The following are the functions of fertilizer nutrients in the soil and plant according to each fertilizer nutrient compositions in forms of Nitrogen (N), Phosphorus (P), and Potassium (K);

1. Nitrogen (N)

Nitrogen is the motor of plant growth. It makes up to 1 to 4 percent of dry matter of the plant. It is taken up from the soil in the form of nitrate (NO₃^–) or ammonium (NH₄⁺).

In the plant it combines with compounds produced by carbohydrate metabolism to form amino acids and proteins.

Being the essential constituent of proteins, it is involved in all the major process of plant development and yield formation.

A good supply of nitrogen for the plant is important also for the uptake of the other nutrients.

2. Phosphorus (P)

It constitutes 0.1 to 0.4 per cent of dry matter of the plant.

It plays a key role in the transfer of energy.

It is essential for photosynthesis and other chemical-physiological process in the plant.

It is indispensable for cell differentiation and for the development of the tissues, which form the growing points of the plant.

3. Potassium (K)

Potassium makes up 1 to 4 percent of the dry matter of the plant.

It activates more than 60 enzymes.

It plays a vital part in carbohydrate and protein synthesis.

Potassium improves the water regime of the plant and increases its tolerance to drought, frost and salinity.

Plants well supplied with K are also less affected by diseases.

4. Magnesium (Mg)

It is a central constituent of chlorophyll, the green pigment of the leaves which functions as acceptor of the energy from the sun, thus, 15 to 20 per cent of the magnesium found in plant is contained in the leaves.

Mg is also involved in enzyme reactions related to the energy transfer of the plant.

5. Sulphur (S)

It is an essential constituent of protein and also involved in the formation of chlorophyll.

In most plants it makes up to 0.2 to 0.3 per cent of dry matter.

6. Calcium (Ca)

Calcium is essential for root growth

It is a constituent of cell wall materials.

7. Micronutrients or Traceelements

These are iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), chlorine (Cl) and boron (B).

They are part of the key substances in plant growth and are comparable with vitamins in human nutrition. Being taken up in minute amounts, their range of optimal supply is very small.

Addition of Plant Nutrients to the Soil

Plant nutrients can be supplied to the soil by adding the following:

Organic manure;

Green manures and other crop residues;

Concentrated organic manures;

Commercial fertilizers;

Soil amendments.

8. Organic Fertilizers/Manures

Farm yard manure, compost, sludge, green manures and other bulky sources of organic matter are known as bulky organic manures. These manures supply plant nutrients in small quantities and organic matter in large quantities.

These manures have a direct effect on plant growth, on the humus content of the soil, so improving its physical properties, and on microbial activities in the soil.

8a. Farmyard manures

The term farmyard manure (FYM) refers to the refuse from all animals of the farm although as a general rule the bulk of this is produced by cattle. The richest and most concentrated manure is poultry manure which is particularly good for vegetable production.

Farm yard manure consists of two components- solids and liquids in a ratio of approximately 3:1. The solid portion is made up straw that has been used for bedding and dung.

Dung is mostly undigested food and urine is a fluid waste product. More than 50% of the organic matter in dung is in form of complex products, often of lignin and protein, which are similar to humus. FYM contain on the average 0.5%N, 0.25% P₂0₅, and 0.5% K₂0. Generally, 30% of N, 30% of the P₂0₅ and 50% of the K₂0₅ in farm manure are available to plants.

Importance of Farmyard Manure

Farmyard manure is source of nutrient especially nitrogen, potash and some trace elements.

It influences the physical properties of the soil.

Farmyard manure increases the humus content and consequently the water holding capacity of the soil.

It improves the structure of the soil by making it more granular, better aerated and better drained. The manure also tends to reduce soil compaction which is often associated with continuous cultivation.

8b. Compost Organic Matter

Compost is well-rotted vegetable matter which is prepared from farm and town refuse.

Compost is prepared in trenches of various sizes and shapes. The accumulated refuse is well mixed and then spread in the trench in a layer of about 0.3 m. this layer is then well moistened by sprinkling over it slurry of cow dung and water, or earth and water.

