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. 

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 (NO3,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, HPO2-4 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+) 

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. 

• 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 

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% N. 

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

- 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, 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.