CROP PRODUCTION PRACTICES AND TECHNIQUES
A. CROP ESTABLISHMENT
1. Site selection
Site can refer to the part of the country (region) where farm should be located
and/or the specific part of the farm a crop is to be established.
Factors to consider when selecting a site for establishing a crop
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Climatic requirements of the main crop(s)
Soil requirements – soil should be good/suitable for cropping (soil pH, soil
structure and texture, salinity)
Nearness to markets –particularly true for perishable crop or crops which require
extensive processing after harvest
Presence or absence of certain/particular pests and diseases e.g in Nigeria, cowpea
pests are prevalent in the wetter southern part of the country and because of this,
large scale production is confined to the drier northern part of the country
Social/anthropological considerations. This criteria include access to cheap
labour, ease of acquiring land and the desire to locate oneself within own ethnic
group – in Zimbabwe and other tropical countries, small-scale
conventional/traditional farmers do not choose in which part of the country they
want to farm. The farm is normally located in their own ethnic region or in their
native village.
Slope: Land to be flat or to slope gently (erosion problems)
water supply ( streams, underground water etc): Irrigation facilities (optional). For
year round crop production
2. Land clearing ( If site has not been cultivated before)
- Use of bulldozers: heavy and cause soil compaction
- Stumping
3. Tillage and land preparation
Tillage – changing a soil condition/position with a tool for man is benefit,
physical manipulation of the soil. (Detailed notes on tillage in AGRO 205)
4. Seed sowing
Seed structure : A seed develops from a fertilised ovule and consists of a seed
coat (testa), the peristem () and the embryo. Some seeds also contain an
endosperm. Different types of seeds show these parts to the various degrees.
Testa: develops from the integuments of the ovule and it protects the embryo
and endosperm. Varies in consistency from being thin and papery (cowpea) and
being thick and impervious (castor bean)
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Embryo: The embryo of a mature seed consists of the seed leaf or leaves called
cotyledons or scutellums, a shoot portion called the plumule and a small root
portion called the radicle. The embryo will develop a plant through cell growth.
Endosperm: Supplies additional food to the embryo during germination in seeds
that have them.
Cotyledons: may constitute the major food storage organ of the seed as in most
leguminous seeds. It may be a very thin leaf like structure enclosed by the
endosperm as in some cereal seeds. The cotyledons are attached to the main
embryo axis consisting of the plumule (shoot axis0 and the radicle (root axis). A
seed may contain one cotyledon (monocotyledonous) or two cotyledons
(dicotyledonous).
Insert of the external and internal morphology of dicot and monocot seeds.
Micropyle: A minute pore. The testa usually covers/encloses the seed entirely
except for the micropyle
Hilum:
Seed storage: To store under appropriate temperature, RH and correct seed
moisture content. Cool dry conditions are desirable to maintain seed viability.
Recommended moisture content for most cereal is 12,5 –14 %.
Each 1% reduction in seed moisture content doubles the life of the seed (this does
not however apply to seed at >14% or <5% MC for maize)
Seed treatment: to control seed borne diseases e.g treatment with captasan (for
maize) and thiram (for groundnuts.
Seed germination: the process (AGRO 101) – seed imbibes water; seeds swell and
various enzyme systems are activated in the seed; food material present in the
seed is hydrolysed: starch converted to sugars, proteins…..amino acids; fats
…..fatty acids and glycerol. Products of hydrolysis get translocated to the growing
points are used for growth new cells and substrates for respiration to supply
energy needed for various metabolic processes.
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Factors affecting germination:
water
oxygen for aerobic respiration to occur to supply energy for the germination
process
appropriate temperature
Types of germination
Epigeal germination: cotyledon are carried above the ground level due to the
elongation of the region just below the cotyledons (hypocotyls): This type of
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germination occurs in most dicots: groundnut, castor, onion, cowpea, melon. The
cotyledons may dry up and wither e.g in cowpea or they may flatten out, become
green and function for along time as foliage leaves (olero and melon)
Hypogeal germination: cotyledons remain at the level where the seed was planted
( they are not carried above the ground). Common in monocots e.g maize, rice.
The broad bean also has this type of germination. Both dicots and monocots fall
in this group just as with epigeal germination.
Seed dormancy
A living seed is said to be dormant when it fails to germinate even when provided
with the normal conditions necessary for germination. Dormancy may also
exhibited by tubers such as yams. They may be incapable of sprouting until some
time after harvesting.
Causes of dormancy
Presence of an impermeable testa
Presence of growth inhibitors
The need for a cold treatment
The need for exposure to certain photoperiods before the seed can germinate
Seed dormancy is important to a farmer because dormant seeds will not readily
germinate when sown, and to facilitate germination the farmer may have to incur
extra costs in breaking the dormancy.
Note: Seeds of most tropical crop plants do not exhibit dormancy to a significant
extent (fortunate for the farmer)
Seed quality
Planting seed should be of good quality if its potential is to fully exploited
It should be genetically pure
It should be physical pure
High germination percentage
Free from pests and diseases
Factors contributing to poor quality seed
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Maturity of seed at harvesting. Immature seed store poorly and often fail to
germinate. Harvesting time is critical. ( Physiological maturity and harvesting
maturity)
Unfavourable conditions during the maturation period(poor seed quality s the
result) and germination is often poor.
Unfavourable conditions during storage (high temperatures, high RH, pests and
disease)
Wholesomeness of the seed. Injury, cracking or breakage of seed impair
germination. The extent to which germination is impaired = f( part of seed which
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is damaged) e.g minute damage of the plumule/radicle axis may lead to failure to
germinate whereas injury to the endosperm or edges of the cotyledons may impair
germination lightly.
Damaged seeds also become susceptible to pest and disease attack.
Most injury to seed occurs during seed processing.
Viability and Germination tests
These determine the worth of a seed lot
Viability tests : Distinguish between living and dead seeds. Such tests can also
distinguish between living and dead portions of the same seed. The tests serve as
a basis for estimating percentage germination.