Subsequent layers of the same thickness of mixed refuse are then spread on the heap and moisten. After about three month it is now fully decomposed and should be taken out of the trenches formed into conical heaps above ground and covered with earth.

After one or two months, the compost will be ready for use. The N, P and K contents of farm compost are on the average 0.5%, 0.15%, 0.5%, respectively, while those of the town compost are 1.4%, 1.0%, 1.4%, respectively.

Advantages of composting organic matter

The carbon: nitrogen ratio is improved because carbon dioxide is released to the air by micro-organisms.

Improve the structure of the soil by making the soil friable, crumbly and easier to handle and work upon.

The heat generated may kill weed seeds and other pathogenic organism.

It is the cheapest source of organic manure.

8c. Green Manures

This is the practice of growing and ploughing in green crops to increase the organic matter content of the soil. Green manure crops are usually fast growing annual legumes and grasses.

They are usually incorporated in the soil when they are green and succulent. Crops which grow rapidly even on poor soils and produce an abundant mass of green leaves and tops can be used as a green manure crop.

Advantages of Green Manure

It increases the organic matter content of the soil.

It improves soil structure.

Makes phosphorus and certain trace elements available to plants.

Checks erosion and leaching.

Helps to control weeds by acting as a smother crop.

In-organic/Commercial fertilizers

What is a fertilizer? Any natural or manufactured material, which contains at least 5 percent of one or more of the three primary nutrients (N P K), can be called fertilizer. Industrially manufactured fertilizers are called mineral fertilizers. Fertilizer may contain one or more of the essential nutrients.

Those that contain only one of the major elements are described as single, simple or straight fertilisers. Those that contain two or more of the major elements are classified as mixed or compound fertilisers.

Nitrogen, phosphorous, and potassium are the main plant nutrients and these three provide the basis for the major groups of fertilisers.

With the rapid increase in population and rise in standard of living there is increasing demand for food and feed grains. To meet up with the food demand it is necessary to intensify field crop production. Achieving and sustaining high crop yield of desired quality is only possible through the use of commercial fertilisers.

Although there have been tremendous increases in fertiliser using in tropical Africa over the years, utilization is still on a very small scale relative to the total needs. There is a wide gap between the national requirements for fertilisers and their actual use by farmers.

Chemical Fertilizers

1. Nitrogenous fertilizers

The nitrogen in many straight and compound fertilizers is in the ammonium (NH₄ ions) form, but this is quickly changed by the bacteria in the soil to the nitrate (NO₃ ions) form.

Most crop plants such as cereals take up and respond to the NO₃ ions faster than to the NH₄ ions, but some crops, such as rice, potatoes and grasses, are equally responsive to both forms.

On the basis of the chemical form in which nitrogen is combined with other elements in a fertilizer, nitrogenous fertilizers may be classified into four groups:

2. Nitrate fertilizers

In these fertilizers, nitrogen is combined in nitrate (NO₃) form with other elements. Such fertilizers are sodium nitrate (N_aNo₃), having 16 percent N, calcium nitrate [Ca (NO₃)₂], having 15.5% N, and potassium nitrate (KNO₃), having 13.4% N and 44% K.

Nitrate fertilisers are quickly dissociated in the soil, releasing the nitrate ion for plant absorption. As such they are readily absorbed and utilised by the plants. The great mobility of the nitrate ions in the soil has the advantage that, even when applied to the surface of the soil, the nitrogen quickly reaches the root zone.

They are therefore very often used as side and top dressings. However, there is also the increased danger of leaching of these fertilisers.

All the nitrate fertilisers are basic in their residual effect on the soil and their continued use may reduce soil acidity.

3. Ammonium fertilizers

In these fertilizers, nitrogen is combined in ammonium (NH⁺₄) form with other elements examples of such fertilizers:

Ammonium sulphate [(NH₄)₂SO₄], having 20% N,

Ammonium phosphate (NH₄H₂PO₄) having 20% N and 20% P or 16% N and 20% P;

Ammonium chloride (NH₄Cl), having 24-26% N,

Anhydrous ammonia, having 82% N and

Aqueous ammonia having 28% N.

