By Dr. S.M. ALAM, NIA, Tandojam.
Nov 18 - 24, 2002



The world's population is increasing tremendously at a time when food production continues to decline. Drought, salinization, unavailability of agricultural inputs and inappropriate utilization of available resources are the main reasons that add to food shortage at the present time. Similar to this, the population of the country is also increasing day by day and at present, it stands at about 150 million people. This situation has caused to think over for the increasing food productivity.

There are various ways to increase agricultural productivity of the country. One of them is through green manuring of the soil. In agricultural productivity of a country, soil organic matter is often treated as one single substance, but in reality there are many different types of organic matters or humus which perform different functions in the improvement of soil and ultimately the crop growth, and these are: i) fresh and incompletely decomposed plant and animal residues "active organic matter"; ii) products of advanced decomposition of organic residues and products, which are re-synthesized thoroughly by microorganisms present in the soil (protein like substances, organic acid, carbohydrates, gum, waxes, fats, tannins, lignins, etc; iii) high molecular humic substances, fulvic acids, humic acids and humin, which are relatively resistant to further biological decomposition.

Organi.c matter is that which contains a considerable amount of plant material, which undergoes into active decomposition in the process of being converted into microbial material and humic substances. Humus produced in this way, is an oxidized part of organic matter. It is the presence of functional groups such as amino, hydroxyl and carboxyl humic compounds, which give rise to a high cation exchange capacity and the ability to form complex with clays, thus contributing to formation of soil structure. They might stimulate or regulate growth, or exists the plants, resistance to pathogenic attacks. Manure is a valuable renewable resource, which completes the nutrient element cycles and allows much of the nitrogen fixed with legumes and harvested as forage to be returned to the soil, where it can become available for growth of subsequent crops.

Organic manuring phenomenon aims to improve the biological, chemical and physical properties of the soil and is important as a source of energy and nutrient elements for the soil ecosystem. Organic matter influences productivity of soils through the mineralization of nutrients, its high cation exchange and water holding capacities, and its ability to improve soil physical properties. Many measurements are used to denote soil structure, including porosity, bulk density, and soil aggregation. The impact of the structure of a soil may be expressed in terms of the content and transmission of water, air and heat as well as soil strength. Some organic substances are combined with the clay fraction in aggregates so that they are not accessible to microbial decomposition. Some of the green manuring materials to be added to soil are:

GREEN MANURE: Green manures comprise plant crops grown on a given soil to a certain stage of development before being ploughed under, while still green. Both leguminous and non-leguminous plants are utilized for this purpose. Leguminous crops are cultivated widely in a variety of agro- climatic zones in the tropics and subtropics. Legumes are used for a wide variety of purposes as a human nutrition, 1ivestock feed, soil improvement, production of oils, gums, wood and fibres. The legume-Rhizobium symbiosis is considered to be the most promising plant-bacteria association for increasing protein yield through BNF process. Green manures serve several distinct purposes for plant growth and soil improvement. They increased the supply of total and available nitrogen in the soil and various leguminous plants are utilized for this purpose. Green manures prevent the nutrient elements of the soil, especially the nitrates from being leached out during the season of the year when no cultivated crops are being grown in the soil. They also increase the supply of organic matter in the soil and protect it against possible erosion. The plants used for green manuring are high in water-soluble constituents, in nitrogen and other nutrient elements, but are comparatively low in cellulose and lignin. As a result, decomposition of a green manure ploughed into the soil and reaction sets in very rapidly. This is accompanied by rapid liberation of the nitrogen and other nutrient elements in available forms. Humus is also produced in this process. As the plants grow older, their ash and nitrogen contents decrease and their cellulose and lignin contents increase.

