Different methods are used by farmers to maintain the fertility of the soil or replace lost nutrients. These methods include:
- Crop rotation.
- Organic manuring.
- Bush fallowing.
- Cover cropping.
- Inorganic manuring or Fertilizers.
a) Crop Rotation
Crop rotation is defined as the planting of different types of crops on the same piece of land in a definite sequence. In other words, it is a system of farming whereby different crops are grown on the same piece of land year after year in a definite manner so as to maintain the fertility of the soil. The cycle is planned in such a way as to restore nutrients removed from the soil. The layout of a four-year crop rotation plan is shown in this table.
The land is divided into sections, usually between 3 – 6, and each year, a different crop is planted in each section. For example, the land in the table is divided into four sections and in each year, different crops like yam, cowpea, cassava and maize are planted.
Crop Rotation as a Method of Replenishing Soil Nutrients
In order to maintain soil fertility, certain principles of crop rotation must be observed.
- Deep-rooted crops like yam and cassava should not follow each other because they will absorb nutrients from the same level in the soil.
- Shallow-rooted crops like rice and maize should also not follow each other because of the same reason as above.
- Crops which have the same diseases should not follow one another.
- Crops which have the same pests should not follow one another.
- Crops of the same family should not follow one another. This is to prevent the uptake of the same nutrients and also to break the life cycle of pests and diseases that might accompany such crops.
- Crop rotation helps to control the growth of weeds which tend to remove nutrient from the soil.
- The inclusion of a legume in crop rotation also adds nutrients to the soil.
- Crop rotation also helps to control erosion which tends to remove nutrients from the soil.
Conditions Necessary for the Practice of Crop Rotation
- Where land is scarce or limited in supply.
- Where population is high.
- The need to have different varieties of crops.
- The need to maintain soil fertility.
- Where the soil is exhausted of nutrients.
b) Organic Manuring: Organic manuring involves the application of organic manure to the soil in order to improve its fertility.
Organic Manuring as a Method of Replenishing Soil Nutrients
Organic manuring as a way of improving soil fertility has the following advantages:
- It promotes the activities of soil living organisms such as earthworms, termites and microbes. These organisms promote aeration of the soil, easy percolation of water, mixing organic materials with soil, form humus and fix nitrogen into the soil. All these help to promote the fertility of the soil.
- Organic manure helps to improve the structure of the soil by building the particles of coarse texture soil together.
- Mineralisation of humus adds nutrients to the soil.
- It reduces rapid soil temperature fluctuations. As a result of its dark colour, humus easily absorbs heat during the day and loses it slowly at night.
- Organic manure helps to conserve moisture and prevent evaporation from the soil.
- Organic manure (humus) has a buffering effect on the soil; that is, it balances the acid-base condition of the soil or soil pH.
- It prevents erosion because it improves the structure of the soil and reduces the speed of the run-off.
- Organic matter increases water-holding capacity of soil.
- It increases the rate of water percolation through clay soil.
- It increases the activities of soil micro-organism.
- It improves the aeration of the soil.
All these advantages of organic manure help to maintain or even promote the availability of nutrients in the soil.
Organic manure refers to the decayed plant and animal products which have been carefully prepared to supply nutrients to plants or crops.
Types of Organic manure: There are three types of organic manure. These are:
- Green manure.
- Farm-yard manure.
- Compost manure.
- Green Manure: Green manure is formed from leguminous crops and other fresh plants which are ploughed into the soil while they are still growing. The plants have to be ploughed in when they are very young, i.e ., before flowering stage so that they will decay rapidly to release the nutrients to the soil. The crops which are suitable for green manure include legumes like mucuna, cowpea, centrosema, calopogonium, pueraria and grasses.
- Farm-Yard Manure: Farm-yard manure is a combination of animal wastes such as animal dung or feaces, urine and animal bedding which collectively undergo series of decomposition before the manure is applied to the soil. Poultry birds, generally, produce manure of a higher nutrient quality than larger animals. Large animals like cattle feed on grasses, which are high in fibre content which cannot easily decompose. Pig, goat and sheep also produce manure which are rich in nutrients. Farm-yard manure must be allowed to complete its decomposition before it can be applied to the soil, otherwise, the heat produced during decomposition would burn the roots of crops.
Compost Manure: Compost manure is the type of manure formed as a result of the rottening down of plants and animal remains in heaps or pits before the residue is applied to the soil.
There are two ways of preparing compost manure. These are:
- Pit Method: This is suitable for areas where rainfall is low such as in the savanna areas.
- Stack Method: This is suitable for areas where rainfall is high such as in the south. Both methods require the same materials and duration.
