutes only a small fraction of
the total matter of most soils, yet the dynamic SOM has a dominant
influence on many soil physical, chemical, and biological properties (Bot
and Benites, 2005; Brady and Weil, 2002). In the current status of land
use in the MD, agricultural practices and cropping patterns have changed
rapidly since the economic reforms in the 1980s (Vo-Tong Xuan and
Matsui, 1998). The cultivation area has increased thanks to man-made
constructions, such as dams, dykes, canal systems, soil rotation systems,
agro-chemicals and the reclamation of so-called ‘problem soils’. These
practices have led to soil reclamation and irrigation activities converting
forest, bare and uncultivated land into agriculture land. Changes in
agriculture are also a big threat to soil fertility and increase the speed of
soil degradation. During the 1980s, intensification of cropping patterns
was introduced to the MD in an attempt to increase food security.
Nowadays, most farmers use modern fertilizers, as an alternative to using
animal manures or a combination of inorganic fertilizers and animal
manures as before, though farmers mostly realise that there will be a
problem with soil fertility, if SOM is completely exhausted. Because of
this, the intensive cropping has tended to reduce the organic matter
content of the soil. In addition, climate of the MD is characterized by
high temperature, humid, and high rainfall, therefore, which accelerates
decomposition rates and a quick release of nutrients into the soil.
Maintaining SOM content requires a balance between inputs and
decomposition rates (Bot and Benites, 2005). The maintenance of SOM
levels and the optimization of nutrient cycling are essential to the
sustained productivity of agricultural systems (Bot and Benites, 2005).
There are several ways to add organic materials to the soil such as
mulches, composts or covering by crop patterns. Especially, Biochar can
be considered as another soil conditioner for improving SOM. Many
studies have shown that biochar can improve soil fertility and increase
crop production. For a deeper discussion on biochar is available in
Lehmann and Rondon (2006), also Shackley et al. (2012), and Yamato et
al. (2006). A recent study conducted by Southavong and Preston T R
(2011) on growth of rice in acid soils amended with biochar, showed that
biochar increased the biomass growth of rice and water holding capacity,
raised soil pH from 4.5 to 5.13 and 5.40.
Water management
Characterised by two distinct seasons, farmers in the MD often irrigate
their plant orchards during the dry season, except during some periods
of drought in the rainy season. The source of their water supply mainly
comes from channels that are linked to the river system. The drains are
used to regulate water level in the cannels. When the soil becomes dry or
plant leaves seem to suffer the symptoms of wilting (usually identified by
feeling the leaf), it is the time to irrigate the plants. Water quality depends
on the water characteristics of the rivers’; but the highest tide is usually
considered as the best one. The method has been commonly applied for
irrigating the citrus orchards is a manually operated irrigation pump. The
pumping system is put on a boat and is moved along the adjacent
cannels to spray water over the top of the plant canopy, because this is
the most convenient and easy way in situ. This research found the
quantity of water is often not uniform on all the raised-beds, even on the
same one. This type of irrigation is more or less similar to a heavy rain
and it may result in easily forming surface runoffs, erosion and soil crust,
if the soil surface lacks a litter or vegetation cover. Also excessive water
21
Pham Van Quang
TRITA LWR PhD Thesis 1073
applications may reduce yield and quality, increase waste of water,
increase the risk of nutrient leaching and lead to pollution of ground
water, all of which are harmful effects on the environment.
In brief, the growers in the region are mostly relying on the use of
intuition or subjective irrigation scheduling methods. This method is
based on instinct, knowledge, and experience gained over many years of
farming. However, this subjective technique is not accurate and
heightens the chances of soil erosion. It is highly recommended that a
scientific irrigation schedule should be developed and implemented.
Improved irrigation water management will lead to water saving which
can reduce irrigation costs, increase plant yields and farmers’ incomes.
Understanding soil moisture dynamic and plant response is of great
importance from an economic and an environmental perspective.
As a component of best management practices, the overall goal of
irrigation water is usually not only to maximise net profit over the long
term, but also to meet sustainable goals of the environment, economy
and society (Lincoln Environmental, 1997). The farmers have therefore
to adopt the new technical and scientific approaches of the appropriate
irrigation management practices. Application of irrigation water needs to
be met the aim of high yields and high water use efficiency (WUE). It
also aims at avoiding environmental hazards. Irrigation scheduling is an
extremely important tool in developing a sustainable plant management
strategy. The core of irrigation water requirements mainly depend on
plants, climate, and soils. A good irrigation schedule must accurately
indicate when to irrigate and how much water to apply.
In fact, there are many irrigation scheduling methods and models
available to help the farmers with decisions in relation to when the plants
require water and how much water needs to be applied. A large number
of techniques and methodologies are introduced in ICID/FAO (1996)
and FAO-56 (Allen et al., 1998). These are shown in Table 5 which can
range from very simple (observing, felling, soil sampling) to intermediate
and high technical methods requiring the participation of multi-lateral
partners such as irrigation engineers, scientist and water resources
engineers, consultants and farmers. High technology equipment, such as
computers and microchips has created devices for monitoring and
recording that can be used to make real-time irrigation scheduling. This
involves monitoring of soil moisture content, plant water status or ET
that describes the actual conditions of the plants currently growing. Realtime monitoring systems are also adaptable to full automation, and
capable of interaction with computer systems as well as the use of
remote access through telecommunication systems (Phene et al., 1990).
