SOIL
MANAGEMENT PRACTICES AFTER PLANTING
After planting, the soil
should be carefully managed so that it remains in good condition suitable to
the needs of the crop with least expenses. This involves maintenance of the
physical condition of the soil, its moisture and nutrient content. A good
system of soil cultivation should ensure:
Weed control and
saving in moisture and nutrients
Very little disturbance to soil and
preventing soil erosion
Reduced cost of cultivation
The soil management
practices undertaken after planting include the following:
Clean culture
Involves regular ploughing
and removal of weeds. Has many disadvantages
Frequent cultivation
depletes humus rapidly leading to the depletion of nutrients
Causes injury to roots
Hard pan is created in the soil
Causes more soil erosion
These defects can be
minimized by avoiding deep and frequent cultivation and avoiding cultivation
when the soil is too wet.
Mulching
Crop residues like straw,
stalks, leaves, sawdust, polythene films are spread in the tree basins and in
inter spaces between trees. Main objectives of mulching are:
To
conserve soil moisture
To control the weed
growth
Keeps soil cool in day;
warm at night hours
Reduces surface run-off
Adds humus to the soil
Prevents soil erosion
Fruits are protected and
kept clean since they fall on the mulches
It allows the absorption
of more rain water
It reduces irrigation
frequency.
Disadvantages of mulching
Dry
materials used as mulches encourage the risk of fire
Thick mulches may act as
places for mice and rodents to live and multiply. These may cause damage to
tree trunks and roots by eating the bark and burrowing to the land.
Mulching localizes roots
on the moist topsoil surface instead of allowing deep penetration. This will
result in lodging due to lack of anchorage
INTERCROPPING
In young widely spaced
crops, there is a long period of time when there is extensive ground area
between planting points which require attention. The main crop should be clean
weeded upto 1 m. radius and the rest of the area used for intercrops.
Intercrops are used for the following reasons:
·
To maximize on land use
·
To nurse the main crop e.g. through modification
of the climate, thereby helping the main crop to establish better. For example,
tall growing temporary trees such as bananas can be used to shade main fruit
species when they are still young.
·
To conserve the soils through protection from
raindrop impact and direct sunlight.
The following problems are
usually encountered in an intercrop/cover crop situation:
·
Competition for resources with the main crop
·
Host of diseases/pests of main crop
·
Makes mechanization difficult
CONSERVATION
AGRICULTURE
Conservation Agriculture (CA) is an
approach to managing agro-ecosystems for improved and sustained productivity,
increased profits and food security while preserving and enhancing the resource
base and the environment. CA is characterized by three related principles,
namely:
q Continuous minimum mechanical soil
disturbance.
q Permanent organic soil cover.
q Crop rotation
In conservation
agriculture, minimum or zero tillage is practiced. Minimum tillage refers to a
reduction of tillage operations to a minimum possible to ensure good seedbed,
rapid germination and favourable growth conditions. Tillage is reduced by
omitting operations which do not give much benefit compared to the cost. Zero
tillage refers to placement of the seed onto the soil without soil preparation.
It is an extreme form of minimum tillage. Primary tillage is completely avoided
and secondary tillage is restricted to land preparation in the row zone only.
In zero tillage, there is direct sowing of the seed directly onto the soil
without any disturbance of the soil.
Advantages of Conservation Agriculture
q Improves soil structure
q Protects the soil against erosion and nutrient
losses by maintaining a permanent soil cover and minimizing soil disturbance.
q Enhance soil organic matter levels and
nutrient availability by utilizing the previous crop residues or growing green
manure/cover crops keeping these residues as a surface mulch rather than
burning.
q Requires significantly less water
use due to increased infiltration and enhanced water holding capacity from crop
residues left on the soil surface.
q Mulches also protect the soil
surface from extreme temperatures and greatly reduce surface evaporation
q Land under no-till is not cleared
before planting and there is less weeding due to establishment of permanent
soil cover
q Soil nutrient supplies and cycling
are enhanced by decomposition of crop residues at the soil surface
q Soil fertility is built up over time
under conservation agriculture, and fewer fertilizers are required to achieve
optimal yields over time.
q Insect pests and other disease
causing organisms are held in check by an abundant and diverse community of
beneficial soil organisms, including predatory wasps, spiders, nematodes,
springtails, mites and beneficial bacteria and fungi, among other species.
q The significant reduction in fossil
fuel use under no-till agriculture results in fewer greenhouse gases being
emitted into the atmosphere and cleaner air in general.
q Reduced applications of agrochemicals under CA
also significantly lessens pollution levels in air, soil and water.
MINERAL
NUTRITION
Fertilizer
placement
Soil
application
Applied to the
soil and taken up by the roots. Less expensive and better suited for large
application rates of the major nutrients and for pre-plant application.
