Impact of Soil Rehabilitation of Tea Soils and Development of an Index
NPSN Bandara1*, UP Abeysekara1, N Navaratna2, AK Prematunga2
MGS Liyanage1, EWTP Prematunga1, DW Vithana1
GP Guneratna3, KG Prematilaka4
1*
Agronomy Division, Tea Research Institute, Ratnapura, Sri Lanka
(Corresponding author, Email: shyamantha.bandara@yahoo.com)
2
Entomology Division, Tea Research Institute, Talawakelle, Sri Lanka
3
Soil & Plant Nutrient Division, Tea Research Institute, Talawakelle, Sri Lanka
4
Uva-Wellassa University of Sri Lanka, Badulla, Sri Lanka
Abstract
A study was undertaken to evaluate four soil parameters, soil bulk density, available
water content, organic carbon content and microbial biomass content, in rehabilitated
field and adjacent old tea field in 40 tea estates, representing all tea growing areas of
Sri Lanka. While BD showed significant reduction after planting grasses, AWC,
MBC and C showed significant increase.
A soil quality index (SQI) was developed after standardization of above parameters
between 0-10 and using the following formula
SQI = 0.36BD + 0.14AWC + 0.34C + 0.16MBC
Accordingly, minimum SQI requirement for skipping the grass planting was
proposed as 5.0 fpr Low elevation, 6.0 for Mid elevations and 7.0 for Uva and Up
elevations in tea growing regions of Sri Lanka. This study proved the beneficial
effects of grass planting prior to replanting and developed a standard index to
evaluate the soil fertility.
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Introduction
Tea cultivation is a major economic growth contributor and most significant agriculture
commodity in Sri Lankan economy (Oxford Business Group 2016). Sri Lanka’s tea production is
now dominated by some 400,000 small scale farmers. Tea small holder sector holds the largest
production percentage (nearly 70%) of Sri Lana tea industry (Ministry of Plantation Industries
2013). Many researchers have shown that decreasing productivity continuously make Sri Lanka
tea industry less market competitive globally (Ganewatta and Edwards 2000, Thushara 2015,
Nilanthi 2016, Wekumbura, Mohotti et al. 2017) and as a result competing countries gain the
market advantage (Thushara 2015). Identified one strategy to counter the declining productivity
is to maintain an annual replanting rate of 2%. Low replanting rate is an issue affecting both small
holder and large corporate sector (Illukpitiya, Shanmugaratnam et al. 2004). The issue is more
affecting the small holder tea growing sector, where replanting rate sometimes fell below 1%
(TSHDA 2015).
Soil rehabilitation prior to replanting is a recommended practice in tea cultivation (Zoysa 2008).
As the rehabilitation technique, Guatemala (Tripsacum laxum) or Mana (Cymbopogon
confertiflorus) is planted for 18-24 months. However, long period of without farm income, (up to
4-5 years), is considered to be a negative factor for replanting tea fields (IFAD 2015). As a result,
there are some demands for skipping the soil rehabilitation process, prior to replanting for the small
holder sector. However, long term productivity analysis has shown that non-rehabilitated tea fields
become economically unproductive within few years (Dissanayake, Udugama et al. 2013)
Hence, there was a need evaluate the impact of soil rehabilitation in tea lands and to develop a
mechanism to identify fertile tea lands, where direct planting can be practised without soil
rehabilitation. after analyzing the soil characters of different soil groups, belongs to different agro-
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ecological regions of tea growing areas , an index, referred as “Soil Quality Index” has been
prepared by Prematilaka et al,(2011) to identify the fertility status of tea lands.
Material and Methods
Soil parameters of soil dry bulk density-BD(g/cm-3), Available water holding content-AWC
(mm/30cm), Organic carbon-C(%) and microbial biomass content-MBC (µg/g of soil) of 40 tea
estates, belongs to all elevations, soil groups and agro-ecological regions. Estates were selected
according to elevational categorization of tea estates in Sri Lanka (Dissanayake, Wijewardena et
al. 1999, Zoysa 2008).
Table 1: Number of tea estates selected for each elevation/region for soil sampling
Elevation/region Altitude (m amsl)
No of estates
Low
0-600
14
Mid
600-1200
6
Up
>1200
9
Uva*
>600
9
* Uva region tea fields belongs to both Mid and Up elevations. Since this is the only region which receives majority
of rainfall from North East monsoon, this region is categorized separately (Watson 1986)
Samples were taken from rehabilitated plots (Mana or Guatemala grass planted for approximately
2 years) and very adjacent old seedling or VP tea field. The tea fields were in the verge of
uprooting. Undisturbed soil core samples were obtained from 0-10, 10-20 & 20-30cm depths
separately for the estimation of BD & AWC. OC and MBC were sampled at 0-15 & 15-30cm
depths.
BD was calculated after oven drying the samples for 1000C overnight. AWC was
measured using the hanging water column method. Walkley & Black method was used to measure
the OC and chloroform fumigation method was used to measure the MBC (Vance, Brookes et al.
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1987, Gonzalez-Quiñones, Stockdale et al. 2011). Six sample points each were analysed from
grass planted field and old tea field and average values were taken for calculation.
Based on the soil parameter values, scores were given according to following table to standardize
the data for applying to SQI calculation (Table 2Table 2Table 2).
