CROP PRODUCTION II: Soil Fertility II

Plant nutrients occur in the soil in form of soluble substances.

These substances are taken in by the plants in different quantities depending on their roles in the plant tissues.

Essential Elements

These are nutrients needed by plants for various uses.

They are divided into two broad categories namely:

• Macronutrients
• Micronutrients.

Macro-nutrients

These are also referred to as major nutrients.

They are required by the plant in relatively large quantities for plant growth. They include;

• Carbon,
• Hydrogen,
• Oxygen,
• Nitrogen,
• Phosphorus,
• Potassium,
• Sulphur,
• Calcium
• Magnesium.

Nitrogen, phosphorus and potassium are referred to as fertilizer elements,

Calcium, magnesium and sulphur, are referred to as liming elements.

Role of Macronutrients in Plants

Nitrogen (NO_3,NH4⁺⁺)

Sources:

• Artificial fertilizers
• Organic matter
• Atmospheric fixation by lightning
• Nitrogen fixing bacteria.

Role of Nitrogen in Plants

• Vegetative growth
• Chlorophyll formation
• Build up of protoplasm.
• Improves leaf quality in leafy crops such as tea and cabbages.

Deficiency Symptoms

• Yellowing of the leaves/chlorosis.
• Stunted growth.
• Premature ripening.
• Premature shedding of the leaves.
• Light seeds.

Effect of Excess Nitrogen

• Scorching of the leaves.
• Delayed maturity.

Loss of Nitrogen From the Soil:

• Soil erosion.
• Leaching.
• Volatilization.
• Crop removal.
• Used by microorganisms.

Phosphorus (H₂ P0₄, HPO^2-₄ P₂O₅)

Sources:

• Organic manures
• Commercial fertilizers
• Phosphate rocks

Role of Phosphorus

Encourages fast growth of the roots.

Deficiency symptoms

• Growth of the plant is slow.
• Maturity is delayed.
• Leaves become grey, purple in colour.
• Yield of grains, fruits and seed is lowered.

Loss of Phosphorus From the Soil

• Soil erosion.
• Leaching
• Crop removal
• Fixation by iron and aluminium oxide.

Potasium (K⁺, K₂O)

Sources;
-Crop residue and organic manures.
-Commercial fertilizers
-Potassium bearing minerals e.g. feldspar and mica.

Role of Potassium in Plants

Potassium deficiency

• Increases plant vigour and disease resistance.
• Increases the size of grains and seeds.
• Reduces the ill-effects due to excess nitrogen.
• Prevents too rapid maturation due to phosphorus.

Deficiency Symptoms

• Plants have short joints and poor growth.
• Plants lodge before maturing.
• Leaves develop a burnt appearance on the margin.
• Leaves at the lower end of the plant become mottled, spotted or streaked.
• In maize, grains and grasses firing start
• Chlorosis at the tip of the leaf and proceeds from the edge usually leaving the midrib green

Loss of Potassium From the Soil

• Crop removal.
• Leaching.
• Soil erosion.
• Fixation in the soil.

Calcium (Ca²+)

Source:

• Crop residues and organic manures.
• Commercial fertilizers.
• weathering of soil minerals.
• Agricultural limes for example dolomite, limestone.

Role of Calcium in Plants

Improves the vigour and stiffness of straw.

Deficiency symptoms

• Young leaves remain closed.
• There are light green bands along the margins of the leaves.
• Leaves in the terminal bud become hooked in appearance there is a die-­back at the tip and along the margins.

Loss of Calcium

• Crop removal
• Leaching
• Soil erosion

Magnesium (Mg2+)

Sources:

• Crop residues and organic manures
• Commercial fertilizers
• Weathering of soil minerals.
• Agricultural limes.

Role of Magnesium in Plants

• Forms part of chlorophyll.
• Promotes the growth of the soil bacteria and enhances the nitrogen fixing power of the legumes.
• Activates the production and transport of carbohydrates and proteins in the growing plant.

Deficiency symptoms

• Loss in green colour which starts from the bottom leaves and gradually moves upwards.
• The veins remain green.
• Leaves curve upwards along the margins.
• Stalks become weak and the plant develops long branched roots.
• The leaves become streaked.

