A fertilizer is any material of either synthetic or natural origin (apart from liming materials) that is applied to the soil or to plant tissues in order to supply one or more plant nutrients essential to the growth of plants. Many fertilizer sources exist, both natural and industrially produced. Management of soil fertility has been the preoccupation of farmers for thousands of years.
Fertilizers can also be said to be simple chemical compounds obtained through mining or chemical processing. They are used in the production of crops, therefore, it is important for us to study and understand these fertilizers.
LEARNING OBJECTIVES
By the end of this lesson, you should be able to:
- Explain the classification of inorganic fertilizers.
- Describe the characteristics of different compounds and straight fertilizers.
- Describe the different methods of fertilizer application.
Inorganic fertilizers can be classified based on the following:
- Nutrients contained, that is, straight or compound fertilizers.
- Effects on soil pH, that is acidic or neutral fertilizers.
- Mode of application, for instance, foliar and top-dressing fertilizers.
- Time of application, that is, planting and top-dressing fertilizers.
Based on the nutrients contained, inorganic fertilizers can be classified into straight or compound fertilizers. Let us look at these fertilizers.
STRAIGHT FERTILIZERS
A straight fertilizer is a fertilizer that contains only one of the primary macronutrients, that is, either Nitrogen (N), phosphorus (P), or potassium (K).
Depending on the nutrient they contain, straight fertilizers are classified as nitrogenous fertilizers, phosphatic fertilizers, and potassic fertilizers.
Nitrogenous fertilizers
These are fertilizers that contain nitrogen. They include sulphate of ammonia (SA), ammonium nitrate (AN), calcium ammonium nitrate (CAN), ammonium sulphate nitrate (ASN) and urea.
Characteristics of nitrogenous fertilizers
- Nitrogenous fertilizers are highly soluble in water.
- They have a scorching or burning effect on crops.
- They are easily leached and hence have a short residual effect.
- They are hygroscopic, that is, they absorb moisture from the atmosphere, making them cake easily.
- They are volatile, that is, they can easily change into gaseous form.
Application and storage of nitrogenous fertilizers
- Nitrogenous fertilizers should be applied on established crops so that the plant makes use of them before they get leached.
- Except for foliar fertilizers, they should not come into contact with any part of the plant, particularly the leaves, due to their scorching effect.
- They need to be applied frequently and in application, as they have a short residual effect.
- They should be stored under dry conditions to avoid caking.
- They should be applied in moist soils to avoid volatilization.
- They should be stored in plastic bags and not metallic containers as they corrode the latter.
Phosphatic fertilizers
These are fertilizers that contain phosphorus. Phosphatic fertilizers are commonly obtained by crushing rock phosphate (apatite). They include single superphosphate (SSP), double superphosphate (DSP), triple superphosphate (TSP), soda phosphate, and basic slag.
Characteristics of phosphatic fertilizers
The characteristics of phosphatic fertilizers include the following:
- They are less liable to leaching.
- They are slightly soluble in water.
- They have a slight scorching effect.
- They have a long residual effect in the soil.
- On dissolving in water, they react with elements in the soil and become fixed, that is, locked up into compounds that are less soluble.
Phosphatic fertilizers are applied at planting time. This is in order to encourage the early formation and development of roots. They also dissolve slowly and are less leached, hence stay longer in the soil to be used by the plants.
Potassic fertilizers
Potassic fertilizers contain potassium. They include potassium chloride or muriate of potash (KCL), potassium sulphate or sulphate of potash and potassium nitrate or nitrate of potash.
Characteristics of potassic fertilizers
- They are moderately soluble.
- They have a moderate scorching effect.
COMPOUND FERTILIZERS
A compound fertilizer is one that contains two or all of the primary macro-nutrients. Compound fertilizers include:
- nitrophos (20:20:0).
- monoammonium phosphate (MAP).
- diammonium phosphate (DAP).
Advantages of application of compound fertilizers
- Saves money and time.
- Mixture ensures improved storage properties and better handling.
Disadvantages of application of compound fertilizers
- They are expensive.
- They can be wasteful.
- The fertilizers may not have been mixed thoroughly leading to non-uniform distribution.
- Some fertilizers are incompatible.
Some of the major advantages of using inorganic fertilizers over organic include: they are fast-acting, and they are not bulky making them easy to use. However, these fertilizers have some disadvantages too, they include: they have a short residual effect, and they are unfriendly to the environment.
Methods of fertilizer application
There are several methods of applying fertilizers. A few of them are:
- Broadcasting. In broadcasting, the fertilizer is applied uniformly on the land surface and then ploughed into the soil before planting.
- Side dressing. The fertilizer is applied after crop emergence. Side dressing can be done through ring application or banding. Ring application involves placing the fertilizer around the crop. Banding is the placement of fertilizer in between the rows at a suitable distance from the crops.
- Foliar application. The fertilizer is applied on the foliage in solution form. The solution can be applied at a high concentration.
- Combined drilling or row placement. This involves drilling fertilizers together with seeds in the planting holes.
- Top-dressing. This is a supplementary application of fertilizers in order to supplement the earlier ones.
- Band placement. This is the placement of fertilizers in bands. This can be done through a hill or row placement. In hill placement, fertilizers in bands are placed close to the plant on one or both sides of the plant. Row placement is done for crops that are sown close together in a row, like maize. The fertilizer is applied in continous bands on one or both sides of the row. This is called row placement.
- Aerial application. In some areas, ground application of fertilizers is not practicable. Fertilizer solutions can be applied through aircraft mainly in forest lands and hilly areas.
- Fertigation. This is the application of fertilizers soluble in water through irrigation water. Nutrients are carried to the soil in solution form.