Table 2: Scores given for each soil parameter based on it's value
Bulk Density
(g/cm-3)
Value
Score
0.7-1.0
10
1.0-1.2
9-10
1.2-1.3
7-9
1.3-1.4
5-7
1.4-1.5
3-5
1.5-1.6
1-3
>1.6
0
Available Water
Content (mm/30cm)
Value
Score
60-70
9-10
50-60
8-9
40-50
7-8
30-40
6-7
20-30
3-6
10-20
1-3
<10
0
Organic Carbon (%)
Value
>5
4-5
3-4
2-3
1-2
0.6-1
<0.6
Score
10
8-10
6-8
4-6
2-4
0-2
0
Microbial Biomass
Content (µg/g of soil)
Value
Score
300-400
10
250-300
8-10
200-250
7-8
150-200
5-7
100-150
3-5
50-100
1-3
<50
0
Principal component analysis method was executed using the SAS University Edition and obtained
the following best fitted equation to calculate the soil quality index
= 0.36
+ 0.14
+ 0.34 + 0.16
Results & Discussion
Pattern of the measured soil parameters in old tea fields, against the field elevation is given in
Figure 1Figure 1Figure 1
Accordingly, there is a weak negative relationship between BD and
elevation and weak positive relationship with AWC, OC and MBC with elevation.
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(a)
1.6
Bulk density (g/cm3)
Available water content (mm/30cm)
1.7
1.5
1.4
1.3
1.2
1.1
1.0
M icrobial biomass content (g/ g soil)
(b)
3.5
Organic carbon (%)
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
500
1000
Elevation (m amsl)
1500
2000
70
(c)
60
50
40
30
20
1800
0
500
1000
1500
(d)
Elevation (m amsl)
1600
2000
1400
1200
1000
800
600
400
200
0
0
500
1000
1500
2000
Elevation (m amsl)
Figure 1: Relationship of BD, AWC, OC and MBC with elevation in tea fields
1. Dry Bulk Density
Difference in the BD among different elevations and tea growing regions were clearly seen among
results (Figure 2Figure 2Figure 2a). According to the graph, a clear negative relationship could be
seen with elevation and the BD even after soil rehabilitation process. The most significant
reduction of bulk density after growing grass for two years in the field was found in Uva region,
where grass planting reduced the BD by 10%. Reduction of the compactness is much lower in low
elevation tea growing areas, where grass planting was able to decrease BD only by 3%.
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2. Available Water Content
Grass planting for a period of two years has improved the water retention of the soils significantly
in low elevations (14%) and Uva region (12%). However, soil rehabilitation has been unable to
improve the water retention of the soils in upper and mid elevation tea growing areas of Sri Lanka.
3. Organic carbon percentage
Organic carbon content increases with the elevation of the tea field.
Soil rehabilitation
significantly improved the organic carbon percentage in low and mid elevations and in
Uva region.
4. Microbial biomass content
Microbial biomass content of the soil increases with the elevation. Among different elevations
and regions, grass planting improved the MBC, except for up elevations. The highest gain of 32%
increase of MBC was observed in Low elevation fields.
Grass planting for two-year period, known as soil rehabilitation contributed to improve the
measured soil parameters, as beneficial for plant growth. However, improvement of some of the
parameters, like AWC has an effect with elevation. Up country elevation (>1200m amsl) is the
least developed through soil rehabilitation with all measured parameters. However, OC and MBC
showed a 20% and 28% increase after soil rehabilitation.
Lower improvement of the soil
parameters, in this elevation could be due to low temperature regimes associated in this elevation
(Watson and Zoysa 2008). Overall most significant improvements were seen from the increase of
OC (28%) and MBC (24%) after soil rehabilitation.
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Avialable water comntent (mm/30cm)
-3
Bry bulk density (g/cm )
1.6
(a)
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
70
(b)
60
50
40
30
20
10
0
3.5
(c)
Organic carbon (%)
3.0
2.5
2.0
1.5
1.0
Microbial biomass content (g/g soil)
0.5
0.0
1400
1200
Grass
Old tea
(d)
1000
800
600
400
200
0
Low
Mid
Up
Uva
Elevation / region
Figure 2: Average soil parameters of bulk density(a), available water content(b), organic carbon
percentage (c) and microbial biomass content(d) according to different tea growing elevations/regions.
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SQI Calculation
The SQI value showed a positive linear relationship with elevation increase (R2 = 0.46, for grass
fields and R2=0.49 for tea fields). Hence, it needs to introduce separate benchmark SQI levels for
different elevations. Accordingly, SQI value of 5.0 has been earmarked for benchmark SQI for
direct planting in Low elevations, 6.0 for Mid elevations and 7.0 for Up elevation & Uva region.
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Grass
Tea
9
Soil Quality Index
8
7
6
5
4
3
2
0
500
1000
1500
2000
Elevation (m amsl)
Figure 3: Calculated soil quality index value for grass and tea fields with elevation. Continuous line shows
the trend line for grass fields and dash line shows the trend line for tea fields.
Application of the SQI for tea industry
Calculation of SQI for estimating tea field fertility for the decision making on direct planting could
have a positive impact on Sri Lanka tea industry sociological and economical terms. According
to the analysed data 10 put of 38 old tea fields were above the elevational bench marks required
for direct planting (Figure 3Figure 3Figure 3). Hence, the subsidy given for replanting could be
saved by about 26% due to the proper implementation of soil quality index.
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Conclusions:
The detailed experiment analyzing the rehabilitated and non-rehabilitated soils showed that,
beneficial effects of soil rehabilitation with grass planting can be visible in all tea growing
elevations/regions at different magnitudes. Hence this is a more sustainable method of improving
the poor quality tea soils which are highly erodible (Dissanayake, Wijewardena et al. 1999).
However to improve the soil replanting rate and to solve the grower concern about the long time
taken for rehabilitation, measuring the soil quality index in given land can be used as an alternative
way of selecting lands for direct planting.
Acknowledgement
The authors wish to acknowledge the Tea Research Board of Sri Lanka for funding the research
work and the Managers and field staff of various estates providing assistant during field operations.
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