Agriculture Form 2 – Soil Fertility II (Inorganic Fertilizers)

Introduction

• Plant nutrients occur in the soil in the form of soluble substances.
• These substances are absorbed by plants in varying quantities depending on their roles in plant tissues.

Essential Elements

• These are nutrients required by plants for various physiological and biochemical functions.
• They are divided into two broad categories:
  • Macronutrients
  • Micronutrients

Macronutrients

• Also known as major nutrients.
• Required by plants in large quantities for growth and development.

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 (NO₃^–, NH₄⁺)

Sources:

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

Role of Nitrogen in Plants

• Promotes vegetative growth
• Essential for chlorophyll formation
• Contributes to the build-up of protoplasm
• Improves leaf quality in leafy crops such as tea and cabbages

Deficiency Symptoms

• Yellowing of leaves (chlorosis)
• Stunted growth
• Premature ripening
• Premature shedding of leaves
• Light seeds

Effect of Excess Nitrogen

• Scorching of leaves
• Delayed maturity

Loss of Nitrogen From the Soil

• Soil erosion
• Leaching
• Volatilization
• Crop removal
• Utilization by microorganisms

Phosphorus (H₂PO₄^–, HPO₄^2-, P₂O₅)

Sources:

• Organic manures
• Commercial fertilizers
• Phosphate rocks

Role of Phosphorus

• Encourages rapid root growth
• Improves overall plant quality
• Accelerates crop maturity
• Influences cell division
• Stimulates nodule formation in legumes

Deficiency Symptoms

• Slow plant growth
• Delayed maturity
• Leaves turn grey or purple
• Reduced yield of grains, fruits, and seeds

Loss of Phosphorus From the Soil

• Soil erosion
• Leaching
• Crop removal
• Fixation by iron and aluminium oxides

Potassium (K⁺, K₂O)

Sources:

• Crop residues and organic manures
• Commercial fertilizers
• Potassium-bearing minerals such as feldspar and mica

Role of Potassium in Plants

• Increases plant vigor and disease resistance
• Enhances the size of grains and seeds
• Reduces negative effects of excess nitrogen
• Prevents overly rapid maturation caused by phosphorus

Deficiency Symptoms

• Short joints and poor growth
• Plants lodge before maturity
• Leaves develop burnt margins
• Lower leaves become mottled, spotted, or streaked
• In maize, grains, and grasses, firing starts at the leaf tip and spreads from the edge, 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 such as dolomite and limestone

Role of Calcium in Plants

• Improves vigor and stiffness of straw
• Neutralizes poisonous secretions within plants
• Assists in grain and seed formation
• Enhances soil structure
• Promotes bacterial activity in soil
• Corrects soil acidity

Deficiency Symptoms

• Young leaves remain closed
• Light green bands along leaf margins
• Terminal bud leaves become hooked; die-back occurs at tips and 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

• Constituent of chlorophyll
• Promotes growth of soil bacteria and enhances nitrogen fixation in legumes
• Activates production and transport of carbohydrates and proteins in growing plants

Deficiency Symptoms

• Loss of green color starting from lower leaves and moving upward
• Veins remain green while leaf tissue yellows
• Leaves curve upward along margins
• Stalks weaken and plants develop long branched roots
• Leaves become streaked

Sulphur (SO₄^2-, SO₂)

Sources:

• Commercial fertilizers
• Soil minerals containing sulphides
• Atmospheric sulphur from industrial emissions
• Rainwater

Role of Sulphur in Plants

• Formation and activation of coenzyme-A
• Constituent of amino acids
• Influences various physiological processes

Deficiency Symptoms

• Stunted growth and small plants
• Poor nodulation in legumes
• Light green to yellowish leaves (chlorosis)
• Delayed maturity

Micronutrients

• Also called trace or minor nutrients
• Required in very small quantities
• Essential for proper growth and development of plants

They include:

• Iron
• Manganese
• Copper
• Boron
• Molybdenum
• Chlorine

Role of Micronutrients and Their Deficiency Symptoms

• Copper
• Involved in oxidation-reduction reactions
• Important for respiration and iron utilization
• Deficiency symptoms: yellowing of young leaves
• Iron
• Essential for protein synthesis
• Participates in oxidation-reduction reactions
• Deficiency symptoms: leaf chlorosis
• Molybdenum
• Involved in nitrogen transformation in plants
• Helps metabolize nitrates into amino acids and proteins
• Deficiency symptoms: leaf curl and scorching
• Manganese – similar role as molybdenum
• Zinc
• Involved in formation of growth hormones
• Important for reproduction processes
• Deficiency symptoms: white bud formation
• Boron
• Facilitates water absorption
• Assists in sugar translocation

Inorganic Fertilizers

• Chemically produced substances added to 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 at planting
• Top dressing after crop emergence
  • Effects on soil pH
  • Acidic fertilizers
  • Neutral fertilizers
  • Basic fertilizers

