Unit 1: Fundamentals of Organic Manures and Fertilizers

I. Orientation

Organic manures and fertilizers are materials added to soil to supply plant nutrients, improve soil properties, or both. Organic manures originate mainly from plant, animal, or microbial residues, whereas fertilizers are nutrient-bearing materials manufactured, processed, or naturally occurring in a form that supplies nutrients to plants. Their value depends on nutrient content, nutrient-release pattern, moisture, maturity, reaction in soil, and uniformity.

• Plant nutrients: Essential elements include primary macronutrients nitrogen (N), phosphorus (P), and potassium (K); secondary nutrients calcium (Ca), magnesium (Mg), and sulfur (S); and micronutrients such as zinc (Zn), iron (Fe), boron (B), copper (Cu), manganese (Mn), molybdenum (Mo), chlorine (Cl), and nickel (Ni).
• Nutrient availability: A nutrient may be present in total form but unavailable until it dissolves, mineralizes, or changes into a plant-absorbable form.
• Organic matter: Carbon-containing material in residues, compost, humus, and living organisms that affects aggregation, water retention, cation exchange, and biological activity.
• Nutrient analysis: Materials are commonly assessed for moisture, organic carbon, total nitrogen, available phosphorus, soluble potassium, pH, electrical conductivity, and contaminant levels.
• Guaranteed analysis: Fertilizer nutrients are generally expressed as percentages by mass, commonly as total nitrogen (N), available phosphate as phosphorus pentoxide equivalent (P₂O₅), and soluble potash as potassium oxide equivalent (K₂O), depending on the applicable standard.
• Agronomic efficiency: The practical value of an input depends on the amount recovered by the crop, the timing of release, placement, soil conditions, and crop requirement.
• Testing basis: Results must identify whether values are reported on an “as received” basis, including moisture, or on a dry-weight basis.

II. Organic Manures — Biological Sources of Nutrients and Soil-Conditioning Matter

Organic manures are decomposed or partially decomposed materials of biological origin applied to soil. Their characteristic feature is that nutrient supply is combined with the addition of organic matter, and release usually depends on microbial decomposition.

A. Introduction to organic manures and fertilizers

This introduction establishes the distinction, relationship, and practical use of organic manures and fertilizers in soil fertility management.

• Organic manure definition: Organic manure is a material such as farmyard manure, compost, green manure, or poultry manure derived from plant or animal residues and applied to improve fertility.
  • Farmyard manure: A mixture of animal dung, urine-absorbed bedding, and litter; its analysis varies with animal species, storage, and handling.
  • Compost: A stabilized product formed by controlled aerobic decomposition of residues.
  • Green manure: A crop, commonly a legume, grown and incorporated while still green to add readily decomposable biomass and nitrogen.
• Organic fertilizer definition: An organic fertilizer is an organic-derived material marketed primarily for its plant-nutrient content, such as oil cakes, bone meal, blood meal, or suitably processed manure.
• Main distinction: Manure is usually bulky and relatively dilute, while fertilizer is generally more concentrated and specified by nutrient analysis. For example, a material containing 1% N supplies 10 kg N per tonne before considering losses or availability.
• Nutrient forms: Organic manures contain nutrients in organic compounds and mineral salts.
  • Organic nitrogen: Present in proteins, amino compounds, and microbial residues; it must generally be mineralized to ammonium (NH₄⁺) and nitrate (NO₃⁻).
  • Mineral nutrients: Soluble ammonium, nitrate, phosphate, or potassium may be immediately available but can also be lost through leaching, volatilization, fixation, or runoff.
• Decomposition: Soil microorganisms break down carbon compounds and release nutrients. A simplified representation of nitrogen mineralization is:

Organic-N → NH4+ → NO3-

Here, Organic-N is nitrogen in organic compounds, NH₄⁺ is ammonium, and NO₃⁻ is nitrate. Nitrification converts ammonium to nitrate under suitable aerobic conditions.

• Carbon-to-nitrogen ratio: The C:N ratio compares organic carbon (C) with total nitrogen (N) in a material.

C:N ratio = mass of organic carbon / mass of total nitrogen

A high C:N material, such as cereal straw, may temporarily immobilize soil nitrogen because microbes require additional nitrogen for decomposition. A lower C:N material, such as well-managed poultry manure, generally decomposes more rapidly.

• Maturity: Mature compost is relatively stable, has reduced phytotoxic compounds, and produces less heat and odor than fresh material.
  • Maturity indicators: Dark, crumbly appearance; earthy odor; temperature close to ambient; and absence of recognizable, actively decomposing feedstock.
  • Testing relevance: Immature manure may show high respiration, unstable ammonium, excess soluble salts, or phytotoxicity despite having measurable nutrients.
• Moisture effect: Water dilutes nutrient concentration on an as-received basis. If a wet manure contains 60% moisture, only 40% is dry matter.