When added to the soil, the ammonium ion is temporarily retained by the colloidal fraction of the soil until it is nitrified. These fertilisers are much more resistant to loss by leaching because the ammonium ions are readily adsorbed on the colloidal complex of the soils. Most ammonium fertilisers have acidic residual effect on the soil.

4. Nitrate and ammonium fertilizers

These are fertilizers that contain nitrogen in both ammonium and nitrate forms. Examples of such fertilizers:

Ammonium nitrate (NH₄NO₃), having 32.5% N,

Ammonium sulphate nitrate (ASN)[ (NH₄)₂SO₄^.NH₄NO₃], having 26% N,

Calcium ammonium nitrate (CAN) [Ca (NH₄NO₃)₂], having 25% N.

These fertilizers are readily soluble in water and suitable for use under variety of soils and cropping conditions.

The nitrate nitrogen of these fertilizers are readily available to plants for rapid growth and the ammonium nitrogen resists leaching losses and can be utilized by the plants at a later stage. These fertilizers are acidic in their residual effect on the soils.

5. Amide fertilizers

These fertilizers are carbon compounds, and so are called organic fertilizers. Important fertilizers in this group are:

Urea [Ca(NH₂)₂], having 46% N and

Calcium cyanamide (CaCN₂), having 22% N.

These fertilisers are readily soluble in water and easily decomposed by micro-organisms in the soil. In the soil they are quickly changed into ammonical nitrogen and then to nitrate form.

General Recommendation on the Use of Nitrogen Fertilizers

For rice, it is recommended to use ammonia-forming fertilizers such as ammonium sulphate, ammonium chloride and urea. In case these fertilizers are not available, ammonium-nitrate fertilizers such as ammonium sulphate nitrate, ammonium nitrate and calcium ammonium nitrate should be used. For the rest of field crops, all nitrogenous fertilizers are equally effective.

Continuous use of ammoniacal or ammonium-forming fertilisers on acidic soil should be avoided as it tends to make the soil more acidic.

All nitrate fertilisers are best suited for side and top dressing.

Since they are easily leached, they should not be applied in large quantities in light sandy soils or during heavy rains.

The entire recommended dose of nitrogen should be applied in 2 or 3 splits.

6. Phosphorus fertilizers

Crop plants absorb phosphorus in the form of negatively charged ions .

Phosphorus fertilizers can be classified into three groups depending on the form in which phosphoric acid is combined with calcium.

Phosphorus fertilizers containing water-soluble phosphoric acid or monocalcium phosphate [Ca (H₂PO₄)₂]: such fertilisers are super phosphate, ordinary or single, having 16-18% P₂O₅; double super phosphate, having 32% P₂O₅; triple super phosphate, having 46-48% P₂O₅ and ammonium phosphate, having 20% N and 20% P₂O₅ or 16% N and 20% P₂O₅.

These fertilizers are quickly absorbed by the plants, since plants absorb phosphorus as H₂PO₄ ions. Water-soluble phosphoric acid is rapidly transformed in the soil into a water-insoluble form. As such there is no danger of loss of nutrients by leaching.

This group of fertilizers should not be used in neutral or alkaline soils and not inacidic soils. Under acidic condition, phosphoric acid is converted into monocalcium phosphate, and there is less chances of the phosphate being fixed as iron or aluminum phosphate.

Fertilizers containing citric-acid, soluble phosphoric acid or dicalcium phosphate [CaHPO₄]; such fertilisers are basic slag, containing 14- 18% P₂O₅: dicalcium phosphate, containing 34-39% P₂O₅ and rhenania phosphate, containing 25-76%. These fertilisers are particularly suitable for acidic soils.

Fertilizers containing insoluble phosphoric [Ca₃(PO₄)₂]: such phosphatic fertilisers are rock phosphate, having 20-40% P₂O₅: raw bonemeal, having 20-25% P₂O₅ and 3-4% N and steamed bone-meal, having 22% P₂O₅.

These fertilisers are well suited for strongly acidic soils or organic soils which require large quantities of phosphorus fertilisers to raise the soil fertility.