Properties and Identification of Fertilizers

Nitrogenous Fertilizers

Characteristics

• Highly soluble in water
• Highly mobile in soil; usually applied as top dress
• Easily leached due to high solubility; no residual effect
• Can scorch young crops during wet seasons
• Volatilizes easily during hot seasons
• Tends to cake under moist conditions
• Hygroscopic; should be stored in dry conditions

Examples:

• Sulphate of Ammonia (NH₄)₂SO₄

Physical appearance:

• White crystals
• Acidic effect
• Contains 20% nitrogen

• Ammonium Sulphate Nitrate [(NH₄)₂SO₄ + NH₄NO₃]
• Granules appear yellow-orange
• Less acidic
• Contains 26% nitrogen

• Calcium Ammonium Nitrate (CAN)
• Greyish granules
• Neutral in nature
• Contains 21% nitrogen

• Urea
• Small whitish granules
• Easily leached or volatilized
• Contains 45-46% nitrogen

Phosphate Fertilizers

• Low solubility and immobile in soil
• Non-scorching
• High residual effect; benefits subsequent season’s crop
• Easy to store; not hygroscopic

Examples:

• Single super-phosphate
• Whitish, creamy white granules
• Contains 20-21% P₂O₅
• Double super-phosphate
• Dark greyish granules
• Contains 40-42% P₂O₅
• Triple super-phosphate
• Small greyish granules
• Contains 44-48% P₂O₅

Potassic Fertilizers

Characteristics:

• Moderate scorching effect
• Moderately soluble in water
• Most Kenyan soils have sufficient potassium

Examples:

• Muriate of Potash (KCl)
• Contains 60-62% K₂O
• Slightly hygroscopic
• Appearance: amorphous white
• Sulphate of Potash (50% K₂O)

Compound or Mixed Fertilizers

• Fertilizers supplying two or more macronutrients

Examples:

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

Advantages of Compound Fertilizers

• Saves time and money
• Mixtures have improved storage properties and better handling

Disadvantages of Compound Fertilizers

• Expensive
• Wasteful if not properly applied
• Mixing may not be thorough
• Possible incompatibility of individual fertilizers

Methods of Fertilizer Application

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

Determination of Fertilizer Rates

Fertilizer contents are expressed as fertilizer grade or fertilizer analysis.

• Fertilizer grade indicates the guaranteed minimum of active ingredients (N, P₂O₅, K₂O) in the mixture.
• Expressed as a percentage on a weight-to-weight basis.

Example: 10:20:0 means that in every 100 kg of the mixture, there are 10 kg of nitrogen, 20 kg of P₂O₅, and 0 kg 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

Calculate the amount of sulphate of ammonia (20% N) required per hectare.

Calculate the amount of double super-phosphate (40% P₂O₅) required per hectare.

Calculate the amount of muriate of potash (50% K₂O) required per hectare.

Answer/Solution

• Sulphate of ammonia (SA) to supply 60 kg/ha N:

  = (60 / 20) × 100 = 300 kg SA

• Double super phosphate (DSP) to supply 60 kg/ha P₂O₅:

  = (60 / 40) × 100 = 150 kg DSP

• Muriate of potash to supply 60 kg/ha K₂O:

  = (60 / 50) × 100 = 120 kg muriate of potash

Example

A farmer applied the following fertilizers:

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

Calculate the amounts of P₂O₅, K₂O, and N applied per hectare.

Solution/Answer

• P₂O₅ applied from 200 kg DSP:

  = (40 / 100) × 200 = 80 kg/ha P₂O₅

• K₂O applied from 150 kg muriate of potash:

  = (60 / 100) × 150 = 90 kg/ha K₂O

• N applied from 150 kg sulphate of ammonia:

  = (20 / 100) × 150 = 30 kg/ha N

Soil Sampling

• Obtaining a small quantity of soil that is representative of the entire farm.

Soil Sampling Procedures

• Clear vegetation over the sampling site.
• Dig soil at depths of 15-25 cm.
• Place the dug soil in a clean container.
• Mix the soil thoroughly in the container.
• Take a sample and send it to the National Agricultural Laboratory for analysis.
• Label the container properly with:
  • Farmer’s name
  • Location
  • District
  • Farmer’s address

Sites to Avoid

• Dead furrows and ditches
• Swamps
• Near manure heaps
• Recently fertilized fields
• Ant hills
• Under large trees
• Near fence lines or footpaths
• Containers contaminated with fertilizers or chemicals should not be used

Methods of Soil Sampling

• Zigzag method
• Traverse method

Soil Testing

• Analysis of soil samples to determine soil qualities and nutrient content.

Importance of Soil Testing

• Determines soil fertility and suitable crops to grow
• Identifies nutrient content to guide fertilizer application
• Determines if soil pH modification is necessary for crop growth

How Soil pH Affects Crop Production

• Influences physical and chemical soil properties
• Affects nutrient availability
• Impacts incidence of soil-borne diseases
• Determines suitable crops for the area

Methods of pH Testing

• Universal indicator solution
• pH meter