Tetrazollium test: (describe the test)
Germination tests
Meant to distinguish between seeds which can germinate immediately and those
which cannot germinate immediately and those which cannot either because they
are dormant or dead
Seed Certification
In most developed and in some developing countries, there are agencies or seed
associations responsible for overseeing seed quality. The duties of such agencies
include:
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Monitoring the production of the seed
Carrying out viability and germination tests
Certifying the seeds
Distributing the seed (in some cases)
In Zimbabwe seed certification is done by the Ministry of Agriculture ( AREX
Department)
PLANTING
Methods of planting
- Direct seeding
- Transplanting
- Machine planting
- Hand planting
- Row planting vs broadcasting
(Brief discussion of each methods; advantages and disadvantages)
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Important aspects of direct seeding/transplanting
 Number of seeds per planting station
This is =f(expected germination %; desired plants per station; easiness to handle
the seed –difficult to be precise on number for tiny seeds ) .
E.g if germination % is 50% and one plant per station is desired, then two seeds
per station will be sown.
At times more seeds than desired per station are deliberately sown and the number
of seeds appearing on each station is higher than desired. When the seedlings are
well established, the extra plants are then removed (thinning)
Thinning provides an opportunity to select out seedlings and also makes sure that
the correct number of seedling plants are is maintained. The process may, may
however be laborious especially where labour is not readily available.
Cotton - It s difficult to be precise on the number of seeds to be placed at a
station, hence more than enough seeds are usually placed. (Cotton is also a weak
germinator). Thinning is then done to maintain the correct seedlings per station.
This done during weeding.
The ‘desirable’ number of plants per station is based on experimental evidence
and depends on the crop type. Large plants e.g cassava require one plant per
station; medium-sized plants e.g maize, can tolerate two plants per station and
small-seeded crops e.g rice, wheat two or plants per station will do.
 Spacing between stations
This is determined by:
Extent of the root and shoot systems of the crop
Environmental potential (soil fertility; rainfall). E.g High populations are
appropriate for early planted crops under high rainfall or irrigated conditions
where management is of good standard e.g for maize, in drought prone areas a
population of 36 000-37 000 plants/ha is recommended whereas in high rainfall
areas, population can go up to as high as 50 000 plants per ha.
For a given crop, optimum population =f(variety) e.g in maize. Some
varietie00000 s
 Plant density/population
It is determined by the inter-row and intra-row spacings or by the number of
planting stations per hectare; number of plants per station.
Number of plants per hectare = (100m/inter –row spacing) x (100 m/intra-row
spacing)
Optimum population for optimum yields. Over populated plants compete for
essential resources (light, water & nutrients)
The stress wheel concept to illustrate effect of plant population on plant
growth/development and yield. The spokes of a bicycle wheel represent the plant
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rows that run from the periphery to the center of the wheel, thus the inter-row
spacings are widest at the periphery and narrowest at the centre. The distance
between plants stations within a row are maintained i.e. the intra-row spacing is
the same along all plant rows. Thus, the plant population decreases form the
periphery towards the centre and as one moves from the periphery to the centre
competition for resources, among plants, increases. The degree of the competition
/ stress is manifested in growth and yield of the plants in the different portions of
the wheel. Growth parameters to observe include, plant height, stem thickness,
lodging, cob placement, cob size, grain yield etc. In the case of maize, for
instance, there will be more lodging of plants at the centre of the wheel, these
plants will also be taller with thin stems, cob placement is higher, cob size smaller
and grain weight lower, than at the periphery.
Some varieties may be susceptible to lodging under high populations. In general
the taller the variety the lower the plant population
Plant population should generally, be higher under optimal conditions than stress
conditions (fertility, water availability) – Graph of relative yield % of maximum
(0-100%) versus population per hectare. Optimum population is lower under
stress conditions e.g drought than under ideal conditions.
For tailoring varieties, lower populations should be used.
Write brief/conscise notes on “Plant competition.”
 Sowing depth
Depth at which seed is placed =f (seed size; type of germination; moisture status
of the soil and soil type
The larger the seed, the greater the depth from which it can emerge, the deeper it
can safely be placed/planted. Rule of thumb is place seed at depth 2-3 times its
diameter. Large seeds have enough quantities of stored food reserves for the
germination and emergence processes. Small seeds deplete their stored food in a
short time
Seeds with epigeal germination have to push the cotyledons to the surface and
therefore have limited ability to emerge from great depths.
Under dry conditions, seeds should be placed/sown deeper in order to place them
in contact with moist soil
Seeds cam emerge from greater depths in sandy soil than in clay soil, all other
factors remaining constant
. Thus planting depth can be adjusted according to soil texture
 Seed rate
Can be expressed as weight of seed per hectare or number of seeds per hectare.
For the same crop variety, the larger the seed the more weight of seed will be
required per unit area.
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Maize = approximately 25-30kg/ha; small grains (5-7kg/ha); soyabean and
groundnuts (90-120 kg/ha). The desired plant population depends the seeding rate.
 Position of seed with respect to land preparation
This is =f(nature of crop and climatic conditions) e.g under water-logged
conditions, seed is normally planted on top of a mound or ridge to remove it
from the high water table. On the other hand planting in the furrow may ensure
greater moisture availability.
Ridges and /raised beds, where they serve a purpose such as moisture
conservation, can be constructed later when the crop has been established rather
than planting the crop on the ridge initially.
Seeds can be planted on the flat.
 Placement of vegetative propagules
Mosst of the principles that apply to seed sowing also apply vegetative
propagules.
Propagules should not be planted more than 5-10cm deep in the soil.
Also cuttings should be planted with the proper orientation or else they will fail to
sprout or perform well.
Planting time
Time to plant is influenced by a number of factors.
1. Rainfall /moisture availability
Crop should be planted during a time when there is adequate soil moisture to
permit germination. Thereafter, there should be enough soil moisture to see
the crop through.
The common practice is to plant when there is adequate moisture in the soil.
Planting methods based on water availability
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Planting with the rains. The farmer plants seed after receiving adequate rains
of after the first effective rains. Normally 50mm is adequate.
Dry planting. The farmer sows seed 2-3 weeks before effective rains are
received. The soil is dry and planting depth should be deeper than normal
planting depth (as used under 1).This is done to ensure that , in the case of
light showers, the planted seed is not wetted. If light showers wet the seed, the
moisture will not be adequate for germination, and all the light rains will do is
to cause rotting of the seeds. Lighter soils are more suitable for dry planting
than the heavier ones because the former soils can easily/quickly dry off if
light showers are received and the danger of seed rotting is reduced. For
maize seeds may be planted at about 7.5 cm instead of the usual 5.0cm depth.