Sulphur (S0₄ ^2- ,SO₂)

Sources:

• Commercial fertilizers.
• Soil mineral containing sulphides
• Atmospheric sulphur from industries.
• Rain water

Role of Sulphur in Plants

• Formation and activation of coenzyme-
• Sulphur is a constituent of amino acids.
• Influence plant physiological processes.

Deficiency Symptoms

• Small plants/stunted growth.
• Poor nodulation in legumes.
• Light green to yellowish leaves/ chlorosis.
• Delayed maturity.

Micro-nutrients

Also referred to as trace or minor nutrients.

They are required in small quantities/traces.

They are essential for proper growth and development of plants.

They include;

• Iron,
• Manganese,
• Copper,
• Boron,
• Molybdenum
• Chlorine.

Role of Micronutrients and Their Deficiency Symptoms

Copper

• Role in oxidation-reduction reactions.
• Respiration and utilization of iron

Deficiency symptoms

• Yellowing of young leaves.

Iron

• Synthesis of proteins.
• Takes part in oxidation-­reduction reactions.

Deficiency symptoms

• Leaf chlorosis

Molybdenum

• Nitrogen transformation in plants.
• Metabolization of nitrates to amino acids and proteins

Deficiency symptoms

• Leaf curl and scathing.

Manganese – Same as molybdenum.

Zinc

• Formation of growth hormone.
• Reproduction process

Deficiency symptoms

• White bud formation.

Boron 

• Absorption of water.
• Translocation of sugar

Inorganic Fertilizers

These are chemically produced substances added to the soil to improve fertility.

Classification according to:

Nutrients contained

• Straight– contain only one macronutrient.
• Compound fertilizers – contain more than one macronutrient

Time of application

• Some applied when planting.
• Top dressing after crop emergence

Effects on the soil pH.

• Acidic fertilizers.
• Neutral fertilizers.
• Basic fertilizers.

Properties and Identification of Fertilizers

Nitrogenous Fertilizers

Characteristics

• Highly soluble in water.
• Highly mobile in the soil hence it is applied as a top dress.
• Easily leached because of the high solubility hence does not have residual effect on the soil.
• Has scorching effect on young crops during wet seasons.
• Easy to volatilize during hot season.
• They have a tendency to cake under moist conditions.
• They are hygroscopic hence should be stored in dry conditions.

Examples:

Sulphate of Ammonia (NH₄) 2 SO₄

Physical appearance:

• White crystals,
• Has acidic effect,
• Contains 20% Nitrogen.

Ammonium Sulphate Nitrate [(NH₄)₂ SO₄+ NH₄ NO₃]

• Colour: granules which appear yellow orange,
• Less acidic,
• Contains 26% Nitrogen.

Calcium Ammonium Nitrate (CAN)

Colour: greyish granules,

Urea

• Colour: small whitish granules
• Easily leached or volatilized,
• contains 45 – 46% Nitrogen.

Phosphate Fertilizers

• Have low solubility and immobile.
• Non-scorching.
• Has a high residual effect hence benefit the next season’s crop.
• Easy to store because they are not hygroscopic.

Examples;

Single super-phosphate

• Appearance: whitish, creamy white granules,
• Contains 20-21 % P₂O

Double super-phosphate

• Appearance: dark greyish granules,
• Contains 40-42% P₂O₅

Triple super-phosphate

• Appearance: small greyish granules,
• Contain 44-48% P2O5

Potassic Fertilizers

Characteristics:

• Has moderate scorching effect.
• Moderately soluble in water.
• Most Ugandan soils have sufficient potassium.

Examples;

Muriate of Potash (KCl)

• Contain 60 – 62% K₂O
• Slightly hygroscopic.
• Appearance: amorphous white.

Sulphate of Potash (50% K₂O)

Compound or Mixed Fertilizers

• These are fertilizers which supply 2 or more of the macronutrients.

Examples;

• Mono ammonium phosphate.
• Di-ammonium phosphate
• 20:20:20, 23:23:23

Advantages of application of compound fertilizers

• Saves time and money.
• Mixture gives improved storage properties and better handling.

Disadvantages of compound fertilizers application

• Expensive.
• Wasteful.
• Mixing may not be thorough.
• Incompatibility of the individual fertilizers.

