Manure and Fertilizer Application in Agronomy

Plant nutrients are supplied through organic and inorganic sources. Organic manures include Farmyard Manure (FYM), compost, vermicompost, and green manures, which improve soil physical properties, water-holding capacity, and microbial activity in addition to supplying macro and micronutrients. Chemical fertilizers are concentrated inorganic salts manufactured industrially to supply essential plant nutrients rapidly.

• Farmyard Manure (FYM): A well-decomposed mixture of cattle dung, urine, and litter. On average, standard FYM contains 0.5% N, 0.2% P₂O₅, and 0.5% K₂O.
• Compost: Produced by aerobic or anaerobic decomposition of farm and urban organic wastes. Indore and Bangalore methods are classical aerobic and anaerobic composting techniques.
• Green Manuring: Incorporating green plant tissues (preferably legumes such as Crotalaria juncea or Sesbania aculeata) into the soil to enrich it with organic matter and biological nitrogen fixation.
• Nitrogenous Fertilizers: Include urea (46% N), ammonium sulphate (20.6% N), and calcium ammonium nitrate (25% N). Urea is the most widely used amide fertilizer in India.
• Phosphatic Fertilizers: Include Single Super Phosphate (SSP containing 16% P₂O₅, 12% S, and 19.5% Ca) and Di-ammonium Phosphate (DAP containing 18% N and 46% P₂O₅).
• Potassic Fertilizers: Muriate of Potash (MOP / Potassium chloride containing 60% K₂O) and Potassium sulphate (50% K₂O).

The method of fertilizer application determines nutrient use efficiency (NUE) and minimizes fixation, volatilization, leaching, and runoff losses. Selection depends on crop root system, soil type, mobility of nutrient in soil, and stage of crop growth.

• Broadcasting: Uniform scattering of fertilizers over the soil surface. Used mainly for basal dressing in dense-seeded crops like wheat and rice before sowing or transplanting.
• Placement: Placing fertilizers close to the seed or plant root zone in bands or pockets. Examples include plow-sole placement and deep placement of urea supergranules (USG) in flooded paddy fields.
• Band Placement: Applying fertilizer in bands either below or to the side of the seed row (starter solutions or row placement). Highly efficient for phosphatic fertilizers to reduce soil contact and fixation.
• Pellet Application: Pressing nitrogenous fertilizers (especially ammonium fertilizers) into mudballs or pellets and placing them deep in reduced soil zones (e.g., lowland rice) to prevent nitrification-denitrification losses.
• Foliar Application: Spraying dilute solutions of water-soluble fertilizers (e.g., 2% urea or 1% potassium nitrate) directly onto plant leaves to correct acute micronutrient or macronutrient deficiencies rapidly.
• Fertigation: Applying dissolved fertilizers through irrigation systems (drip or sprinkler irrigation). It ensures high fertilizer use efficiency, uniform distribution, and precise nutrient delivery directly to the active root zone.

Nutrient Use Efficiency represents the proportion of applied nutrients taken up and utilized by the crop. Low NUE causes economic loss and environmental degradation such as eutrophication and groundwater contamination.

• Nitrogen Losses: Nitrogen is lost primarily through ammonia volatilization (high pH soils), denitrification (anaerobic waterlogged soils), nitrate leaching (coarse-textured sandy soils), and surface runoff.
• Phosphorus Fixation: Phosphorus reacts with iron and aluminum oxides in acidic soils, and with calcium carbonates in alkaline soils, forming insoluble complexes that reduce availability.
• Potassium Fixation: Potassium can become fixed within the lattice structure of expanding 2:1 clay minerals such as illite and vermiculite.
• Enhancing NUE: Use of nitrification inhibitors (e.g., nitrapyrin, neem-coated urea), urease inhibitors, split applications of nitrogen, balanced fertilization, and localized band placement.

Agronomists and farm managers must calculate exact commercial fertilizer quantities based on recommended nutrient doses (N:P₂O₅:K₂O) and fertilizer grades.

• Formula for Fertilizer Quantity:
  Quantity of Fertilizer (kg) = [Recommended Nutrient Dose (kg/ha) × 100] / Percentage of Nutrient in Fertilizer
• Urea Calculation Example: To apply 120 kg N per hectare using Urea (46% N):
  Quantity of Urea = (120 × 100) / 46 = 260.87 kg/ha
• DAP and Urea Combination for N and P: When applying DAP (18% N, 46% P₂O₅), the nitrogen supplied by DAP must be subtracted from the total recommended nitrogen before calculating the remaining urea requirement.
• Sulphur and Calcium in SSP: Single Super Phosphate provides secondary nutrients (12% sulphur and 19.5% calcium), making it superior to DAP in sulphur-deficient soils.

Integrated Nutrient Management is the maintenance or adjustment of soil fertility and plant nutrient supply at an optimum level for sustaining crop productivity through the optimized use of all available organic, inorganic, and biological sources in an integrated manner.

• Objectives: To maintain long-term soil health, maximize economic yield, minimize environmental pollution, and reduce dependence on expensive chemical fertilizers.
• Components: Chemical fertilizers, organic manures, crop residues, green manures, and biofertilizers (e.g., Rhizobium, Azotobacter, Azospirillum, and Phosphate Solubilizing Bacteria like Bacillus megaterium).
• Biofertilizers: Living microorganisms that colonize the rhizosphere or interior of plants and promote growth by increasing the supply or availability of primary nutrients to the host plant.