Introduction
Soil fertility is the ability of the soil to provide crops with the required nutrients in their proper proportions.
Characteristics of a Fertile Soil
Good depth - Good soils give roots greater volume to obtain plant nutrients and provide strong anchorage.
Good aeration - for the respiration of plant roots and use by soil organisms.
Good water holding capacity - ensures provision of adequate water for plant growth.
Proper drainage - ensures provision of adequate air for plant growth.
Correct soil pH - different crops have different soil pH requirements.
Adequate nutrients supply - it should supply the required nutrients in the correct amounts and in a form available to plants.
Free from excessive infestation of soil borne pests and diseases.
How soil loses fertility
Leaching: vertical movement of dissolved minerals from the top to the lower horizons of the soil profile.
Soil erosion - The removal and carrying away of the top fertile soil from one place to another.
Monocropping - This is the practice of growing one type of crop on a piece' of a land over a long time.
Continuous cropping - crops take away a lot of nutrients from the soil which are never returned.
Growing crops continuously without giving the soil time to rest makes the soil infertile.
Change in soil pH - changes in soil pH affect the activity of soil microorganisms as well as the availability of soil nutrients.
Burning of vegetation - burning of vegetation cover destroys organic matter. It also exposes the soil to the agents of soil erosion.
Accumulation of salts - soils with a lot of salts are said to be saline. State of having too much salt in the soil is referred to as soil salinity.
Salts accumulation cause water deficiency in plants. It may also lead to change in soil pH.
Maintenance of Soil Fertility
Soil fertility is maintained through the following methods:
Control of Soil Erosion ;
- Terracing,
- Contour cultivation,
- Strip cropping,
- Cut off drains
- Planting cover crops.
Crop Rotation ;
- Practice of growing different crops on the same field in different seasons in an orderly sequence.
Control of Soil pH :
- Application of liming materials such as limestone, quicklime, magnesium carbonate and slaked lime if the soil is acidic.
- Application of acidic fertilizers if the soil is alkaline.
- Application of manures.
Proper drainage;
Done through:
- Breaking hard pan.
- Construction of water channels.
- Growing crops on cambered bed
- Pumping out water from the soil.
Weed control:
- Use of herbicides.
- Slashing
- Uprooting.
- Mulching
- Use of proper farming practices such as early planting, correct spacing and cover crops.
Intercropping
- Farming practice where different crops species are grown together in the field.
Minimum Tillage;
- Use of herbicides.
- Uprooting of weeds.
- Slashing weeds
- Mulching
- Strip cultivation.
Use of Inorganic Fertilizer ;
- Chemical compounds manufactured to apply specific plant nutrients for example calcium ammonium nitrate (CAN).
Use of Manure;
- Well decomposed manures release nutrients into the soil and increase its water holding capacity.
Organic Manures
Manures are derived from plants and animal remains.
They supply organic matter to the soil which after decomposition releases plant nutrients.
The end product of this decomposition is known as humus.
It influences soil chemical properties and soil temperature.
Manures supply a wide range of essential plant nutrients.
Importance of Organic Matter in the Soil
Increases the soil water holding capacity of the soil.
Improves soil fertility by releasing a wide range of nutrients into the soil.
Provides food and shelter for soil micro-organisms.
Improves the soil structure.
Buffers soil pH/moderates soil pH.
Reduces the toxicity of plant poisons in the soil.
Moderates soil temperature by its dark colour.
Limitations in the Use of Manure
They are bulky - low nutritive value per unit volume.
Laborious in application and transport.
They spread diseases, pests and weeds.
Loss of nutrients if poorly stored.
If not fully decomposed crops may not benefit from them.
Types of Organic Manures
Green manure.
Farm yard manure.
Compost manure
Green Manure
Made from green plants which are grown for the purpose of incorporating into the soil.
Characteristics of plants used for preparation for green manure:
Have fast growth rates.
Have high nitrogen content.
Capable of rotting quickly.
Capable of growing in poor conditions.
Preparation of Green Manure
Plant the green manure crop in the field.
Allow the crop to grow up to flowering stage.
Incorporate it into the soil through ploughing.
Allow the crop to decompose for two weeks.
Prepare the field for planting the major crop.
Reasons why green manure is not commonly used/limitations:
Most of the plants used as green manure are food crops.
Green manure crops may use most of the soil moisture.
Most of the nutrients are used up by soil micro-organisms in the process of decomposing the green manure.
Planting of the major crop is delayed.
Farm Yard Manure (FYM)
Is a mixture of animal waste and crop residues used as beddings in animal houses.
Factors that Determine the Quality of FYM
The types of the animals used.
Types of food eaten
Types of litter used.
Method of storage.
Age of farmyard manure.
Age of the animals used.
Preparation of FYM
Provide beddings in the houses of farm animals.
Animals deposit their droppings and urine on the beddings.
Animals mix them through trampling.
The beddings together with dung are removed and heaped under shed to decompose.
After sometime, the materials decompose and FYM is formed.
It can then be used in the farm
Compost Manure
Is manure prepared from heaped (composted) organic materials.
Factors to consider in selecting site for making compost manure:
A well drained place.
Direction of the prevailing wind.
Size of the farm.
Accessibility.
Preparation of Compost Manure
Two methods:
Four heaps method
Indore Method (Pit Method)
Indore Method (Pit Method)
Procedure ;
Select a sheltered place with a shade and near the field.
Dig a pit with the dimension 1.2m x 1.2m x 1.2m.
Place the materials in the following order:
Hedge cuttings or maize stalks to a depth of 30cm as a foundation
A layer of grass, green weeds or leaves and kitchen wastes to 30cm.
A well rotten manure/poultry droppings.
Wood ash and phosphatic fertilizers.
A layer of topsoil to introduce micro¬organism for the decomposition of organic remains.
Note: Some water should be sprinkled to the materials to initiate the decomposition process and regulate temperatures.