Soil Fertility and its factors

Soil fertility is defined as the ability of the soil to produce and sustain high crop yields indefinitely. In other words, fertility is a measure of the available nutrients in the soil and the proportion in which they occur. The soil is able to do this only if it can adequately supply the nutritional requirement of the crop plants in question.

A.     Factors contributing towards soil fertility

Some of the factors are as listed below:

1. Soil depth- deep soils afford plant roots greater volume to exploit.
2. Good drainage to avoid water logging.
3. Good aeration to promote healthy root development and functioning.
4. High water-holding capacity
5. High level of nutrients
6. Soil pH.
7. Freedom from pests and diseases

In farming soil fertility may be lost in many ways. The common ways include the following:

1. Soil erosion by either water or wind
2. Soil capping: formation of an impervious layer of soil on the surface of the soil which obstructs rain infiltration leading to run off.
3. Development of hard pans short distance below the surface of the soil which impedes water percolation as well as root penetration. Hard pans may be caused by repeatedly ploughing at the same depth.
4. Loss of organic matter through rapid oxidation by soil micro-organisms due to unduly too frequent cultivations.
5. Leaching, especially serious with such nutrients as nitrogen which are highly soluble in water.
6. Loss of nutrients through sale of farm produce off the land without replenishing the nutrients removed in the produce.
7. Weeds. These compete for nutrients and moisture.
8. Alteration of soil pH, such as through misuse of certain fertilisers.
9. Build-up of soil pests and diseases by practicing certain systems of farming such as monoculture.

B. Plant Nutrients

In plant nutrients, you may discover that certain elements are essential for plant growth. Those being required in the largest quantity are known as major or macro elements while those needed in small amounts are known as minor or micro elements, and those needed in very minute quantities are known as trace.

The elements needed in large amounts are carbon, hydrogen and oxygen, obtained mainly from air and water. Nitrogen, Phosphorus, potassium, calcium, magnesium, sulphur and iron are obtained from the soil dissolved in water, or from application of foliar sprays. Essential trace elements are copper, manganese, zinc, molybdenum, boron and chlorine which can be made available to plants either through their roots or their leaves.

The table below summarises the importance of major and minor nutrients to the plants, deficiency symptoms and their sources.

Table 3.3

NUTRIENT	IMPORTANCE TO PLANT	DEFICIENCY SYMPTOMS	SOURCES
NITROGEN	Constitutes of proteins	Stunted growth in plants	Ammonium nitrate Urea Ammonium sulphate
PHOSPHORUS	Protein formation and good root formation	Stunted growth. Purple colour in the leaves	Single and triple super phosphate
POTASSIUM	For flower and fruit formation	Yellow leaves with brown edges	Potassium sulphate Potassium chloride
CALCIUM	Formation of cell wall	Weak plants	Ground limestone
MAGNESIUM	Formation of chlorophyll	Yellowing of leaves	Magnesium sulphate
SULPHUR	Formation of proteins	Yellowing of plants	Ammonium sulphate Calcium sulphate

C. Inorganic fertilisers

As for Inorganic Fertilisers, these are artificially made fertilisers which are manufactured to supply the chemical nutrient needs of plants. The proportions of the main contents are stated on the container. Fertilisers which supply one main element such as nitrogen, phosphorus and potassium are known as straight fertilisers. Those containing mixtures in definite proportions are known as compound fertilisers.

The table below shows examples of straight fertilisers:

(a) Nitrogen

Fertiliser	% of Nitrogen
Ammonium sulphate	21
Ammonium Nitrate	34
Urea	46

Phosphorus

Fertiliser	% of Phosphorus
Single super phosphate	19
Triple super phosphate	44

(c).Potassium

Fertiliser	% of Potassium
Muriate of potash (KC1)	60
Sulphate of potash (KS04)	50

It is necessary to note that top dressing fertiliser is sometimes applied with fertilisers of nitrogen. Plants, which are adequately supplied with potassium, are better able to withstand drought conditions than those suffering from a shortage.

On the other hand, Compound fertilisers are fertilisers containing more than two major elements along with minor elements. They are mostly applied as basal dressing fertilisers.

The table below shows the percentage constituents of Nitrogen, Phosphorus and potassium present in the different compound fertilisers

Compounds	% N	% P20S	% K20	Other
A	2	18	15	65
C	6	18	12	64
D	10	20	10	60
R	20	20	0	60
V	4	18	15	63
X	20	10	5	65

D. Organic Fertilisers

As for organics, these are the waste and residues of plants and animals. These organics can be in many forms such as poultry manure, cow dung/animal manures, farm yard manure, green manure and compost. They are natural manures and their decomposition is achieved by bacteria to form humus. Organic manures have bad and good effects on the soil.