Water planting with water.
In this case, the soil is initially dry. The farmer artificially supplies water to
the planting stations or rows. A hole is dug (planting station). Soaking water is
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administered to the hole until it subsides. The hole is slightly covered with dry
soil and seed is then placed. The hole is then covered with dry soil (soil
mulch)
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Water planting without water
The farmer relies/uses residual soil moisture. Seed is planted on a wet (from
residual moisture) soil. The crop is established before the start of the rain
season. Common practice in dambos/ wet lands/vleis.
2. Temperature
Not critical in the tropics unlike in the temperate regions. It is of appreciable
importance at high altitudes.
Germination and growth of crops is affected by T0C. Planting should be timed
in such a way that critical stages such germination, vernalisation, flowering
coincide with conducive T0C
3. Day length /Photo-period (plant response to day length)
May influence planting time. The crop should be planted at a time that will
allow the appropriate photoperiod to exist at the flowering or timbering stage
SDP –Flowering is promoted by day lengths shorter than a critical maximum
and is usually affected by other environmental factors such as temperature.
E.g tobacco and soyabean
SLDP e.g night jasmine (estrum nocturnum).
LDP- Flowering is promoted by day lengths longer than a critical minimum.
E.g barley)
LSDP
DNPs – Flowering is insensitive to photoperiodism, but is associated with an
age factor. In general flowering starts after a minimum age or size has been
attained. E.g: tomatoes, dandelion, buck wheat. Many plants of tropical origin
are DN, but others are SDP e.g tropical soyabean cultivars.
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4. Occurrence of pests and diseases
The strategy is to adjust the time of planting so that the crop is in the field at a
time when pests and diseases of which it is host, are least prevalent.
In Zimbabwe, maize stalk-borer and maize streak virus are more prevalent on
late-planted crops.
Cowpea production in Southern Nigeria is influenced by this factor. If grown
early in the rainy season, the crop is severely attacked by pests and diseases.
Planting is therefore normally delayed to the latter half of the rainy season to
reduce the incidence of pests and diseases.
Early planted butternuts are at low risk of infestation by downy mildew
5. Marketing
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The strategy is to time planting such that harvesting occurs when the crop can
fetch a good market price. This is particularly so for horticultural
crops/produce which cannot be stored for a long time. The first batch of crops
such as tomatoes, green mealies and leafy vegetables usually fetch the highest
price and as such, most farmers aim to get their produce first on the market.
Hence they go for early planting requiring them to (in most cases), produce
using irrigation.
6. Cropping system
Planting of a given crop in a cropping cycle =f(place of that crop in a rotation
or cropping cycle)
E.g: In Western Africa, cassava is usually planted in the latter part of the rainy
season after some of the earlier intercrops such as maize, okra and melon have
been harvested. The planting of cassava is delayed until the harvesting of the
earlier intercrops creates enough space between the yam plants ???????
7. Availability of inputs ( labour, seed, fertilizer etc) and equipment
Note: In Zimbabwe the problem of draft power shortage is almost perennial in
some areas, especially communal areas and the effect of this is delayed
planting and reduced yield. Draft power sources are animals and tractors. Cost
of hiring tractors is often prohibitive. (What is the current cost)
With enough labour and equipment, it should be possible to grow more than
one crop per season, is the crop(s) is a short season crop [multiple cropping]
8.
Legislation:
There are stipulated planting dates for certain crops in this country. ( Which
crops; what dates and why?)
Emergence and seedling vigour
Emergence refers to the appearance of the seedling above the ground. It is not
synonymous to germination.
Emergence % - the farmer assesses the effectiveness of the seeding operation. It
is the number of emerged seedlings expressed as a percentage of the expected
number seedlings.
Photosynthesis begins
Important aspects of seedling emergence:
 Time from planting to emergence = f ( soil temperature; time to germination;
sowing depth,; nature of soil e.g soil capping; vigour of the seedlings)
 Ideally the time to emergence should be as short as possible
 Final % emergence refers to the % of seeds sown that eventually emerge. If
too low, the farmer may re-plant. Poor germination is often the cause of low
seedling emergence. However germinated seeds may fail to emerge because
of very low vigour of the seedlings; sowing at too great depth; attack by pests
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and diseases; extreme temperatures which severely retard seedling growth
and; soil capping
Uniformity of emergence: If uniform, it allows accurate programming of
operations such as fertilizer application and harvesting.
Non-uniformity indicates different times of emergence
Seedling vigour: An indication of the health status of the seedlings and the
likelihood that will yield well.
It can be lowered by small seed size; the presence of pathogens in the seed or in
the soil; long periods of storage (protracted seed storage) and adverse
environmental conditions during storage and during germination.
C. CROP FERTILIZATION (FERTILIZER APPLICATION)
Definition of a fertilizer:
To fertilize is to render fertile. Any substance supplied directly or indirectly to
crops in order to promote their growth, increase their yield and / or improve their
quality (market value; nutritional quality etc).
The use of fertilizers has contributed immensely to yield increases in Zimbabwe
and the world over, Intensive use of fertilizers was part of the Green Revolution
package. In Zimbabwe, maize grain yields have increased by mare than 200%
over the past 40-50 years (Tattersfield, 1984)
Background information on nutrient requirements of crops (micro and macro) and
associated deficiency systems is required).
Types of fertilizer / Classification of fertilizers
There is a wide diversity of fertilizers. These can be classified as follows:
a) According to type of origin:
Natural vs artificial fertilizers
Natural: formed in nature. They are used in the form in which they occur
without or with little processing e.g manure (fresh or decomposed); peat; leaf
litter; ash; lime etc
Artificial: Synthetic [‘man-made fertilizers’] –produced in factories. May
involve chemical changes of natural products (case with most P and K
fertilizers) or completely synthetically from simple source materials (e.g most
N-fertilizers)
b) According to source: Farm manures vs commercial fertilizers
Manures (solid; semi-liquid); composite, humus
Commercial – obtained through trade channels
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c) According to mode of action: (Direct acting vs indirect acting)
Direct:
These are often referred to as plant fertilizers.
Contain essential components of available plant nutrients and thus supply them
directly to the plant
Mostly commercial N-P-K fertilizers as well as liquid and semi-liquid manures
from the farm.