The Pit Method
Size of the Pit: The size of the pit will depend on the quantity of compost needed, but a dimension of 180cm x 120cm x 60cm could be appropriate.
Materials Required: Three pits (A,B,C) are dug. Grasses and legumes to form the compost, ash or urine – to remove traces of acidity, animal dung-to introduce bacteria of decomposition, and a little water – to provide a moist environment for the agent for decomposition.
Procedure: In pit A, a layer of grass and legume is put at the bottom. Then a layer of animal dung is added followed by a thin layer of ash. Water is further sprinkled on the materials to make it moist but not wet. The whole process is repeated. Layers are added in this manner until the pit is filled up. Cover each layer up and place a long stick called a tester at the side.
Check if the operation is successful, by feeling the tester. If the tester is hot, you can continue, but if cold, it means the operation is a failure and the whole process has to be repeated.
After two weeks, the materials are turned with a shovel and packed into pit B. This turning provides air for the compost so that the bacteria continue to work on the decaying materials. Pit A is filled with fresh materials. After another two weeks, pit B is turned into C and A turned into B. This process continues until the last pit is reached. Compost prepared in this manner can be applied directly to the soil but planting cannot be done; otherwise, the roots of the seedlings may be burnt.
c) Bush Fallowing: Bush fallowing is the practice in which farmlands are left to lie fallow after one or two years of cultivation. The purpose of this is to allow nutrients to revert to the soil. When a piece of land is placed on continuous cropping, year after year, the nutrients are completely used up from the soil without replacement.
Fallowing as a Method of Replenishing Soil Nutrients
- When land is allowed to fallow, plant food has time to form in the soil as a result of humus accumulation.
- The system helps to check some plant diseases because when farms are left to fallow for many years, disease organisms lose their hosts and die.
- Fallowing helps to check erosion.
- Land becomes relatively cheap to replenish with lost nutrients as farmers may not need fertilizers.
- Fallowing helps to dislodge pest from farm plots due to the absence of hosts during fallowing.
- Fallowing also improves soil physical properties like soil structure, texture, etc.
- Fallowing improves the activities of soil flora and fauna.
The period of allowing the land to rest in order to re-grow back to bush and rebuild the used-up nutrients is called fallow period.
d) Cover Cropping
Cover cropping is a process of planting certain plants mainly to cover soil surface. By so doing, the nutrients are conserved in the soil.
Some commonly used cover crops are Mucuna utilis, Pueraria phaseoloides, Centrosema pubescens, Crotalaria juncea, Calopoginium mucunoides and cowpea. All these are leguminous crops.
Cover Cropping as a Method of Replenishing Soil Nutrients
Cover cropping has many advantages,
especially in the recovering of lost nutrients to the soil. These advantages are:
- It reduces erosive capacity of moving water, thus checking erosion.
- It adds organic matter to the soil when dead.
- Leguminous cover crops increase the nitrogen level of the soil through the bacteria in the root nodules.
- It increases water – holding capacity of the soil. That is, it influences soil water supply.
- It improves soil development.
- Cover cropping suppresses weeds.
- It protects the surface of the soil from direct heat of the sun.
- It reduces leaching of the soil nutrients.
- It acts as wind breaks and cuts down wind movement.
- It provides a suitable cover for soil organisms, e.g ., earthworms and millipede.
Liming refers to the process whereby calcium or magnesium containing compounds are added to the soil to reduce soil acidity.
Some common liming materials are:
- Limestone – CaCO3.
- Quicklime – CaO.
- Slaked lime – Ca(OH).
- Basic slag – Ca2(OHSiO3)2.
- Dolomite or calcite – CaCO3MgCO3.
- Calcium bicarbonate – Ca(HCO3)
- Gypsum – CaSO4.2H,0
- Wood ash – Ca(PO4),SiO4.
Generally, most crops perform well between pH range of 6 – 8.
Uses of Lime in Agriculture
Application of lime helps to maintain soil fertility in the following ways:
- It helps to reduce or neutralise soil acidity.
- Lime application helps to increase the activities of soil living organisms.
- Liming makes nutrients like calcium and phosphorus more readily available for tissue development.
- Liming also improves the soil structure.
- It also increases the availability of phosphates and molybdenum ions.
- Lime increases the rate of water percolation in clay soil.
- It reduces the toxicity of dissolved copper and manganese in the soil.
- It serves as additional sources of calcium for bone development in fish.
- It reduces turbidity in fish ponds through flocculation property.
f) Application of Inorganic Fertilizers: Fertilizers are chemical substances, generally in the form of powder, granules, pellets or crystals which can be added to the soil to increase its fertility. The use of fertilizers is the surest way of replenishing lost nutrients from the soil and increase the productivity of crops.
