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

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  • 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.


  • 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 (NO3, NH4+)


  • 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, HPO42-, P2O5)


  • 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)


  • 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+)


  • 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+)


  • 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)


  • 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.


  • 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 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


  • 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.


  • Sulphate of Ammonia (NH4) 2 SO4·
    • Physical appearance: white crystals,
    • Has acidic effect,
    • Contains 20% N.
  • Ammonium Sulphate Nitrate [(NH4)2SO+ 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.


  • 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


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


  • 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.


  • 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?


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

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?


  • P2O5  applied per hectare from 200kg of DSP
    = 40  × 200 = 80kg/ha P25
  • K2O5   applied per hectare from 150kg of muriate of potash
    =60   × 150 =90kg/ha    K2O
  • 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

  • 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

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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