Decomposition of younger plants results in the liberation of some of the nitrogen and ammonia, the younger the plant, the higher the nitrogen contents, the more rapidly is the ammonia liberated and in larger amounts. Young plants, low in lignin and cellulose, but high in water-soluble minerals and nitrogen decompose much more rapidly than do mature plants. They leave smaller residue in the form of humus, in which only a small part of the original nitrogen is stored away. The age of the plants used for green manuring exerts an important influence upon the amount and the rate of liberation of nutrient elements in available forms. Generally, green manures play a very important role in the design of crop rotation for bio-fertilizer farming systems. The main benefits of green manuring are: i) maintaining carbon and nitrogen accumulation; ii) reduction of nutrient elements leaching; iii) reduction of soil erosion; iv) improving water utilization; v) providing aeration of the soil; vi) weed and pest control; vii) providing cost savings in the crop rotation as a result of lowering fertilizer use, improving nutrient elements utilization, easier cultivation and reducing plant protection requirements.

FARMYARD MANURE: The main types of livestock wastes which are found in farms are farmyard manure, either fresh or stockpiled, slurry, liquid manures (urine). Farmyard manure consists of three main groups of components; bedding or litter, solid of the animals and liquid urine. Urine is high in nitrogen and minerals. Solid excrete contains considerable amounts of proteins, and thus tend to give a more balanced medium for the growth of microorganisms. Sheep manure is high in protein and low in cellulose. Horse manure is low in protein and high in cellulose. Low manure is intermediate in its properties. Chicken and pigeon manures are highest in nitrogen, phosphorus and potassium as well as the most important nutrient elements required for plant growth. Cattle and horse manures contain the lowest quantities of these essential ingredients. Considerable quantities of humus are formed during the decomposition of farmyard manure, whether in composts or in soils. The normal rate of application is about 10 tons of farmyard per hectare.

COMPOST: Composts consist of a variety of organic materials that have been subjected to decomposition before being added to soil. The chief advantage of composting before mixing with soil in one of physical condition. Compost of plant residues with mineral fertilizer results in a product similar in every respect to that obtained from farmyard manure. Composting has a wide range of objectives: Suppression of unpleasant odours, improvement of hygienic conditions, reduction of the germination capacity of weeds, increase the biological activity of soils, minimum loss of nutrient elements during application in the soil. Cereal straw and other plant residues are utilized to prepare artificial manures similar to chemical composition and their effects upon the soil to compost commonly obtained from farmyard manure. These are supplemented with an organic source of nitrogen and to some extent, also with available P and lime. When they are properly moistened, decomposition of the composts sets in immediately and is accompanied by a rise in temperature.

The compost heap is usually turned up and down several times especially after its temperature reaching 65-70C. Turning must be frequent enough for proper aeration, which the development of aerobic fungi, actinomycetes and bacteria. The moisture content of the composts must be adjusted to about 75- 85%. If excess water is added, an aerobic condition which retard decomposition are initiated.

SLURRY AND LIQUID MANURES: Slurry is a difficult material to manage the provision of safe, long-term storage is expensive, thus spreading is often carried out in an appropriate time of the year to make room. This has a serious pollution impact and wastes valuable nutrient elements. In arable areas, where straw is plentiful, compost production using slurry and straw provides an alternative way of managing slurry and liquid manures. Typical quantities are 20 tons of straw to 100,000 litres of slurry, or in some cases, sewage effluents. A starter such as cold compost or chicken manure might also be helpful.

SEWAGE SLUDGE: Sewage sludge and other municipal and agricultural wastes hold promise of benefiting soil organic matter and contains N and P, but little potash, must of which and some of the N are lost in the liquid during separation. The organic matter obtained through such process can make useful contribution to soil improvement. If sewage sludge is used in bio-fortified farming, then it must be applied with extreme caution. Large quantities of these materials have traditionally been incinerated or deposited in landfills.

The composition of waste materials must be determined before land application, though to avoid potentially hazardous high levels of trace metals and toxic compounds.

GARBAGE MANURE: The organic fraction of household refuse in several towns is sorted and composted. The mature compost has been shown to be of high quality and low to heavy metal contaminants compared to mineral fertilizers.

Green manuring has been practiced in Pakistan in different forms like burying a legume crop in situ before planting the next crop, simultaneous cultivation of a green manure and a main crop and adding leaves and cuttings of annual and perennial plants and trees growing elsewhere before or after planting a crop. Of the various benefits credited to green manuring, increase in available plant nutrients and in the organic matter content of the soil have been studied in both short and long-term experiments.