Indirect: ‘soil fertilizers’
Primarily improve the nutrient substrate (soil in field crops), although they have a
certain additional significance as a source of nutrient e.g limes, peat, straw.
d) According to speed of action: (Fast acting versus slow acting).
Fast acting: fertilizers are immediately available to the plants (e.g water
soluble N and K fertilizers), or improve the soil within a short time.
Slow acting: effective only after conversion in the soil.
e) According to type of chemical compound (Organic versus inorganic)
Organic: usually a mixture of a number of organic compounds e.g the natural
organic fertilizers, such as manure and peat.
May be definite single compounds like some especially slow acting N
fertilizers or urea.
f) Inorganic fertilizers/mineral fertilizers
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Consist of one or more inorganic compounds (salts, oxides etc)
Mostly contain mineral nutrient or yield them upon conversion
Also include some compounds that are organic according to their chemistry,
but are rapidly converted into mineral substances in the soil e.g urea?????
g) According to number of nutrient elements Single-nutrient element versus
multiple-nutrient fertilizers
Single-nutrient/single fertilizers: contain one essential nutrient element e.g N
fertilizers. They are also termed straight fertilizers.
Multiple fertilizers: contain for than one nutrient element. E.g compound
fertilizers. Sometimes termed “complete fertilizers”.
h) According to amounts required by the plant
Macro/major nutrient fertilizers:
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Contain the major (essential) plant nutrients N, P and K. They may also contain
micronutrients since they are not chemically pure substances.??
Micronutrient fertilizers:
Contain micronutrient elements mostly applied in small quantities.
i) According to state of aggregation/formulation/physical state
solid; liquid; gaseous e.g ammonia
j) According to time of application
Basal versus top dressing
External factors affecting nutrient availability
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Natural supply of nutrients in the soil which is closely tied-up to the parent
material of that soil and vegetation under which it developed.
Soil pH as it affects nutrient release. It influences rate of nutrient release through
its influence on decomposition, Cation Exchange Capacity(CEC) and solubility of
materials.
Decomposition of organic matter is the source of N and S ( Decomposition is
fasted between 6-8 pH range)
Relative activity of microorganisms. They play a role in the release of nutrients
and mycorrhiza
Fertility addition in the form of commercial fertilizer, animal manure and green
manure etc
Soil temperature, moisture and aeration.
Critical aspects in fertilizer application:
1. How much to apply? (Amount/quantities to apply)
The amount of fertilizer to add/apply depends on:
 The target yield (variety and expected/prevailing moisture conditions)
 Crop’s requirements
 Nutrient status of the soil (soil tests and /or plant analysis)
2. How to apply? (Methods of application)
Choice of method =f ( available equipment; size of field; labour availability;
physical properties of the fertilizers; planting method used etc)
 Banding
 Spot application (Cup method; dopping)
 Foliar application – Limited amount of nutrient is supplied, but plant recovery
rate (from deficient symptoms) is high. Nutrients can/are absorbed through
micro-pores of the leaves. Water-soluble fertilizers are used.
 Fertigation (sprinkler and drip are appropriate)
 Broadcasting and incorporate; broadcast(lime; manure; cpd fertilizer) and
leave on surface
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Vicon
Injection : used for solution and gaseous fertilizers e.g ammonia . Application
should be such that the nutrients penetrate into the root zone and become
optimally effective there.
3. When to apply (timing).
Time of application is aimed at providing nutrients in sufficient quantities to meet
the crop demand and at the same time avoiding excess availability and leaching
losses.
Timing is dependent on:
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Crop uptake pattern and purpose of the fertilizer e.g for early growth, a plant
requires high N; and P for root development.
N, P and K are taken in large quantities in early stages of crop growth. E.g in
finger millet, 95, 86 and 68% of N, P and K uptake is completed by panicle
initiation stage. N is necessary for the synthesis of proteins which are
necessary for the development of tissues.
Uptake of N is slow at the later stages of growth which is generally met from
the soil by mineralization
Legumes require nitrogen at early growth stages until root nodules are formed.
K is taken gradually throughout the growth and development of the crop
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Soil properties and nature of fertilizers
E. g Nitrogenous fertilizers are lost into deeper layers beyond the root zone if
the entire quantity of fertilizer is applied, especially in a light textured soil.
This is so because N fertilizers are soluble and highly mobile in soil. On the
other end, phosphatic fertilizers are highly reactive and are fixed in the soil
and become immobile. K fertilizers are less mobile since they are adsorbed on
the clay complex. As a result, the entire quantity of phosphatic and potash
fertilizers are therefore applied in one dose at the time of sowing. (But K is
also applied as a top dressing fertilizer e.g in tomato)
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End product of the crop.
The levels of carbohydrates and nitrogen in the plants are inversely
proportional. When nitrogenous fertilizers are applied in large quantities, the
level of carbohydrates in plants decreases. When N is less in plants,
carbohydrate level in the plants increase. Under a sufficient level of N, in the
plant, carbohydrates are utilized for the synthesis of proteins. The
assimilation of N requires energy which is obtained from light or the
breakdown of carbohydrates.
E.g in fodder crops, leafy succulent crop with higher level of proteins are
preferred compared to fibrous crop with higher carbohydrates, therefore,
nitrogen in several splits is necessary. On the hand if the fodder crop is for
silage, it should have higher carbohydrates just before cutting for better
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quality silage. And therefore in this case, nitrogen application should be
curtailed in the last stage ( Reddi and Reddy, 1992).
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Availability of soil moisture (the nutrients need to be in solution for them to
be taken up the plants).
Diagnosis of deficiency symptoms
Effects of fertilizer use on the environment (Environmental pollution)
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effects on soil (pH, soil structure, soil life, toxicity
effects on water (river and water bodies). Artificial fertilizers from farmlands
are the sources of nutrient pollutants.
P and N are the two elements of main concern. N concentrations greater than
10 ppm (nitrate nitrogen) cause methmoglobin ( caused by reduction of
haemoglobin to methemoglobin)
Eutrophication results (excessive growth of algae and water weeds, which
eventually lead to depletion of oxygen for other forms of life in these water
sources). Most natural water has adequate dissolved nutrient quantities to
support algal growth except N, P and sulphur. These elements are present in
most compound fertilizers used locally.
