- 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 ;
- 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.
- Breaking hard pan.
- Construction of water channels.
- Growing crops on cambered bed
- Pumping out water from the soil.
- Use of herbicides.
- Use of proper farming practices such as early planting, correct spacing and cover crops.
- Farming practice where different crops species are grown together in the field.
- Use of herbicides.
- Uprooting of weeds.
- Slashing weeds
- 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.
- 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
- 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
- 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.
Preparation of Compost Manure
- Four heaps method
- Indore Method (Pit Method)
Indore Method (Pit Method)
- 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 microorganism for the decomposition of organic remains.
- Note: Some water should be sprinkled to the materials to initiate the decomposition process and regulate temperatures.
Four heaps method:
- Clear the site.
- Level the site
- Four posts 2m high are fixed 1.2m apart from four corners of the heap.
- Fix wood planks on the sides.
- Materials are placed in two heaps as in the pit method,
- The two heaps make up heap 1.
- After 3-4 weeks, the decomposed material from heap 1 is transferred to heap II.
- After another 3 – 4 weeks the material is transferred to heap III.
- After 3-4 weeks it is ready for use in the farm.
Indicators of well decomposed manure
- Absence of bad odour.
- Materials are lighter.
- Manure is brown in colour.
Advantages of Compost Manure
- One does not have to own livestock in order to prepare it.
- A lot of manure can be produced within a short time.
- A variety of materials can be used in its preparation.
- Uses locally available materials thus cheaper than the artificial fertilizers.
- Improves the soil structure.
Limitations of Compost Manure
- It releases nutrients slowly into the soil.
- Large quantities of compost manure are required to supply enough plant nutrients.
- Its preparation is labour intensive.
- It may induce soil-borne pests and diseases.