Methods of fertilizer application

• Broadcasting – random scattering of the fertilizers on the ground.
• Placement method – application of fertilizers in the planting holes.
• Side dressing – fertilizer is placed at the side of the plant within the root zone, in bands or spot-rings.
• Foliar spraying – specially formulated fertilizer solution applied on the foliage in spray form.
• Drip method– applied through irrigation water.

Determination of Fertilizer Rates

Contents of fertilizers are expressed as fertilizer grade or fertilizer analysis.

• Fertilizer grade indicate the guaranteed minimum of the active ingredients (N, P₂O₅, K ₂O) in the mixture.
• It is expressed as a percentage on a weight to weight basis or percentage by weigh

Example

10:20:0 means for every 10kg of the mixture there are 10kg of nitrogen, 20kg of P₂O ₅ and 0kg of K₂O.

Example

A farmer was asked to apply fertilizers as follows:

• 60 kg/ha nitrogen (top dressing)
• 60 kg/ha P₂O₅ (in planting hole).
• 60 kg/ha K₂O.

1. How much sulphate of ammonia (20%) would be required per hectare?
2. How much double super-phosphate (40%) P₂O₅would be required per hectare?
3. How much muriate of potash (50% K₂O) would be required per hectare?

Answer/Solution

1. Sulphate of ammonia (SA) which gives 60kg/ha N

= 60

20x 100 =300kg SA

2. Double super phosphate (40% P₂O₅)which gives 60kg/ha P₂O₅

60

= 40x 100 =150kg DSP

3. Muriate of potash (60% K₂O) which gives 60kg/hK₂O

= 60 x 100=100kg muriate of potash

60

Example

A farmer was asked to apply fertilizers as follows:

• 200kg/ha of DSP (40% P₂O₅
• 150kg/ha of muriate of potash (60% K₂O)
• 150kg/ha of sulphate of ammonia (20% N)

1. How much P₂O₅ did the farmer apply per acre?
2. How much K₂O did the farmer apply per hectare?
3. How much N did the farmer apply per hectare?

Solution/Answer

1. P₂O₅ applied per hectare from 200kg of DSP

40

= 100x 200= 80kg/ha P₂O ₅

2. K₂O₅ applied per hectare from 150kg of muriate of potash

60

= 100×150=90kg/ha K₂O

3. N applied per hectare from 150kg/ha sulphate of ammonia

20

= 100 x 150= 30kg/ha N

Soil Sampling

Refers to obtaining of small quantity of soil that is representative in all aspects of the entire farm.

Soil Sampling Procedures

1. Clear the vegetation over the site.
2. Dig out soil at depths of 15-25cm.
3. Place the dug out soil in a clean container.
4. Mix thoroughly the soil in the container.
5. Take a sample and send it to National Agricultural Laboratory for analysis.
6. The container carrying the sample should be properly labeled as follows:

• Name of the farmer,
• Location,
• District
• Address of the farmer.

Sites to Avoid

• Dead furrows, ditches.
• Swamps
• Near manure heaps.
• Recently fertilized fields
• Ant hills.
• Under big trees.
• Near fence lines or foot paths.
• Do not put them in containers which are contaminated with fertilizers or other chemical containers.

Methods Of Soil Sampling

• Zigzag method
• Traverse method

Soil Testing

• Soil testing is the analyzing of the soil sample to determine certain qualities of the soil.

Importance of Soil testing

• To determine the value of the soil hence determine the crop to grow.
• To determine the nutrient content hence find out the type of fertilizer to apply.
• To determine whether it is necessary to modify the soil pH for a crop.

How Soil pH affects Crop Production

• Influences the physical and chemical properties of the soil.
• Affects the availability of nutrients.
• Influences the incidences of soil borne diseases.
• Determine the type of crop to be grown at a given area.

Methods of pH Testing.

• Universal indicator solution
• pH meter
• Know the course of action to be taken in the event of a disease and maintenance of good health.
• Know the prevalent diseases.
• Calculate the cost of treatment.

Marketing Records – show commodities sold, quantities and value of all the sales.

Labour Records – show labour utilization and labour costs.

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