Agricultural land is often the source of nitrates while both Nitrates and
Phosphates are present in domestic effluent. P is sparingly soluble, S is more
mobile and nitrate very soluble from manure and artificial fertilizers ( Grant,
1975 cited by Nyakanda, 19..)??
Sewage sludge is used as a fertilizer and is also a source of water pollution
(pathogens; trace metal elements; nitrates and phosphates
Fertilizer losses that can occur from the soil
1. Leaching
N and K are easily leached . For N, the nitrate (NO3) form is the most readily
leached form.
Both NH4+ and NO3- are soluble in water, but the positively charged NH4+ are
held to cation exchange sites and resist leaching.
Leaching losses increase with quantities of percolation water
Split application reduces losses through leaching
2. Erosion
3. Denitrification
Soil nitrogen can be lost through this process. It is the change by bacteria of
NO3- to a nitrogen gas (mostly N2, some N2O and other oxides)
It is usually the most extensive gaseous N loss.
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It is common in poorly aerated soils. In such situations, the amount of free
oxygen is limited such that specifically adapted bacteria use the nitrogen in
NO3- as an electron acceptor. The end products are N2 or N2O) and these
gases volatilize into the air.
Favourable conditions for denitrification are:
 lack of free gaseous oxygen
 any energy source of oxidizable Organic Matter –food for the bacteria)
 warm and slightly acidic conditions.
4. Volatilization (Ammonia volatilization)
Occurs when NH4+ is in a basic solution
Greatest loss occurs from surface applications of any NH4+ or Urea fertilizer
on alkaline soils
It is minimized by covering the fertilizer with soil or leach it in with irrigation
or rainfall.
Volatilization of urea
Urea, plus the enzyme, urease, in solution forms ammonium carbonate. In
basic soils calcium hydroxide [Ca(OH)2] reacts with the ammonium
carbonate[ (NH4)2CO3] to form ammonium hydroxide.
Ca(OH)2 + (NH4)2CO3---2NH4OH + CaCO3
NH4OH-----NH3 +H2O
-
Losses of ammonia gas are greatest on high pH calcareous soils
Losses are also great when fertilizer is left on the soil surface
Losses increase with temperature, especially as surface soil dries out
after being wetted (drying concentrates ammonia)
Losses are great in soils of low CEC.
5. Removal of crop residues
6. Fixation: especially for phosphorus and also of potassium makes these
nutrients unavailable to plants although they may be within the rooting zone.
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D. WEED CONTROL
A weed may be defined as a plant growing where man does not want it to be.
Thus any kind of plant can be a wed for as long as it is growing where it is
considered undesirable.
Effects of weeds on crop plants:
1. Reduce crop yields and this is an easily observable effect.
The reduction in yield is brought about due to the fact that the weeds compete with crop
plants for water, mineral
nutrients, light and space. Usually the competition for water and mineral nutrients is most severe
and crop yields are most depressed when either of these factors is in short supply.
Competition for light = f(canopy structure of the crop and the weed and upon their
relative times of establishment) E.g a tall growing weed in a prostrate crop such
as goat weed in pumpkin or cowpea, will compete more severely for light as
compared to the reverse situation (a prostrate weed in a tall crop e.g Portulaca
oleracea on maize).
Competitive ability of the crop against weeds varies with crop stage ( It is low
during the seedling and early vegetative stages. Certain stages are particularly
sensitive to weed competition e.g for maize, the most critical period is between
emergence and tasseling while for yams it is the first 2-3 months after
emergence/establishment.
Competitive ability is also influenced by plant density. A high crop density may
control weed seedlings by depriving them of light.
2. Allelopathy: Weeds may compete with plants by producing substances that
inhibit growth and development of the crop. The harmful substances can be
produced in the soil when the weed is still alive or when the roots are decaying.
Example: the roots of Agropyron repens (quack grass), release substances which
inhibit germination and growth of various crop seedlings.
3. Weeds can be parasitic on crop plants e.g witch weed (Striga spp). And the
dodder plant (Cuscuta spp). The two weeds attack a number of crops including
maize, sugarcane and sorghum.
4. Weeds can habour pests and diseases – they save as reservoirs for these
organisms. This especially important/critical in the carry-over of disease
organisms from one season to the next. Viz, the importance of crop residues at the
end of the season, but weeds can serve as host for the pests/diseases during offseason.
Examples:
Weed acting as alternate hosts
1. Echnochloa & Panicum spp
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Pest/disease
Stem-borer
Crop
rice
Page 16
2. Agropyron repens
3. Saccharum spontaneum
black rust
downey mildew
wheat
maize
The weed may serve as alternate host so that the organism cannot complete its
life cycle unless that weed spp is present.
5. Weeds can reduce the quality of the harvested crop (weed seeds in grain; black
jack weed on cotton etc)
6. Some weeds can be poisonous or can cause discomfort(e.g spiny weeds) to
livestock. Lantana camara is poisonous to animals
7. Weeds may clog irrigation canals
Examples of common arable weeds found in Zimbabwe.
Weeds can be classified according to:
a) Seed type --------1) monocots[grasses and sedges]
2) dicots
[ most broad-leaved weeds]
b) Life cycle (annuals; biennials; perennials)
c) Habitat (cultivated land weeds; wasteland weeds; orchard weeds; aquatic
weeds
d) Origin (indigenous; exotic)
e) Crop-weed relationship/competition (parasitic; non-parasitic weeds)
Yield losses (due to the effects of weeds) of up to 30% have been reported in
Zimbabwe (Chivinge, 1980???).
Common name followed by botanical name and then vernacular name
1. Sabi morning glory; Ipomoea plebia; Katewe
Heart shaped leaves; large V-shaped cotyledons.
2. Black jack; Bidens pilosa; tsine/Kanzota/mhuwu
Dark stem, dark green leaves’. Sometimes dark purple under cotyledons.
3. Mexican marigold/Khaki bos/Tail khaki weed/stinking roger; Tagetes minuta;
Kambanje/Jerimani/Mbande
Leaves more feathery, jagged, khaki bos smell when rubbed between fingers.
4. Dwarf marigold; Suhkria pinnata; Rukarwa
Leaves finned (pinnata)
5. Spindle pod; Cleome monophylla; Mujakari
Light green stem, first true leaves long and pointed. Cotyledons shorter and
rounded.
