SOIL FERTILITY AND MANAGEMENT

SOIL FERTILITY AND MANAGEMENT

A soil is regarded as being fertile when crops can be successfully grown on it and give high yields.

It has already been established that the level of soil fertility may fall in successive years of use. This is because crops remove large quantities of plant food from the soil. Cultivation also disturbs some of the important physical characteristics of the soil.

It has been stated that fertile soils should be neither too sandy or contain too much clay; they should also be properly aerated and contain an adequate supply of plant nutrients. Fertile soils must not be water-logged; good soil structure and texture are also important.

When soil fertility falls, methods of improving the soil productive capacity must be found. In agriculture, fertility is maintained using one or a combination of the following:

(a) Rotational cropping (crop rotation)
(b) Fallowing
(c) Cover cropping
(d) Organic Manures
(e) Fertilizers.

Fertility Management

(a) Rotational cropping (continuous cropping)

This is the practice of growing different crops in a particular order on the same piece of farmland for many years. The cycle is planned in such a way as to restore nutrients removed from the soil. This situation is similar to an individual who withdraws money continuously from his savings account without putting some more money back into the savings. A system of crop changing is therefore necessary and must be planned in such a way as to ensure that the nutrients are not completely used up.

The length of rotation is the number of years it takes a cycle to be completed, that is the number of years that will pass before a given crop is grown again on the same plot of land; 3, 4, or 5 year rotation are the commonest.

An example of a 4 year course rotation is as follows:

Year	Plot 1	Plot 2	Plot 3	Plot 4
1	Yam	Maize	Cassava	Groundnut
2	Maize	Cassava	Groundnut	Yam
3	Cassava	Groundnut	Yam	Maize
4	Groundnut	Yam	Maize	Cassava

Different crops need different amounts of macro-nutrients. Crops also have different root systems; some are shallow like cereals and grasses, while some are deeper like yams and groundnut.  Similar crops that are attacked by the same pests and diseases should not follow one another on a piece of land. The disease-causing organisms should be allowed to die before the crop is planted in the same soil again.

Principles of crop rotation

1. Manures should be applied at least once in a rotation of four years.
2. The rotation may be one of a 3, 4 or 5 year course.
3. Shallow rooted crops should follow deep-rooted crops.
4. Crops for a rotation should be adaptable to the area's soil and climate.
5. Crops that are affected by same disease or insect attack should not follow each other.
6. The rotation should include a legume, since legumes help to increase soil fertility through the increase of soil nitrogen.
7. Crops of the same family should not be made to follow each other.

Advantages of crop rotation

1. It helps to control pests (weeds, insects and diseases).
2. It facilitates the efficient and economic use of soil nutrient.
3. It is adopted where there is scarcity of land.

4. It helps to maintain the fertility of the soil through the cultivation of leguminous crops in the rotation.

5. The grasses and legumes in the rotation help to check erosion.

6. Helps farmers to produce different kinds of crops.

7. Acts as an insurance against crop failure.

Disadvantages

1. Exhaustion.
2. Erosion and
3. Low productivity.

(b) Fallowing

Under bush-fallow system it involves growing crops on a piece of land until it is exhausted, when the land is allowed to go back to bush for 6-12 years and is used again. It is a modified form of shifting cultivation. It is still practised in many parts of the tropics, especially in sparsely populated districts. When land is allowed to lie fallow, plant food has time to form in the soil as a result of humus accumulation. The soil gradually builds up its fertility chiefly through the activities of higher plants and micro-organisms, plant food are returned to the soil.

Advantages

(a) It is a simple method of bringing back soil fertility.

(b) Does not require any technical knowledge.

(c) Plants which grow during the fallow period improve the soil structure.

(d) Prevents the spread of crop diseases, pests and weeds, since their activity is reduced during the fallow period.

Disadvantages

(a) Thick bushes are difficult and expensive to clear.

(b) Large amount of land which could be put to productive use is left fallow.

(c) Does not encourage large scale farming. Owing to these disadvantages, fallowing method of restoring fertility has become less popular.

(c) Cover cropping

Cover crops are close growth of plants which cover the soil surface. The soil is protected from the action of the sun and they also prevent erosion arising as a result of heavy rainfall. Examples of such plants include Centrosema pubescens, Calopogonium mucunoides and Pueraria phaseoloides.

Advantages

(a) They reduce soil erosion by protecting the surface from heavy rain.

(b) Enrich the soil with fallen leaves and stems.

(c) Improving beneficial chemical activity by minimizing temperature extremes due to shading.

(d) Improving and stabilising humidity by reducing soil evaporation.

(e) Reducing weeds by over-shadowing.

(f) Increasing the population of the soil micro-organisms by making the habitat more tolerable.

Disadvantages

1. Compete with the crops for water particularly during the dry season.

2. Increases the rate of water loss from the soil through transpiration.

These use of cover-crops in the tropics is yet very much limited due to some difficulties.

(a) Lack of machinery is ploughing is being done with tractors.

(b) The cost of the exercise is not all that cheap.

(c) There is pressure on land due to increase in population.

