Organic Manures: Meaning, Classification and Reactions

After reading this article you will learn about:- 1. Meaning of Organic Manures 2. Classification of Organic Manures 3. Characteristics 4. Factors Affecting the Composition 5. Reactions in Soil.

Meaning of Organic Manures:

Manures may be defined as materials which are organic in origin, bulky and concentrated in nature and capable of supplying plant nutrients and improving soil physical environment having no definite chemical composition with low analytical value produced from animal, plant and other organic wastes and by products.

Organic manures are included well rotten farm yard manure (FYM), compost, green manures etc. Generally farm yard manures and composts are the decomposed products of agricultural by-products (animals and crops). Whereas green manures may be defined as materials which are un-decomposed green plant tissues susceptible to decomposition in the soil after incorporation.

Classification of Organic Manures:

We know that organic manures are of different types i.e. mainly bulky and concentrated in nature.

A simple classification scheme of organic manures is being presented below:

A. Bulky Organic Manures:

Bulky organic manures generally contain fewer amounts of plant nutrients as compared to concentrated organic manures. The concentrated organic manures are mainly derived from raw materials of animal or plant origin.

The amount of nutrients content varies with the nature and kind of oil cakes. No definite composition of NPK and other micro-nutrients can be given. However, all oil cakes either edible or non-edible contains differential amount of N, P and K etc.

1. Farm Yard Manure (FYM):

The term farm yard manure refers to the well-decomposed mixture of dung, urine, farm litter (bedding material) and left over or used up materials from roughages or fodder fed to the cattle. The FYM collected daily from the cattle shed consisting of raw dung and part of the urine absorbed in the refuse.

Newly collected and stored FYM is fresh as against well decomposed FYM which has been stored for a sufficient period of time to allow its decomposition to completion.

Farm yard manure consists of two components—solid phase, dung and liquid phase, urine. On an average, the animals give out three parts by weight of dung and one part by weight of urine. However, this ratio of dung and urine varies with the kind of animals.

Horses, cows and bullocks give out more dung and less urine than that of sheep, goats and pigs. An average nutrient and moisture content of different kinds of animals is given in Table 24.1.

Urine of all animals contains more amounts of N and P as compared to dung. The urine of cows, bullocks and horses contain practically nil or traces of P. Similarly, the dung of all animals except pig is low in P.

2. Green Manuring:

Green manuring can be defined as a practice of ploughing or turning into the soil un-decomposed green plant tissues for the purpose of improving soil physical chemical and biological environments.

Kinds of Green Manuring:

The practice of green manuring is performed in different ways according to suitable soil and climatic conditions of particular area. Broadly the practice of green manuring in India can be divided into two types—Green manuring in situ and Green manuring by collecting green leaves and tender twigs from some other places.

i. Green Manuring in Situ:

It can be defined as a system by which green manure crops are grown and incorporated into the soil of the same field that is to be green manured, either as a pure crop or an intercrop with the main crop. Common green manure crops in this system sun hemp (crotolariajuncea) dhaincha (sesbaniaaculeata and sesbaniarostrata), guar (cyamopsistetragonoloba), etc.

ii. Green Manuring through Collection of green Plant Tissues from other Places:

It refers to turning into the soil green leaves and tender green twigs collected from outside the field to be green manured. The common green manure crops, are Glyricidia (Glyricidiamaculata), Karanja (Pongamiapinnata) etc.

Advantages of Green Manuring:

There are various advantages of green manuring in relation to soil fertility which are as follows:

(i) It increases the organic matter regime of the soil and there by modifies soil physical, chemical and biological environments. In fact, this stimulates the activity of soil micro-organisms.

(ii) The green manure crops help for returning the different plant nutrients to the surface soil layer from the sub-surface soil layer.

(iii) It improves the soil structure, aeration status, permeability and infiltration capacity of soil.

(iv) It reduces the soil loss caused by run-off and erosion.

(v) Due to green manuring the nutrient regimes can be improved and restored otherwise be lost by leaching.

(vi) Green manure crops have some residual effect in relation to supply of different plant nutrient and thereby it helps for the better growth to the next crop.

Disadvantages of Green Manuring:

The adoption of green manuring in a improper way and application or incorporation of green manure crops without proper soil and water management leads to the following deleterious effects:

(i) Under rainfed conditions where rainfall is limiting, the proper decomposition of the green manure crops may not take place and thereby benefits of green manuring may not be achieved satisfactorily.

(ii) Sometimes the cost of green manuring crops may be more than that of chemical nitrogenous fertilizers and in that situation green manuring may not be economical.

(iii) There is a change of occurring diseases and insects in the field crops.