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6. Stinkblaar; Jimson weed; Datura stramonium, thorn apple; Chowa
Distinct smell, reddish stem, cotyledons long, narrow and pointed. First true
leaves shorter.
→→→→→→→→→→→→→→→→→→→→→→
30.
Some of the characteristics of weeds which enable them to compete
effectively with crop plants.
 ability to flower and produce seeds in a short space of time (short growth cycle) –
can complete cycle even if favorable season for growth is short
 ability to produce large numbers of seeds.
 Presence of efficient mechanisms for seed dispersal (main agents of dispersal are
wind, animals and water). [Water dispersal – light seed + membranous structures
or cork on testa]
 Presence of allelo-chemicals that inhibit growth of crops or other weeds.
 Possession of specialized dormancy mechanisms (e.g impervious coats, growth
inhibitors etc). Net effect is that seed germination occurs only under conditions
that will enable them to complete their life cycles.
 Ability to survive in a wide range of environments, this way the weed can grow in
a number of cropping situations
 Presence of perennating organs (perennial weeds have this additional means of
survival). The organs enable the wed to survive from season to season and allow
re-growth ( multiplication of weeds during mechanical weed control).
 Ability to propagate vegetatively.
Management of Weeds
-
Includes concepts of prevention, control and eradication
Prevention: concerned with efforts to prevent the introduction and establishment
of weed species into an area where they do not exit.
Control: Any procedure that reduces the infestation or vigor of weeds to a level
that makes them less harmful to crops.
Eradication: Total/complete/permanent removal of a given weed species from the
ecosystem. It is difficult to achieve; often impractical when large areas are
infested.
Weed Control Methods
For successful control one has to consider/know the following points:
i)
ii)
iii)
iv)
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Life cycle of the weeds
Characteristics of the weed
Mode of reproduction
Soil condition
Page 18
v)
Habitat and locationarm practices / cropping systems e.g sole vs intercropping; effect of herbicide on crop to come
1. Cultural control: Involves use of sound agricultural practices e.g use of
weed free crop seed or planting materials; crop competition





Crop competition: ( an ecological method) – control is brought about by
managing the crop in a manner that gives them a competitive advantage
over weeds
dense crop stand (can adjust plant density to achieve maximum
competition with the weeds
can select crops that are more competitive for water, nutrients and
available light
If main crop is a poor competitor, an aggressive intercrop plant planted at
high density may be used to suppress weeds.
Avoid introduction of weeds in weed free (with respect to a particular ) or
new areas
Clearing of equipment thoroughly after its use in a weedy field before
moving to another field to prevent spread[Weeds are also dispersed by
adhering to equipment]
Grazing :animals move from one field to another-potential means of weed
dispersal: seed adhering to their bodies and through their dung
(Chidhongi) Upright Starbur (Acanthospermum hispidum)
Rotations
To prevent or reduce the occurrence of weed species that are associated
with particular crops growth habits and life cycles of these crops are
normally quite to weeds also with them.
2. Mechanical/physical control

Hand pulling
To remove weeds very close to the crop.
Suitable in home gardens and for potted plants or where weed spp occurs
in very isolated stands
Important to make sure that removed weed is not left in a position or
condition in which it will easily re-establish.
 Use of hand –held implements
Short and long-handled hoes
Other operations can be carried out during hoeing e.g mending mounds,
thinning
Suitable where cropped areas are relatively small
Labour intensive
Other implements (besides the hoe) are the machete and cutlass.
Short handled hoes require too much stooping
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 Machine tillage:
Can use plough, harrow or cultivator
Ploughing brings buried weed seeds to the soil surface and they begin to
germinate. Shallow tillage done shortly after seedling emergence destroys
weed seedling (Good control method for annual weeds).
Repeated tillage at relatively short intervals may be necessary for perennial
weeds.
Tillage to aim at destroying the weed plants before they reach the stage of
seed-setting.
Weeds in unploughed fields or fallow may produce seeds which get dispersed
in the cropped fields. These which are not dispersed pose a problem when the
fallow land is cropped in subsequent years as the seed may remain viable for a
number of years
 Mowing
Practised between rows of tree crops in pastures and along roadsides
One of the main purposes is to prevent the weed from producing seeds.
 Fire
Mostly used to destroy/remove plant growth and plant material prior to
cropping viz; use in shifting cultivation
Removes existing weed plant on the plot as well as destroying weed seeds
lying close to the surface
Flaming –Directed burners/flames are used to control weeds in growing crops
e.g onion; cotton
 Mulching:
A mulch is layer of non-living material placed over the surface of soil for
various purposes(as indicated below).
A mulch can smother the weeds and it also cuts them off from direct sunlight.
Mulch should be resistant to plant (weed) penetration and in the case of grass
or crop-residue mulch, the mulch should be relatively thick. Mulch made of
continuous layer of paper or opaque plastic is usually very effective for weed
control.
Normally a mulch is not put up for purposes of weed control. Its other
purposes include: conservation of moisture through reduction of evaporation
rates; preventing excess heating of the soil during the day (temperature
moderation) and excessive heat loss at night.; reducing soil loss/soil wash by
breaking the impact of rain-drops; providing organic matter to the soil as
mulch decays.
3.Biological weed control
Definition: The use of living organisms to control weeds and to reduce crop
competition below economic levels
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Examples: Use of natural enemies of certain weed spp. The natural enemy has to
be introduced or encouraged (build –up population to a level that will keep the
weed population below economic levels). The introduced insect must have
considerable adaptability to a wide range of environments so that it can multiply
rapidly and control the weed effectively =regulate.
Method is most effective/suitable where one weed spp is dominant.
Limitations and /or problems: (reasons for limited scope for biological control
the method poses potential danger in that the natural weed enemy may end up
attacking/ feeding on the crop in the event that the insect pest runs out of food.
The need to find an insect that is specific in its choice of its host so hat crops are
not endangered.
Examples :
i) Use of weed eating insects:
a) In Australia the control of the prickly pear (Cactus opuntia spp), which in
1925 had infested about 24 million hectares by the moth borer Cactoblastis
spp, introduced from Argentina.