(d) The rate of decomposition of organic matter is very high.

(d) Organic manure

In order to prevent the soil from being exhausted and subsequently barren we add various materials which replace the nutritional elements in the soil. Any such material which is added to make up the deficiencies is called a manure.

Organic manures are manures prepared from plant and animal refuse. Such substances help to improve the soil condition for plant growth. They are natural sources of nutrients. They contain high levels of humus; and are generally quite cheap. Organic manures contain several kinds of plant nutrients, particularly nitrogen and phosphorus in addition to small quantities of potassium and trace elements although these are present considerably smaller quantities than they are in inorganic fertilizers.

There are four main types of organic manure:

(a) Green manure.
(b) Animal or Farm Yard Manure (FYM)
(c) Compost
(d) Humus.

Green manure

When crop plants, particularly leguminous plants, are ploughed into the soil while they are still living, it is referred to as green manuring. There, the materials decay and become manure in about a fortnight later. Green manure derives its name from the green succulent plant materials from which it is prepared.

Advantages of green manure

(a) Increases the humus content of the soil and so improves the physical condition, soil aggregate and bulk density of the soil.

(b) Nitrogen is added into the soil.

(c) Nutrients from the subsoil are returned to top soil when the crops are ploughed.

(d) Reduce rate of evaporation from the soil surfaces by providing cover.

(e) Reduce the loss of nutrients through soil wash and leaching.

Disadvantages of green manure

(a) Some of them may secrete injurious substances which may harm subsequent crops.

(b) Fertilizers may be cheaper to buy than growing green manure.

(c) It may bring about the incidence of pests or diseases.

(d) May absorb the nutrients but may not release them in time for the succeeding crops.

Certain conditions especially in dry areas must be fulfilled before growing green manure crops:
(i) There must be adequate supply of water.
(ii) There should be enough water for decomposition of dug-in material.

Conditions of green manure crops

(a) Growth should be rapid and crops ploughed in when succulent.

(b) Should be free from insect pests and diseases.

(c) Should not require much fertilizer.

(d) Often used as food and feed.

(e) Where crops grown are legumes, they produce enough nodules for nitrogen fixation.

(f) Grown at a time when no other crop is grown in the farm.

(g) Seeds should be able to germinate without treating it with chemical.

(h) Crops should be able to produce seeds easily, rapidly and cheaply.

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Animal or Farm Yard Manure (FYM) or Pen manure

Definition

Farm yard manure (FYM) consists of faeces, or excreta, urine and other waste products of domesticated animals. These are known as animal droppings. Together with a bedding of grass, animal droppings are deposited in a shaded place which has been specially prepared for this purpose.

The solid part of manure usually contains more nitrogen, phosphorus and potassium than the liquid part. It also improves the structure of the soil. It also improves the process of nitrification in the soil.

Advantages

(a) Has a fairly lasting effect on the soil.

(b) Considerably cheap and easy to produce.

(c) Contain all substances required by plants, but it is bulky.

(d) It binds sandy soil and loosens clayey soil.

Major problems of using farm-yard manure

(a) It is bulky in nature.

(b) There is need for transport and handling.

(c) Time taken by labour to dig it in.

(d) Exposure facilitates disintegration.

(e) Rain wash away some nutrients.

Compost

Compost manure is manufactured by placing organic waste materials such as crop residues from the farm and garden, weeds, leaves, grass, kitchen and compound sweepings in a heap and letting the mixture decompose.

Chapter 12 — SOIL FERTILITY AND MANAGEMENT

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Making a compost: Materials required

Organic waste material, such as all vegetables and crops, remains from the farm and vegetable gardens, weeds, leaves of trees, grass, clippings from the lawn, trimmings from hedges, compound and kitchen refuse; farm-yard manure or old compost or nitrogenous fertilizer such as sulphate of ammonia, ordinary wood ash from the kitchen or burnt crop residues and urine.

Farm yard manure or old compost or nitrogenous fertilizer may be used as a starter to start the fermentation which is the first stage of the decomposition which takes place in the compost heap. Also, urine or good top soil from the garden or bush can be used as starters.

Fig. 32: Laying out compost heaps or stacks

Methods of composting

There are two main methods of composting:

(a) Stack and
(b) Pit

Benefits of compost to the farmer

(a) Improves water conservation in soil.
(b) Returns of nutrients to soil.
(c) Encourages biotic activity in soil e.g. worms, bacteria.
(d) It improves texture and crumb structure.
(e) It prevents extremes of pH in the top soil.
(f) Prevents erosion due to improvement in soil texture.

Humus

When plants and animals die they gradually decay and form a dark brown substance known as humus. The rate at which decay takes place depends on:

(a) Temperature: Bacteria can work faster if the temperature is high.
(b) The amount of oxygen which helps decay.
(c) Moisture: Bacteria need some moisture.
(d) The presence of bacteria (which feed on the matter and break it down).

Humus plays an important part in the carbon cycle. Carbon is the basis of all organic matter, and as plants and animals decay, they gradually break down into carbon again. The steady release of nutrients is only part of the benefit of humus in a soil.