Chemical Fertilizers – Inorganic Fertilizer Manufacturers

Fertilizer requirements of cereal crops depend mainly on two things: the fertility status of the soil (after harvesting the previous crop) and the yield targets of the farmer. Soils of low fertility status require more fertilizer as do higher yield targets. At each harvest, nutrients taken up from the soil by the growing crop are exported from the field in the form of grain or stover (see Table 1), further depleting the productive capacity of the soil. Thus, arable soils require external nutrient replenishment in the form of fertilizers (organic and mineral). Nutrient mining and insufficient supply of plant nutrients are some of the major problems contributing to declining crop yields. Maize and wheat, like most field crops, require a good supply of primary nutrients (i.e. nitrogen – N, phosphorus – P, and potassium – K) with the addition of other essential elements (e.g. magnesium – Mg, sulfur – S, zinc – Zn, and boron – B) in smaller quantities where necessary.

How to choose the fertilizer

Fertilizer use in cereal systems aims at providing the crop with balanced proportions of essential nutrients when required. Crop growth is dependent on a sufficient supply of each nutrient, and yield is limited by the nutrient in shortest supply. To determine fertilizer needs for your crop, it is essential to know which nutrients are required for the desired target yield. Nitrogen is the most important nutrient in plant growth and is involved in all major processes of plant development and yield formation. Loss of green color is the most typical hunger symptom of N. Yellow discoloration of leaves from midrib backwards and premature death of lower leaves are also indicators of N deficiency. In the early stages of a plant growth, P is important in laying down the foundation for its productivity. P deficiency is accompanied by a marked reduction in plant growth and leaves develop a dark bluish-green and purple color. A K deficiency is accompanied by a weakening of the stalk, resulting in lodging and crop loss. Symptoms of K deficiency include drying along the tips and edges of lower leaves.

Figure 1. Careful fertilizer application is essential to prevent overuse and reduce the overall fertilizer costs.

The recommended fertilizer application rate depends on a number of factors:

• Crop variety and yield potential: Varieties differ in their response to fertilizers depending on their yield potential. Improved cereal varieties with high yield potential will require more nutrients in order to achieve their yield potential. Their yields will be significantly reduced if fertilizer rates are low.
• Crop and fertilizer management: Efficient use of applied nutrients depends on soil and crop management. Fertilizer application methods and timing affect nutrient availability. Timely weeding reduces competition for nutrients by weeds. Application of irrigation water (in terms of both amount and timing) influences nutrient movement in the soil and the uptake by crop.
• Soil type: Soil fertility is determined by soil type, which in turn is based upon its depth, organic matter content and texture. N, P and S reserves in inherently poor soils with low organic matter content, coarse texture and a history of continuous cropping for many years, are usually very low while there may be sufficient K for 50-75% of crop needs.
• Balance of other fertilizer elements: A sufficient supply of all nutrients (including micronutrients) is important for a good, healthy crop and the efficient use of each applied nutrient. If the nutrient is present in soil, but insufficient in quantity, the plant growth and uptake of other nutrients is limited.
• Weather conditions: Response of rainfed crops to fertilizer also depends on soil moisture. Highly variable rainfall patterns make optimum yield and fertilizer requirements difficult to predict, so fertilizer use is a risky investment. When drought risk is high, split applications of nitrogen fertilizers may be advisable, with adjustments throughout the season based upon evolving weather conditions. Generally P applied in the previous season retains some residual availability for the next crop since it is not easily leached.

Blanket fertilizer recommendation packages for different agro-ecological zones based on a ‘normal’ season’s yield potential are available from most national agricultural research institutions. However, many of these recommendations do not take into account the variations in soil type, different crop management practices, and weather conditions.

Timing and methods of fertilizer application

One of the most important aspects of fertilizer usage is to know when and how fertilizers should be applied. It depends primarily on the type of crop grown and the mobility of the particular nutrient applied to soil. With nutrients that are stored efficiently in soil (i.e. P, K, S), fertilizers can be broadcast annually at or before planting, or banded below the seed. P is immobile in soil and it should therefore be incorporated into the root zone (e.g. during sowing). N application should be timed to coincide with periods of peak demand and rates adjusted according to rainfall received during the season via split application (particularly in sandy soils) to reduce leaching losses.

Without a means of anticipating the nature of the upcoming season, the best a farmer can do is to apply fertilizer tailored for a typical season. To minimize losses, farmers can employ ‘response farming’ strategies where applications are conditional upon rainfall and are adjusted according to the evolution of the season. Application of nitrogen shall be split to at least two (heavy textured soils) or three (sandy soils) doses in a season. Nitrogen should not be applied if there is severe wilting.

Figure 2. Maintaining crop residues in the field contributes to natural cycle of nutrients and reduces the need for fertilizer.