Agriculture And It Principles & Methods of conserving and renewing soil fertility



For the greater part of the million or more years since Man evolved, he has fitted into the ecosystems where he chose to live.

He obtained food by collecting plants or their products and by capturing animals. Thus his effect on a balanced ecosystem was probably little different from that of any other omnivorous mammal. It is thought that 100 km² of territory would support only three or four people who lived by hunting and gathering food and this was probably an important factor in limiting the growth of the human population. The development of agriculture allowed a rapid increase in population and in the last 200-300 years man has become so numerous that his activities threaten to destroy the ecological balance. This chapter Considers firstly the principles underlying agricultural practice and secondly the threat to ecological stability of intensive agriculture and related activities.


Principles Of Agriculture 

1. Arable farming. The principles of arable farming involve the removal of the natural vegetation from the soil in the first place.

Then (a) the soil is planted with seeds of a single plant species, (b) their growth is encouraged by improving the soil fertility, e.g. adding water and mineral salts, and (c) plant competitors and parasites are eliminated by hoeing or spraying. In this way, a limited area of soil is made to produce a much larger quantity of edible plant material than if it were left to develop its natural vegetation such as grass or forest trees.

(a) The plant species cuitivated today must have had their origins in wild plants, but by careful selection and cross- breeding over many hundreds of years they have become plants with large seeds (e.g. maize), edible fruits (e.g. pineapple) or substantial root or stem tubers (e.g. yam and potato). The process of breeding and selection continue even nmore in- tensively today on a scientific basis to try and find plants that yield more food, are resistant to disease or can survive in adverse conditions such as drought.

The agricultural practice of growing large colonies of a single species of plant is called monoculture.

(A) The soil used for agriculture is "improved" mechanically by ploughing. This loosens the soil, admits air, allows water to drain and makes penetration by plant roots easier. The nutritive value of the soil is increased by adding natural fertilizers such as farmyard manure, or artificial fertilizers such as ammonium sulphate. Soil structure and fertility are both improved by the practice of crop rotation. If water is scarce it can be added to the soil by various methods of irrigation, and if the soil holds too much water it can be drained by digging ditches or placing pipes beneath the soil.

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(c) In the favourable conditions provided by cultivation, plants other than the crop plants will grow and compete with them for water, root space, nutrients and light. Such plants are collectively called weeds. The weeds are kept in check firstly by ploughing the soil before planting and then by hoeing between the rows as the crops grow. Certain chemicals, herbicides, can be used to reduce weeds both before and after planting.

  Fungus diseases of the crop plants are suppressed by selecting resistant strains of plant and by spraying chemical Jungicides on the crop. Insect pests which eat or damage crop plants or their seeds are reduced by spraying the crops with insecticides and by dipping the seeds in similar chemicals before planting.

2. Animal husbandry. Animal protcin contains more essential amino acids than does plant protein and man, generally, finds animal products particularly palatable. In consequence, man uses some of his crops to feed animals and then eats the animals or their produce such as milk and eggs.

The principles of animal husbandry are much the same as those applied in growing crops. A limited number of species is used. One such is the coW, which after years of selective breeding has high milk yield, produces much meat or is resistant to disease or adverse conditions. The animals are allowed to grow in as near ideal conditions as possible and pests and competitors are eliminated. Only about 10 per cent of the food given to animals appears as flesh, or eggs or milk. The rest is used for energy to keep the animal alive, maintain its temperature and enable it to move about. The logical step in modern animal husbandry is therefore to reduce the energy losses toa minimum. In "factory farming, the animals are kept indoors at a constant temperature to reduce heat losses, and their movements are restricted so that the food they eat is used for making flesh rather than being used for energy production.

If the world population continues to increase at its present rate, it will become necessary to use more plant products directly for food instead of losing 90 per cent of their energy value by feeding them to animals.

Methods of conserving and renewing soil fertility

In natural conditions soil fertility is maintained by the activity of the organisms living on it or in it. For example, the plant roots maintain the soil's crumb structure and the burrows of arthropods enhance its drainage. Although plants remove mineral salts they are replaced by the death and decomposition plant and animal bodies. The practice of agriculture interrupts natural cycles by removing the crops at harvest but not returning to the soil the dead remains of either the plants or the animals which eat them. This practice and the loss of soluble salts washed out by rain greatly reduces the yield from crops.

What has been grown and harvested on an experimental strip d at a British experimental station for over 100 years without anything being added to the soil. In this time, the yield has dropped from 147 to 6-9 kg per 100 m-. The soil nitrogen, however, has remained at a steady concentration over the nitrogen-fixing bacteria and other micro-organisms present in the soil. Similar results have been obtained in Kenya, where the yield of maize from an experimental plot dropped from 5800 kg per hectare to 2240 kg per hectare in only three years.