Dry matter (%) = 100 - moisture (%)

For 1,000 kg of manure at 60% moisture, dry matter is 400 kg. Nutrient comparisons must therefore state the reporting basis.

• Organic carbon: Organic carbon contributes to soil organic matter and is often estimated from laboratory measurements. A commonly used approximation is:

Organic matter (%) ≈ organic carbon (%) × 1.724

The factor 1.724 assumes organic matter contains about 58% carbon; actual conversion may vary with material and method.

• Soil benefits: Regular use can improve aggregation, infiltration, aeration, water-holding capacity, microbial activity, and cation exchange capacity. These effects are especially important in soils low in organic matter.
• Nutrient limitations: Nutrient ratios in manure may not match crop demand. Applying enough manure to meet phosphorus needs may supply excess nitrogen, while applying enough nitrogen may add excessive phosphorus, potassium, salts, or metals.
• Potential losses: Nitrogen may be lost as ammonia (NH₃), nitrate may leach, and phosphorus may move with eroded soil or runoff.
  • Ammonia loss: Greater when ammonium-rich manure is left on the surface, exposed to warmth and wind, or placed on alkaline soil.
  • Storage loss: Rainfall can leach soluble nutrients, while anaerobic storage can alter odor, nitrogen form, and organic matter quality.
• Sanitation: Raw manure may contain pathogens, weed seeds, or residues of veterinary medicines. Composting, adequate storage, and appropriate application intervals reduce but do not automatically eliminate every hazard.
• Heavy metals and contaminants: Repeated application of industrially contaminated compost or sewage-derived products can increase elements such as cadmium (Cd), lead (Pb), chromium (Cr), or arsenic (As). Testing is necessary where contamination is possible.
• Sampling principle: A manure pile is heterogeneous. Representative sampling requires collecting several small portions from different locations, mixing them thoroughly, and submitting a composite sample in a clean, labeled container.
• Laboratory measurements: Important tests include:
  • Moisture and dry matter: Establish the concentration basis.
  • pH: Indicates acidity or alkalinity and influences nutrient reactions.
  • Electrical conductivity (EC): Indicates soluble salt concentration; excessive EC can impair germination and root water uptake.
  • Total N: Measures all nitrogen forms included by the method, not necessarily immediately available nitrogen.
  • Available P and soluble K: Estimate nutrient fractions more closely related to short-term plant supply.
  • Organic carbon and organic matter: Indicate carbon contribution and degree of stabilization.
  • C:N ratio: Helps interpret decomposition and possible nitrogen immobilization.
• Application calculation: The mass of material required for a target nutrient rate is calculated from nutrient percentage.

Material required (kg/ha) =
target nutrient rate (kg/ha) ÷ nutrient fraction

For 100 kg N/ha from manure containing 2% total N, the theoretical quantity is:

100 ÷ 0.02 = 5,000 kg/ha

This is 5 tonnes/ha before accounting for mineralization efficiency, field losses, and residual effects.

• Available nutrient correction: If only a fraction of total nutrient becomes available during the crop season, the effective supply is lower.

Available nutrient =
total nutrient applied × availability fraction

If 100 kg total N/ha is applied and the estimated first-season availability is 30%, available N is 30 kg/ha. The fraction depends on material type, maturity, incorporation, soil, weather, and crop.

• Integrated use: Organic manures and mineral fertilizers can complement each other. Manure supplies carbon and slow-release nutrients, while fertilizer can correct a specific nutrient deficiency accurately and rapidly.
• Practical limitation: Bulky materials require greater transport, storage, labor, and spreading capacity. Their analysis should guide application rather than assuming a fixed nutrient composition.
• Environmental principle: Application should be based on crop nutrient demand, soil-test results, manure analysis, and risk of runoff or leaching. More material does not necessarily mean greater fertility.
• Quality interpretation: A high total nutrient value is not sufficient evidence of high agronomic quality. A useful interpretation combines concentration, availability, maturity, contaminant status, moisture, and uniformity.
• Fertilizer comparison: Inorganic fertilizers such as urea, ammonium sulfate, single superphosphate, or muriate of potash usually have more predictable nutrient concentrations. Organic fertilizers may provide slower release and broader soil benefits but often show greater batch-to-batch variation.
• Classification by nutrient supply: Materials may be grouped as:
  • Nitrogenous: Blood meal, oil cakes, poultry manure, and legume residues.
  • Phosphatic: Bone meal and certain processed organic residues.
  • Potassic: Plant ash and selected crop residues, subject to soluble salt content.
  • Mixed organic amendments: Compost and farmyard manure, which supply several nutrients at lower concentrations.
• Nutrient balance: Nitrogen, phosphorus, and potassium should be considered separately because one material rarely supplies them in the exact crop-required ratio.
• Testing conclusion: Reliable manure and fertilizer testing connects a laboratory result with a field decision: what material is present, how much nutrient it contains, how much is likely to become available, and whether its use creates soil, crop, health, or environmental risks.