Applications by broadcasting, banding or top dressing
Circle
banding: A furrow is cut, 20 cm wide and 30 cm deep, around the tree in a
circle beneath the outer canopy
Strip band application.
Parallel furrows are cut, 20 cm wide and 30 cm deep, between the rows of
the trees. This method is suitable for old orchards, where the trees are more
than 10 years old because of greater root spread.
Hole
placement. Four or five holes are dug beneath the outer canopy of each
tree. The holes should be 15-20 cm in diameter, and at least 30 cm deep.
Broadcasting:
Fertilizer broadcast on the surface under the spread of the branches at
least 15 cm from the trunk, since injury can occur if placed too close.
Foliar
applications
Absorption of
nutrients is very rapid, and plant’s response is evident in 1-2 days. However,
application must be more frequent than soil application because of little
residual effect. Used for applying nutrients immobile in soil (Fe and Zn) or
those immobile in plants (Ca and B)
Boron:
applied to increase level at flowering
Calcium and Iron:
applied at post-bloom to harvest period to increase level in the fruit.
Application of
macronutrients to quickly correct deficiency
Fertigation
Refers
to application of soluble fertilizers through an irrigation system. In
fertigation, a fertilizer concentrate is prepared and mixed in the tank. This
fertilizer concentrate is sucked and mixed with the irrigation water at set
ratios e.g. 1:100, 1:200. As the water and fertilizer mixes, the concentrate
gets diluted and gets to the plant at acceptable levels.
Benefits
over other fertilizing methods include:
q allows accurate and uniform application of nutrients at the
right concentration in order to meet the actual nutritional requirement of the
crop throughout the growing season
q Allows for change of the fertilizer program during the
growing season in order to adjust for fruit, flower, leaf and root development.
q increases the efficiency of fertilizer application thereby
reducing the amount of applied fertilizer and production costs
q lessens the potential of groundwater pollution caused by
fertilizer leaching
q Saves on labor
q the supply of nutrients can be more carefully regulated and
monitored.
q Crop foliage is kept dry thus avoiding leaf scorching and
delaying the development of plant pathogens.
q Allows application of small concentrations of micronutrients
which are otherwise very difficult to apply accurately to the soil by soil
application
Phosphorus fertilization
Specific growth factors associated with
phosphorus
stimulate root development
Increases stem strength and thickness
allowing plants to withstand strong winds
improves flower formation and seed
production
More uniform and earlier crop maturity
increases resistance to plant
diseases.
Deficiency symptoms
Phosphorus
deficiency is difficult to diagnose. Crops usually display no obvious symptoms
of phosphorus deficiency, other than a general stunting of the plant during
early growth and dull-green or blue-green color. Plants become slender, with
thin stems and fruit cluster development is poor. Red or purple color forms on
the underside of the leaves, including the veins, delayed maturity and roots
become brown and develop few lateral branches.
Soil pH and P
Soil
pH is a measure of the concentration of hydrogen ions (H+) in the soil
solution. The concentration of H+ ions in the soil solution determines whether
the soil is acid, neutral or alkaline (basic). A pH of 7 is neutral, below 7 is
acid and above 7 is alkaline (or basic). Plant nutrients are most available at
a pH varying from 5.5 to 7.0. A soil containing a pH less than 5 is strongly
acidic and injurious to plant growth. Similarly, pH above 7.5 is strongly
alkaline and also injurious to growth.
Continuous use of fertilizers
alters the soil pH. Acid-forming
fertilizers include fertilizers that
contain sulphur, fertilizers containing ½ the content as nitrogen and ⅓
the content as P. Examples are urea, DAP, ammonium phosphate, ammonium nitrate,
ammonium sulphate and phosphoric acid. Increased acidity causes the following
effects in crop production:
v Increases the solubility of Fe, Al and Mn in the soil to a
level which is toxic to plant growth
v Causes nitrogen deficiency because the nitrogen-fixing
bacteria will not thrive under acidic conditions.
v Causes phosphorous deficiency due to conversion of P to
insoluble forms. Under acid conditions, phosphorus reacts with iron or
aluminium in the soil to form iron or aluminium phosphates which are of low
solubility.
v Causes Ca and Mg deficiency because the soluble forms in
insufficient quantities
Low soil pH is remedied by
liming with CaCO3, dolomite (Mg,CaCO3)2 or
CaO. Liming removes the H+ from solution and raises the pH.
NITROGEN FERTILIZATION
Roles in plants
An essential component of proteins,
which make up the protoplasm and enzymes
Nitrogen is also present as a part of
nucleoprotein, amino acids, amines, amino sugar, polypeptides, chlorophyll and
other organic compounds in plants.