(iv) Due to decomposition of green manure crops, various toxic substances like organic acids e.g. butyric acid, propionic acid etc. and toxic gases like methane (CH₄) and others etc. are liberated which affects the root growth of growing plants and thereby affect the growth and yield through inhibition of nutrient absorption by the plant.

B. Concentrated Organic Manures:

Concentrated organic manure may be defined as a material of organic origin derived from raw materials of animal or plant, without bulky in nature having no definite composition of plant nutrients.

Some most common such organic manures are oil cakes—edible to cattle (e.g. mustard oil cake, groundnut oil cake, till oil cake etc.) and non-edible to cattle (e.g. neem oil cake, mahua oil cakes etc.); blood-meal, fish manure, bone meal etc.

Since the sources of availability of concentrated organic manures are different and hence they do not contain definite amount of nutrient elements. However, concentrated organic manures are easy to handle and have relatively higher plant nutritive value as compared to bulky organic manures. Besides these, it is quick-acting organic manure when incorporated into the soil.

Characteristics of Organic Manures:

All these manures are bulky in nature as well as concentrated nature and supply:

(i) Plant nutrients in small quantities and

(ii) Organic matter in large amounts.

Since it contains two components (plant nutrients and organic matter), when it is applied into the soil it will act as follows:

1. Organic manures supply primary, secondary and micro-nutrients to plants which are liberated in an available forms during the process of mineralization carried out by different micro-organisms.

2. Organic manures also supply organic matter to the soil and hence improve the physical condition of the soil like soil structure, aeration, water holding capacity etc.

3. It also stimulates the activity of different soil micro-organisms through the supply of energy.

4. It improves the buffering and exchange capacities of soil and also influences the solubility of soil minerals as well as mineral nutrients in soil.

5. It also forms chelates which also help for the nutrition of plants.

6. It also regulates the thermal regimes of the soil.

Factors Affecting the Composition of Organic Manures:

There are various factors which can affect the composition as follows:

(i) Origin of Manure:

Sheep and poultry manures are somewhat richer in plant nutrients than cow, horse and pig manures.

(ii) Types of Food Consumed by Animals:

This is one of the most important factors that determine the manure quality. As for example, the richer the food in proteins, the richer will be the manure in nitrogen.

(iii) Age and Condition of Animals:

The manure of young animals is not so rich like that of matured animals because young animals retain more nutrients for their growth than that of old or matured ones.

(iv) Species of Animals:

The composition of nutrient contents varies with the ruminant and non-ruminant animals.

(v) Nature and Amount of Litter:

The composition of FYM varies with the nature and amount of litter used for animals (e.g. paddy straws, wheat straws etc.)

(vi) Function of the Animal:

Animals producing milk and wool absorb large amount of nutrients from their food than that of working draft animals. Therefore, manure from bullocks generally contains more nutrients as compared to milch cows.

(vii) Handling and Storage of Manures:

Loss of potash occurs if any drainage is allowed to escape from the manure heap. Therefore, improper handling and storage leads to losses of plant nutrients from the manures.

Reactions of Organic Manures in Soils:

It is found that both bulky and concentrated organic manures contain some amount of plant nutrients including macro- and micro-nutrients, of which organic nitrogen content is likely to be dominant. The organic forms of soil nitrogen occur as consolidated amino acids, proteins, amino sugars etc.

When Organic manures like FYM, composts, oil cakes, green manures etc. are added to the soil, the microbial attack to these materials takes place and results complete disappearance of the organic protein with the remainder of the nitrogen being changed into inorganic form of nitrogen through the process of mineralization.

The released ammonium (NH₄⁺) is subject to following changes:

1. It may be converted to nitrites and nitrates through the process of nitrification carried out by micro-organisms,

2. It may be absorbed directly by the plants.

3. It may be utilized by hetero-trophic organisms in further decomposing organic carbon residues.

4. It may be fixed in the lattice of certain expanding-type clay minerals.

5. It could be slowly released back to the atmosphere as elemental nitrogen.

Mineralization and immobilization of nitrogen or any other nutrient elements occur continuously in microbial metabolism and the magnitude and direction of the net effect are greatly influenced by the nature and amount of organic manures added.

Normally organic manures are applied to the soil as a source of fertilizer nitrogen should contain about 1.5 to 2.0 per cent of the dry weight of the manures in order to meet the needs of the soil micro­organisms, otherwise little or no nitrogen will be released for the use of plants.

The carbon nitrogen ratio (C: N ratio) in the organic manures remaining in the soil after consuming by the soil micro-organisms is approximately 10: 1.

Therefore, different organic manures containing variety of organically bound nutrients like P, S and other micro-nutrients etc. are subject to transformation in soils similar to that of mineralization and immobilization processes of nitrogen and releases inorganic forms of nutrients in soils which become available to plants.