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To make good the losses from the soil, the farmer adds either farmyard manure or artificial fertilizers.

Manure. Manure is the collected faeces of farm animals, usually mixed with straw or other litter and allowed to decay for several months. When it is ploughed into the soil it provides organic matter which (a) decays to give the nutrient salts needed by plants and (b) provides the material needed to form humus.

Untreated human sewage, although it would help to complete the nitrogen cycle, IS not used on crops because of the dangers of spreading intestinal diseases such as cholera, typhoid and dysentery, but treated sewage and sewage sludge which has been fermented at a high temperature can be a safe and useful fertilizer for the soil.

Artificial fertilizers. Although small mixed farms may produce enough manure to maintain soil fertility, large-scale arable farms do not and the minerals must be replaced by more direct chemical means. The artificial fertilizers are made in factories from sulphuric acid, ammonia, lime, slag from steel processing, and other industrial wastes.

i) Sulphate of ammonia (ammonium sulphate) is made from ammonia and sulphurie acid. It proVides nitrogen and sulphur.

ii) Basic slag is made by grinding the slag from steel-making furnaces. It contains lime and phosphorus.

(ii) Superphosphate. Basic slag or natural rock, rich in phosphates, is crushed and treated with sulphuric acid. It supplies phosphorus.

Crop rotation. Different crops make differing demands on the soil; e.g. maize takes more than average amounts of phosphates and nitrates from the soil, and their roots exploit different depths. By changing the crop grown on a certain field from year to year the soil is not depleted of one particular group of minerals. Leguminous crops such as beans and ground-nuts may help to restore the nitrogen content of the soil with their root nodules containing nitrogen-fixing bacteria, provided some of their vegetation is ploughed back into the soil as "green manure" A year or two of grass (lying fallow) greatly improves the soil's crumb structure.

Rotating the crops also reduces the competition from weeds and the hazards of insect or fungus infestations that are characteristic of one kind of crop. For example, successive crops of potatoes on a soil will increase the population of the fungus causing the disease potato blight". A field freed for a few years from potatoes will show a reduced incidence of this disease.

Bush fallow cultivation has a similar effect to crop rotation. An area of bush is cut down and planted with crops, In a few years the mineral salts are depleted and the soil structure has degenerated so that the yields are poor. The farmer abandons this plot and allows it to revert to bush while he cultivates another area. In 8-10 years' time the soil structure and fertility of the original plot are sufficiently restored to make it worth cultivating again. In the practice of shifting cultivation the exhausted plot was abandoned permanently and allowed to revert to bush while the farmer moved to a new area. The increase in population has made this method of cultivation impracticable.

The cultivation of maize. The principles outlined above can be illustrated by describing the method of cultivating maize. Maize is an important and widely distributed cereal crop. The grain is a concentrated source of nutrients, easy to harvest and store.

Being a high yielding crop, maize removes a large amount of nitrate and phosphate from the soil, particularly from the upper layers because of its relatively shallow rooting system.

Consequently it is best to rotate maize with soya beans or ground-nuts whose roots occupy different areas of the soil and whose demand for nitrates is much less. In fact, the yield over a long period can be doubled if such a rotation is practised. In some tropical small holdings a mixed crop of maize with either beans, cotton or ground-nuts is raised.

At the start of the rainy season the seeds are planted about 5 cm deep in moist soil and 12 cm in dry soils with gaps of about 30 cm between seeds. (The grain and its germination are described). A well-aerated, warm, moist soil is preferred and digging or ploughing is kept to a minimum in order to preserve the soil's crumb structure. The fertilizer-used should be rich in nitrate, phosphate and potassium; one-third of it is applied at sowing to hasten the early growth and the rest at about the time the male flowers appear, which is the period of maximum uptake. (Pollination) Weeds are removed from between the plants to reduce competition for nutrients and water and the soil surface is broken up by hoeingg to prevent it forming a hard crust which would cause rain to run off without penetrating to the soil beneath. From 100 to 200 days after planting the maise is ready for harvesting. The cobs can be cut and collected over a period of several weeks since they are well protected from insects and birds by their covering of bractsMaize is subject to a number of fungus diseases, one of which is smut caused by a fungus called Ustilago may is. he fungus causes gall-like growths on the cob and considerably reduces the yield. There seems to be no effective means or controlling the disease but resistant varieties of maize are available. If the plants are damaged by careless hoeing, the Tungus is able to enter the plants from the soil. If the disease is present in the crop, affected areas should be cut off and burned and a different crop should be grown in the following year or two because fungus spores might still be present in the soil.



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