To date, the weed is controlled by cochineal insect (Dactylopius tomentosus)
in India
b) In India the grass Saccharum spontaneum is controlled by growing basket
grass. The latter’s roots excrete substances which are inhibit growth of
Saccharum spontaneum
c) John’s wort/Klamath weed (Hypericum perforatum) which infests the ranges
west of the USA) in the United States of America is controlled by a beetle
Chrysolina spp.
d) Aquatic weeds are controlled by snails, fishes etc.
ii) Use of catch crops and trap crops for the control of parasitic weeds such as
witch weed
Use of catch crop: Plant and allow a crop that is host to the parasitic weed to
establish and then plough under both the catch crop and the weed before the
parasite produces seed. E.g of crops that can be used as catch crops:
Use of trap crop: Crop used is not a host to the parasitic weed, but induces it
(weed) to germinate; the weed will not be able to complete its life cycle, since
the trap crop will support the weed. Examples:
iii) Selective/preferential grazing: E.g geese are used for the selective grazing in
cotton. Young geese will graze grasses and sedges the field without damaging
the crop.
Sheep- graze plants close to the ground, thus continuous grazing may
considerably weaken certain perennial weeds and prevent others from
producing seed.
Goats- in veldts/natural range can control bush
4. Chemical control
Use of chemicals called herbicides. These can be classified according to:
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Page 21

Time of application
Pre-planting application – done before crop is planted. Herbicide can be
sprayed on the foliage of existing weeds e.g paraquat (other name) or
incorporated into the soil during tillage operation e.g Trifluralin.
Pre-emergence application- done before the crop has emerged. Can be
applied during sowing (done/used with seed drills). At time of application,
weeds may or may not have emerged. Herbicide to be used will depend on
this e.g if weeds have not emerged, diuron, simazine or ametryne can be
used ( these can act on unemerged seedlings). If weeds have emerged,
herbicide which kills emerged seedlings can be used e.g ???????
Pre-emergence herbicides are more effective if applied to damp soil.
Post –emergence application: done when crop has emerged. Weeds may or
may not have emerged by then. Herbicide should not kill the crop. Can be
achieved by use of selective herbicides (these will kill the weed and not
the crop)e.g simazine in maize or else a directed spray will be needed.
(Note: Pre or post emergence application can also be with respect to the
weed, according to certain literature)

-
Whether or not they are selective (selective versus non-selective)
Ideal situation – predominant weed spp are killed by the herbicide
Selectivity may be based on: amounts of the chemical that are intercepted,
retained absorbed and translocated by the weeds and the crop.
Formulation of the herbicide can also influence selectivity
Selectivity also = f(ability of crop to tolerate a particular herbicide better
than the weeds)

Site of herbicide action:/ mode of action
Contact herbicide
- Kill the tissues at or very close to the point where they touch / get in
contact with the plant
- Weeds should be thoroughly covered with the herbicide. Usually effective
in eliminating annual weeds while perennials and plants with underground
reserves may re-grow at a later stage.
- May be selective or non-selective.
- Paraquat is contact non-selective herbicide and such herbicides are useful
for total weed control along roads, railroad tracks and irrigation ditches.
Systemic herbicides – Are absorbed into the plant and translocated to various
parts of the plant.
- Can thus kill both shoot and root
- Selective or non-selective
- Selectivity is based on ability of weed and crop to absorb, translocate and
detoxify the herbicide. E.g is 2,4 –D a systemic herbicide that kills broadleaved weeds but spares grassy weeds and crops
- Particularly useful in controlling perennial weeds since underground
organs and roots are killed in addition to the shoot
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Soil acting herbicides:
- Primarily act in the soil
- Inhibit or retard the germination of weeds
- Normally have long “residual action” This way they can prevent growth of
weeds for a significant part of the growing/cropping season
- Residual effect also has an effect on the subsequent crop in the cropping
cycle. When choosing a herbicide to use one has to consider its residual
effect and the effect on the following crop e.g ????????????

Chemical nature:
Classification is by chemical composition of the herbicide
Examples
Chemical
group
Carbamates
Phenoxy-cpds
Substituted
ureas
Herbicide characteristics
Examples
Pre-planting or preemergence. Some are
volatile and require soil
incorporation
Post-emergence control of
broad-leaved weeds
Pre-emergence weed
control; most effective
before weeds have emerged.
High persistence.
Vernolate; asulam; IPC
2,4D; MCPA
Ametryne; atrazine and
simazine.
Residual effect of herbicides
-
-
-
There is possibility that the crop takes up the herbicide which may then appear in
the harvested product.
Extent to which take up and appearance in harvested product occurs = f(herbicide
type, crop type, time of herbicide application). Thus before a herbicide is
recommended, the extent and nature of the herbicide residues in the crop is
determined through rigorous experimentation
Permissible residue concentration =f(level of toxicity of the residue and the use of
the harvested product/plant part).E.g A higher level of residue would be
permissible in cotton lint as compared to produce used for food e.g carrots or
cabbage.
Residual effect in the soil(as explained earlier).
E. PEST AND DISEASE CONTROL
Insect Pests
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Page 23
-
Depending on numbers, nature of damage and crop stage, insect pest can cause
economic crop losses. Hence the need for control.
Methods of control
a) Collection and destruction (physical destruction)
Suitable in countries with primitive agriculture; for small areas and where
there is over-abundance of labour
Larva and eggs can be collected and physically destroyed.
b) Cultural practices
 Time of sowing
 Sanitation e.g removal of weeds(alternative hosts
e.g…………………………………???); destruction and burning of stover or
ploughing it in.
 E.g Pink boll-worm larvae in cotton; destruction of or ploughing under the
stalks and residues has been found to destroyed 85% of the larvae
 Crop rotation – rotate with non-host crops
 Sowing trap crops
 Use of resistant varieties: Breeding for insect resistance has rece4ived far less
attention than for disease resistance because insect-plant/host relationships is
far less specific and more complex than is the case with pathogens.
Successes - maize varieties resistant to the European corn-borer; Hessian fly
resistant wheat varieties and jassid –resistant varieties of cotton.
 Tolerance: Certain plant characteristics may repel or deter insect pests e.g
hardness, hairiness or may prevent feeding and oviposition.
c) Chemical control (Use of insecticides).
By far the most widely used method. If well implemented it is quite effective.
Quite expensive. It possess a threat to the environment just like the use of
herbicides =environmental pollution
When inappropriately used e.g use of wrong dosages, it will result in
resistance build-up by the insect pest. To reduce chances of resistance build –
up for some problem pests, insecticide rotations are practices e.g the acaricide
rotation in cotton and pyrethroid rotation (horticultural crops.