Principles of Effective Utilization of Phosphorus Fertilizers

Granular fertilizers with a high degree of water-solubility are more effective on acid and neutral soils than powdered fertilisers.

On acid and neutral soils, band application of powdered fertiliser with a high degree of water solubility will give better results than mixing the fertilisers with the soil.

Water soluble fertilisers give the greatest response when applied in band.

To get optimum response from addition of phosphorus fertilisers other nutrients must be in adequate quantities.

7. Potassium fertilizers

All potassium fertilisers consist essentially of potassium in combination with chloride, sulphate, or nitrate. Almost all potassium fertilisers are water soluble. The following are examples of potassium fertilisers:

Potassium chloride [KCl] or muriate of potash, having 60-63% K₂O.

Potassium sulphate [K₂SO₄], having 50 – 53 % K₂O₅ and 18% of sulphur.

Potassium-magnesium sulphate [K₂SO₄. MgSO₄] having 22% ^K2^O5.

Potassium nitrate [KNO₃], having 13% nitrogen and 44% K₂O_5.

Potassium metaphosphate [KPO₃] having 40% K₂O and 60% ^P2^O5.

Principles of Effective Utilization of Potassium Fertilizer

All potassium fertilizers are equally available to plants because all of them are readily soluble in water.

Potassium fertilizers containing sulphur, magnesium or sodium have some additional agronomic importance on some soil because of the presence of other elements.

Potassium fertilisers containing chlorine or sulphur should be used with caution as they may be injurious to some crops.

8. Compound fertilizers

Compound fertilizers supply two or three of the major plant nutrient elements (i.e. nitrogen, phosphorus, and potassium).

They are produced by mixing the straight fertilizers such as ammonium nitrate, ammonium phosphate and muriate of potassium or by more complex chemical processes. The chemical composition of compound fertilizers is usually given as the ratio of nitrogen, phosphorus and potassium expressed as elemental N, P₂O₅ and K₂O respectively.

A 15:20:10 compound fertiliser therefore contains 15% N, 20% phosphorus P₂O₅ and 10% potassium expressed as K₂O.

Advantages of Compound Fertilizers

The mixture is usually dry, fine and well mixed and can be applied by hand as well as through a fertilizer drill.

The mixture usually contains all major plant nutrients.

It saves the farmer time and labour.

It does not form lumps or deteriorate in any way if it is not used immediately.

Grades of Compound Fertilizer

Low grade mixtures are 6:12:6, 5:10:10, and 9:9:0 etc. and high grade mixtures are15:15:15, 20:20:20. Using high grade fertilizers have some advantages like:

Low cost per unit of plant nutrient.

Lower cost of transportation, labour, and storage.

Increased the speed of application in the field.

9. Slow release fertilizers

Slow or controlled release fertilizers contain a plant nutrient (usually nitrogen) in a form, which after application delays its availability for plant uptake significantly longer than a common fertilizer.

This effect is obtained either by coating a common (nitrogen or NPK) fertilizer with sulphur or with a semi-permeable polymer material or by special chemical nitrogen compound formulations.

Advantages of slow release fertilizer

Labour saving, instead of several split application only one for the whole growing periods.

Reduces toxicity to seedlings even with high application rates.

Disadvantages of slow release fertilizer

The cost per unit of nutrient is considerably high than that in common fertilizers.

10. Nitrification and urease inhibitors

Nitrification inhibitors are compounds which, when added to nitrogen fertilizers (containing the nitrogen in form of ammonia (NH₄ ⁺⁴) causes delay in the transformation of the ammonium-ion (NH ⁺) held by the adsorption complex into nitrite (NO₂) and further to nitrate (NO₃^–) through the activities of soil bacteria, thus preventing leaching of nitrate not taken up immediately by the crop.

Urease inhibitors are compounds that depress the transformation of the amide-N in urea into ammonium for about 10 to 12 days; thus preventing, or reducing, evaporation losses of ammonia to the air when the weather stays dry or the urea cannot be incorporated into the soil immediately after application.

Both nitrification and urease inhibitors are strongly mixed with the nitrogen fertilizers before spreading and then spread together in the mixture.

Functions of Fertilizer Nutrients in the Soil and Plants