Soil Fertility II (Inorganic Fertilizers) - Agriculture Form 2 Notes

• Introduction
• Essential Elements
  • Macro-nutrients
  • Micro-nutrients
• Inorganic Fertilizers
  • Classification of Fertilizers
  • Properties and Identification of Fertilizers
  • Methods of Fertilizer Application
  • Determination of Fertilizer Rates
• Soil Sampling
  • Soil Sampling Procedures
  • Sites to Avoid
  • Methods of Soil Sampling:
• Soil Testing
  • Importance of Soil Testing
  • How Soil pH Affects Crop Production
  • Methods of pH Testing

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Introduction

• 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.

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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 large quantities.
• They include;
  • carbon,
  • hydrogen,
  • oxygen,
  • nitrogen,
  • phophorus,
  • 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 (H₂P0₄, HPO₄^2-, P₂O₅)

Sources:

• Organic manures
• Commercial fertilizers
• Phosphate rocks

Role of Phosphorus

• Encourages fast growth of the roots.
• Improves the quality of the plant.
• Hastens maturity of the crops.
• Influences cell division.
• Stimulates nodule formation in legumes.

 

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

• 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 starts 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+)

Sources:

• 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.
• Neutralizes the poisonous secretions of the plants.
• Helps in grain and seed formation.
• Improves the soil structure.
• Promotes bacterial activity in the soil.
• Corrects the soil acidity.

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-A.
• 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

 

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Inorganic Fertilizers

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

Classification of Fertilizers

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% N.
• Ammonium Sulphate Nitrate [(NH₄)₂SO₄ + NH₄NO₃]
  • Colour: granules which appear yellow orange,
  • less acidic,
  • contains 26% N.
• Calcium Ammonium Nitrate (CAN)
  • Colour: greyish granules,
  • neutral in nature,
  • contains 21 % N.
• Urea
  • Colour: small whitish granules
  • Easily leached or volatilized,
  • contains 45- 46%N.

 

Phosphate Fertilizers

• Has 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 % P₂O₅
• 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 Kenyan 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, K₂O) 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 P₂O₅ and 0 kg of K₂O.

Example 1

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.

How much sulphate of ammonia (20%) would be required per hectare?

How much double super-phosphate (40%) P₂O₅would be required per hectare?

How much muriate of potash (50% K₂O) would be required per hectare?

Answer/Solution

• Sulphate of ammonia (SA) which gives 60kg/ha N
  = 60 × 100 = 300kg SA
     20
• Double super phosphate (40%  P₂O₅)which gives 60kg/ha P₂O₅
  = 60 × 100 =150kg DSP
     40
• Muriate of potash (60% K₂O) which gives 60kg/hK₂O
  = 60 × 100=100kg muriate of potash
     60

Example 2

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)

How much P₂O₅ did the farmer apply per acre?

How much K₂O did the farmer apply per hectare?

How much N did the farmer apply per hectare?

Solution/Answer

• P₂O₅  applied per hectare from 200kg of DSP
  = 40  × 200 = 80kg/ha P₂O ₅
    100
• K₂O₅   applied per hectare from 150kg of muriate of potash
  =60   × 150 =90kg/ha    K₂O
    100
• N  applied per hectare from 150kg/ha sulphate of ammonia
  = 20
    100 x 150= 30kg/ha N

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Soil Sampling

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

Soil Sampling Procedures

• Clear the vegetation over the site.
• Dig out soil at depths of 15-25cm.
• Place the dug out soil in a clean container.
• Mix thoroughly the soil in the container.
• Take a sample and send it to National Agricultural Laboratory for analysis.
• 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

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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.

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