Acaricide rotation:
Pyrethroid rotation:
Insecticides and any other pesticides should be used safely as they are a
potential hazard to the user and the environment viz:
 The label and the information contained should be read and understood –
what information does the label contain?
 Protective clothing;
 No drinking and smoking when spraying;
 Timing of the operation e.g calm and cool conditions are ideal;
 Cleaning of equipment after spraying
 Storage of chemicals(where and under what conditions);
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 Dealing with spilled chemical;
 Disposing of empty containers etc;
 First aid in the case of poisoning. etc
d) Biological control
E.g microbial control: The use of Bacillus thuringiensis (a bacterium in the
control of the European corn borer and the control of semi-loopers in
soyabean and other lepidopterous insect pests by the use nuclear-polyhydrosis
virus
e) Preventive control
E.g surveillance control of locusts especially the Red locust (Nomadaeris
septemfasciata) and of the African locust (Locusta migratoria). The foci of
infestation of these pests are in relatively restricted areas. By maintaining a
constant watch over these potential outbreak areas it is possible to reduce
dangers of large scale invasions
Desert locust – nomadic; supervision of the seasonal movements of swarms in
the areas that frequently are frequently infested.
Armyworm –successively dealt with through surveillance. Taking action
when pest is noticed in the field may be late as the pest feeds very fast.
Surveillance is critical so that effective control operations can be planned at
earlier stage.
f) Integrated Control - Integrated Pest management (IPM) –IPPM
FAO (1967) defined Integrated Control as “A pest management system that,
in the context of the associated environment and the population dynamics of
the pest species, utilizes all suitable techniques and methods in as compatible
a manner as possible and maintains the pest population at levels below those
causing economic injury”.
It is an ecological approach to pest control.
Economic injury level: The lowest population density that will cause
economic damage.
Economic threshold: The pest population level at which control measures
should be started to prevent the pest population from reaching the economic
injury level.
Economic damage : The injury done to the crop which will justify the cost of
artificial control measure.
Characteristic features of the IPM
1. Orientation /scope: The entire pest population or a relatively large portion of
it, rather than localized infestations.
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2.
3.
4.
5.
Population to be managed is more often international . There is a high
degree of co-orperation both locally and internationally.
Immediate objective: Lower the population density of the pest so that the
frequency of the fluctuations (spatial and temporal), above the ET is reduced
or eliminated
Method or combination of methods is chosen to supplement the effects of
natural control agents where possible and is designed to :
give maximum long-term reliability of protection
give minimum expenditure of efforts and money
give least negative effects on the ecosystem
Significance: alleviation of problem is broad/general and long-term versus
being localized and temporary.
Philosophy: To manage the pest population rather than attempt to eradicate it.
For years, IPM has been a rather new concept, more developed theoretically
rather than in practice especially with regard to use of computer models. The gap
between theory and practice is now being narrowed.
IPM Programme in Zimbabwe: (Highlights – brief notes)
Disease control
Control methods aim at:
a) Preventing the build-up of inoculum
b) Evading infection
c) Increasing the resistance of the crop to infection and improving its ability to
recover from infection.
Cultural methods
-
Use disease free planting material
Sowing dates e.g early planting in wheat allows crop to escape damage from rust;
early maturing varieties of the crop may escape damage from rust.
Rotations
Sanitation e.g weeding destroying/burying crop residues; also sanitation in the
granaries
Use of resistant / tolerant varieties
Storing under appropriate conditions 9conditions that do not favour disease/ pest
build up e.g high humidity and high temperature).
Maintaining a health crop, e.g by effective use of fertilizers. This assists crop in
recovering from effects of diseases. Too much fertilizer causes rank growth which
promotes disease infestation.
Use of chemicals
E.g fungicides and bactericides ( e.g seed treatment, spraying infected plants etc)
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F. HARVESTING
Harvesting time: has an effect on product quality and harvest losses incurred.
Most suitable time to harvest in when plants have attained physiological maturity.
(Difference between harvest maturity and physiological maturity). In the case of
grain crops, when crop is mature, kernels reach the hard dough stage. Maturity
indicators for selected crops:
Maize – development of a black layer at the point of attachment of grain to the
cob. Moisture content at Physiological maturity is approximately 30%.
Groundnuts – insides of the kernels darken; colour of seed skin; firmness of the
skin when rubbed off between fingers.; approximately 80-90% leaf fall; colour of
leaves
Sunflower: colour of the bracts etc
Harvesting equipment: machine or hand harvesting. Machine harvested produce
is normally of lower quality compared to hand harvesting, however, the latter is
labour intensity. Harvest losses are also relatively higher for machine harvesting.
Harvest losses:
Preliminary surveys in Zimbabwe have shown that harvest losses of up to 10%
can be incurred.
These are =f (method used to harvest; time of harvesting; crop type and /or
variety). Harvesting in the morning reduces shattering losses in a crop like
soyabean. The crop should also be harvested before the crop starts to shatter.
Choice of variety in this case also determines amount of losses incurred. Lodging
also contributes to yield losses during harvesting especially machine harvesting.
For soyabean, pod clearance is critical in determining harvest losses especially if
a combine harvester is used.
G. PROCESSING
Methods employed depend on the crop in question e.g
Cereals ------------------------dry; thresh; winnow
Groundnuts--------------------dry/cure; pick/pluck; shell
Sunflower----------------------dry; thresh; winnow
Tobacco------------------------cure (curing is a process by which the harvested leaf
is made ready for the market. It is essentially a drying process whereby most of
the moisture in the harvested leaf is removed. It is done in such a way that certain
well defined and desirable qualities in types of tobacco are produced.
H. GRADING AND MARKETING
Grading : Size; Uniformity; quality; foreign matter content; stage of ripeness
(particularly for horticultural crops)
Different grades fetch different prices
Marketing of produce may be delayed until prices improves (Farmer holds on to
his produce)
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Transport is often a big problem in marketing of produce.
Some commodities have a controlled market (maize and wheat)
I. STORAGE
-
Idea is to keep the produce in good quality for a long period. (for different
purposes).
Storage length =f(crop type; variety; storage environment)
Detailed notes in Principles of crop production II (AGRO 205 /NRM 212)
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