Uses of Fertilizers in Agriculture
- Fertilizers help to increase the yield of crops.
- They improve the structure of the soil.
- They increase the activities of soil organisms.
- They also improve soil aeration.
- They increase the fertility of the soil.
- They increase the growth of planktons in fish ponds.
Types of Fertilizers
Fertilizers are grouped into two major classes based on their composition:
1) Straight or Single Fertilizers: These fertilizers contain only one element, Such element may be nitrogen, phosphorus and potassium.
i) Fertilizers which are rich in nitrogen are called nitrogen fertilizers and examples include:
- Ammonium sulphate – (NH4)2SO4 21%N, 24%S.
- Ammonium nitrate – NH4NO3 33.5%N.
- Urea – CO(NH2)2 46%N.
- Sodium nitrate – NaNO3 16%N.
- Potassium nitrate.
- Anhydrous ammonia.
- Ammonia liquor.
- Calcium ammonium nitrate.
- Diammonium phosphate.
ii) Fertilizers which are rich in potassium are called potassium fertilizer and example is muriate of potash – KCl 50% K2O.
iii) Fertilizers which are rich in phosphorus are called phosphorus fertilizers and examples include:
- Single super phosphate – CaHPO4 43.7%P.
- Triple super phosphate – 45%.
- Rock phosphate.
- Basic slag.
- Dicalcium phosphate.
- Diammonium phosphate.
- Guano (poultry/bird droppings).
2) Complete or Mixed or Compound Fertilizers: These are fertilizers which contain two or more nutrients. A good example is N.P.K. fertilizer, which contains nitrogen, phosphorus and potassium. Compound fertilizer can exist in various combinations such as 15-15-15, 20-5-10 and 5-10-5. 15-15-15 grade would have a 1:1:1 ratio of N: P2O5:K2O. 15.15.15. means equal amount of nitrogen, phosphorus and potassium.
Methods of Fertilizer Application
Different methods are used to apply fertilizers to the soil. These include:
- Broadcasting: This is the spreading of fertilizers evenly on the ground before they are ploughed into the soil.
- Ring Method: A circular hole is made round the crop and fertilizer is placed inside the hole after which the hole is covered. The circular hole must not come too close to the plant, otherwise, fertilizer will touch the plant and burn it.
- Row or Side Placement: Make a hole a few centimetres from each plant and apply a teaspoonful of fertilizer per plant in each hole, then cover it up with soil.
- Top Dressing: This refers to the second application of fertilizer several weeks after the first dose had been applied. The second dose is to supplement the first application.
- Folial Spray: This is the method in which some trace elements or micro-nutrients are dissolved in water and sprayed on the crop directly. The leaves are capable of absorbing the nutrients directly into their body.
WAYS IN WHICH SOIL NUTRIENTS CAN BE LOST
a) Crop Removal
- Nutrients are removed from the soil by crops for growth, development and production.
- When the crops are harvested, the nutrients contained in the plants are never returned to the soil.
- The rapid removal of nutrients from the soil by continuous cropping completely deprives the soil of such nutrients.
- Heavy rainfall causes the washing or carrying away of top soil which is rich in plant nutrients.
- Top soil can also be blown away by winds, resulting in nutrient reduction in the soil.
- This is the removal of nutrients from the top soil to the inner parts of the soil beyond the reach of the roots of plant.
- It results in the loss of nutrients such as calcium, magnesium, potassium from the top soil in solution.
- It also results in the accumulation of aluminium and hydrogen ions which become acidic and toxic to plants.
d) pH: The degree of acidity or alkalinity of the soil affects the availability of nutrients both in the soil and also to plants.
- A low pH (high acidity) will encourage the disintegration of clay minerals like calcium, iron and aluminium ions which are leached away from the soil.
- At high pH (high alkalinity), calcium and magnesium ions accumulate in the soil which affects the growth of plants.
- A low pH also reduces the activities of soil living organisms which aid the decomposition of organic matter.
e) Excess of Other Nutrients
- The presence of certain element(s) in high concentration may prevent the absorption or utilisation of other elements.
- The concentration of nitrogen and phosphorus in the soil results in the non-availability of potassium.
- This condition results in retarded growth, low yield or even death of the plant.
f) Oxidation and Reduction of Organic Materials
- Some compounds such as ammonium radicals are oxidised to gaseous ammonia.
- Nitrates are also reduced to molecular nitrogen or oxides of nitrogen by denitrifying bacteria.
- These products i.e, ammonium radicals and nitrates which escape into the atmosphere in form of gases make the soil become poorer in nutrients.