Although these high technological methods can bring many benefits, the
appropriate monitoring devices are also needed. These devices are not
commonly available commercially in Vietnam, and so far are only being
used for scientific experiments.
Due to the benefits to be gained from irrigation scheduling, it is
recommended that promoting the use of automatic irrigation systems in
the MD is necessary now, especially in orchards and upland cropping
areas. This action has the potential to contribute to sustainable
agriculture development in the MD. Nevertheless, implementing this is
also a challenge for the following reasons:
• The area cultivated per household is often small;
• The level of farmers’ knowledge and skills is low and needs to be
improved;
22
Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods
•
Equipment for monitoring is expensive because at present it is
imported from other countries and not yet manufactured in
Vietnam.
C ONCLUSIONS AND RECOMMENDATIONS
Orchards on the raised-bed system in the Mekong delta (MD) have
considerably been affected by the tendency of soil deterioration
processes. In the present study, it represented that the soils have been
deteriorated on both chemical and physical properties with the aging of
raised-beds. This research showed that raised-beds for growing citrus
fruit trees in the Mekong Delta show clear symptoms of soil
deterioration and it established a relationship between the aging of the
raised-beds and a decline in their physical and chemical characteristics.
These declines were indicated through low pH soil, nutrient deficiencies
and/or imbalances, increase in dry bulk density and decrease in hydraulic
conductivity, decline in organic matter, and increase in soil penetration
resistance. However, the dynamic studies need to be followed by the
ageing processes of the raised-beds to find further explanations of the
impact on the clay content.
As indicated, by using a portable electronic penetrometer (or
penetrologger) combined with registration of soil water content,
measurements can be easily and speedily possible to assess the soil
quality in terms of the constraints for root growth. This is one of the
commonly used non-destructive methods. The penetrologger is able to
store and process a great amount of data and the procedure can be
repeated at many positions within a short period of time.
This research also raises some concerns regarding the poor fertilizer and
irrigation management practices of farmers. Because soil is a nonrenewable resource over the long term scales and is dynamic and prone
to rapid degradation due to land misuse or poor management practices,
appropriate approaches are needed to help farmers develop better
management practices. Good management practices must be based on
plant, soil and water conservation techniques as well as the imitation of
the natural environment. In this way, one may prevent the many
symptoms of soil and plant degradation developing instead of needed to
react to them after they develop. On the basis of the findings from this
study, the following recommendations can be made:
• Fertilizer application needs to be based on the plant requirements
and the actual nutrient availability of the soil, provided that the soil
pH is within a proper range, i.e. the soil fertility status is managed to
maintain optimal pH levels which make available sufficient nutrients
for plant growth but without excess, which may lead to water
pollution. No more fertilizer and nutrients should be applied than
the plant can use and the soil can store.
• Nitrogen and phosphorus need to be monitored because both can
result in environmental damage when soils are too rich with respect
to these nutrients.
• Soil organic matter (SOM) must be maintained at suitable level
(3.4% SOM - threshold value) through appropriate practices.
• Comprehensive soil and plant tests need to be performed regularly
to identify possible nutrient deficiencies (at least every two years).
• Observing and recording the variability in plant yield and soil tests
for each field should take place regularly in order to follow soil
health changes and take corrective actions.
23
Pham Van Quang
TRITA LWR PhD Thesis 1073
•
Develop and promote irrigation management that are founded on
the technical and scientific approaches such as monitoring of soil
moisture content, plant water status and evapotranspiration.
• Continue to improve our understanding of soil fertility and its
associated problems in order to develop better and more suitable
management techniques.
To achieve sustainable soil management targets, it is essential that several
different entities cooperate, particularly farmers, scientists, and extension
personnel. Agricultural researchers play pivotal roles in developing
agricultural technologies and management practices. Extension
personnel can be regarded as a bridge for researchers and farmers to
make sure that the desired technologies and management practices
smoothly transfer to the farmers. Eventually, farmers directly work on
the land and implement and adapt new technologies and practices.
Farmers have some knowledge and skills of technologies and practices
but not enough. For example, farmers may know more about what
happens above ground than about interactions and processes below
ground. A good education, information and skills training program can
improve knowledge and management abilities of farmers, and they can
adapt to current and new technologies and techniques. Moreover,
knowledge gap between scientists and farmers also need to be identified
to promote a better communication among the researchers, extension
workers, and farmers. This is an essential factor for improving
transferability of knowledge, management skills, and technology.
F UTURE WORK
Agricultural management is a term which refers to many activities closely
related to the farming management, in which, using optimal nutrient and
water management practices are required in order to grow healthy plants,
improve and retain soil productivity. Enhancing soil organic matter,
avoiding excessive tillage, managing pests and nutrients efficiently,
keeping the surface of the ground covered, preventing soil compaction
and erosion, and diversifying cropping pattern systems are good
procedures to maintain soil and plant conservation. Soil nutrients and
water management practices in the MD are still restricted and more
appropriate practices need to be developed.
Therefore, my future research should emphasize:
• Research on fertilizer recommendations that combine general
requirements and site-specific requirements;
• Studies of biochar as a soil conditioner in order to determine its
applicability within the Mekong delta conditions;
• Studies of the feasibility of irrigation scheduling and the use of
automatic irrigation systems;
• Studies of the soil water dynamics based on simulation modelling.
24
Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods
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