Needed for cell division and
reproduction.
Increases vegetative growth
Plays a role in flowering, fruit set,
fruit growth and fruit size
Deficiency symptoms
New leaves are small, thin and light
green in color.
Mature green leaves slowly turn to irregular
green and yellow pattern, become entirely yellow, and then are shed.
Plants may appear nearly normal but are
undersize.
Plants have poor flowering and carry
little or no fruit load. They are also highly erratic in bearing.
Nitrogen fertilizer sources
Urea,
Calcium ammonium nitrate, Ammonium sulphate, Ammonium sulphate nitrate, Calcium
nitrate, Potassium nitrate, Magnesium nitrate, Ammonium nitrate, Urea
phosphate, Sodium nitrate, Nitric acid
Potassium
Physiological
roles
Maintenance of water turgor in leaves
Regulates opening and closing of
stomata
Balances charge between negatively and
positively charged ions within plant cells.
Regulation of turgor pressure protect
plant cells from disease invasion.
Enzyme activation
Protein synthesis
Facilitates cell expansion
Improves fruit color
Improves resistance to strong winds and
drought.
Potassium
deficiency is likely to occur on sandy soils due to leaching. Lack of soil
moisture also reduces K uptake. Supply to plants may be decreased by soils are
very high in Ca and Mg or by heavy application of N.
Potassium deficiency
Moderately
low K cause a general reduction in growth without visual deficiency symptoms.
Presence of visual deficiency symptoms means that production has already been
seriously impaired.
Potassium
deficiency in leaves
Fruits accumulate large amounts of K,
so leaf symptoms are more likely and most severe as fruit approaches maturity
during heavy crop years.
Symptoms appear first on older leaves
because K is mobile but may affect young leaves in severe cases.
Deficiencies result initially in
yellowing of tissue along leaf margins.
The leaf margins later dry up
completely while the inner parts of the leaves remain green
Deficiency effects on fruits
Reduction in fruit size over time
Premature shedding of fruit.
The fruits have lower acid
concentration hence low fruit quality
Decreased yield
Other deficiency effects
Deficiency causes a reduction in the
rate of photosynthesis leading to reduced production of carbohydrates.
Increased susceptibility to disease
outbreaks, drought and cold
Potassium
fertilizer sources
Potassium nitrate
Potassium sulphate
Potassium chloride (muriate of potash)
Monopotassium phosphate
PRUNING
AND TRAINING
Training is the
practice of directing tree growth into a particular shape and form. Pruning is the removal of parts of a
plant to correct or maintain the formed tree structure. Correct training
and pruning are essential for early production, sustained high yields, optimum
fruit quality and efficient management. Symptoms of unsatisfactory growth that
suggest adjustments in the type and/or amount of pruning include the following:
Failure to
control pests or diseases may indicate that the trees are too high or too thick
for good spray coverage.
Poor fruit color may
be due to shading or excessive vegetative vigor.
Small fruits may be
due to Low plant vigor
Oversize fruit may
be due to high vigor which is often a response to over pruning.
Harvesting problems
because the trees are too tall or too thick
Low yields may be
due to few bearing branches
Training and
Pruning objectives
1. To control
size of the plant. Unpruned avocado or mango trees can attain heights
and widths of over 20 meters. Such trees bear fruits at the top of the canopy,
making harvesting difficult. The large
sizes also make spraying difficult
2. To control
the form of the tree with respect to the angle of branching. Remove
narrow-angled branches and leave wide-angled branches which should be well
spaced and distributed around the tree. Narrow angled branches are weak and
tend to break under heavy fruit load.
3. To open the
center of the tree to light exposure
4. To facilitate
air movement to the center of the tree. Opening the tree canopy permits
adequate air movement through the tree, which promotes rapid drying to minimize
disease infection and allows thorough pesticide penetration.
5. Improve health through removal of
diseased, dead or damaged parts. This allows the plant’s energy to be
channeled to productive branches.
6. Removal of
suckers growing below the grafting point
7. To aid in
establishment of transplants. Root and shoot pruning reduces transpiration
shock and promotes successful plant establishment.
Pruning
of taproot encourages the development of fibrous root system
Shoot pruning in
seedlings conserves moisture by reducing the transpiration surface
8. To balance
between shoot and fruit production. The main aim when pruning is to obtain a good
crop of fruits rather than a tree with abundant lush but unproductive foliage.
Unpruned trees tend to produce a large crop of small fruits often damaged by
pests and diseases and much of the crop is out of reach at the top of the tree.
Overpruned trees tend to produce light crops of large fruits. Pruning is
therefore carried out to achieve a balance between shoot growth and fruit
production.