The chemical substances supplied to the crops to increase their productivity and crop yield are called fertilizers. They contain the essential nutrients required by the plants to grow- nitrogen, potassium, and phosphorus. Fertilizers are chemical substances supplied to the crops to increase their productivity. These are used by the farmers daily to increase the crop yield. The fertilizers contain the essential nutrients required by the plants, including . They enhance the water retention capacity of the soil and also increase its fertility.

Pros

• increase plants’ tolerance towards pests. This has
• reduced reliance on insecticides and herbicides, thereby, producing healthier crops.
• Consequently, diseases have reduced, providing aesthetic value to the crops.
• improve the water holding capacity of the plants and increase root depth.
• The potassium content present in the fertilizers strengthens the straws and stalks of the plants.
• The phosphorus present in the fertilizers helps in the faster development of roots and formation of seeds in the plants.
• Nitrogen in the fertilizers enhances the growth of the plants

Cons

• They are expensive.
• The ingredients in the fertilizers are toxic to the skin and respiratory system.
• Excessive use of fertilizers damages the plants and reduces soil fertility.
• Leaching occurs and the fertilizers reach the rivers causing eutrophication.
• Long term use reduces the microbial activity and disturbs the pH of the soil.

Eutrophication

Eutrophication is the gradual increase in the concentration of phosphorus, nitrogen, and other plant nutrients in an aquatic ecosystem.  These substances enter the ecosystem primarily by runoff( runoff includes not only the waters that travel over the land surface and through channels to reach a stream but also interflow, the water that infiltrates the soil surface and travels by means of gravity) from land that carries debris and products of the reproduction and death of terrestrial organisms. Rainfall washes off the fertilizers from field into water. The amount of organic material that can be broken down into nutrients increases thereby enriching the ecosystem. This creates suitable living conditions for algae and microbes to grow and proliferate. Algae often develop on the surface, preventing the light penetration and oxygen absorption necessary for underwater life. Eutrophic waters are often murky and may support fewer large animals, such as fish and birds, than non-eutrophic waters.

The phenomenon of lack of oxygen in body which leads to unconsciousness and death is called Asphyxiation. This poses as the main reason of mass killing of fish. According to reports, causes of natural fish kills are:

■ Lack of oxygen in the water (anoxia). The majority of natural kills are caused by this problem.
■ Naturally toxic water. Acidic water containing toxic levels of aluminium leached from acid-sulphate soils
has been identified on the floodplains of Magela Creek and Nourlangie Creek.
■ Disease, especially red spot disease, often kills fish but this only rarely results in a large number of deaths at
one time.

During the day, aquatic plants and algae produce oxygen as a by-product of photosynthesis, raising the
levels of oxygen in the water. At night, the oxygen is used up by respiration of all the aquatic biota, reaching a minimum level at dawn. The decay of dead plant material at the end of the Wet season can aggravate this effect. With high temperatures and declining water levels at the end of the Wet and the Dry season, there may be little or no oxygen left in the predawn period. This causes oxygen stress to the fish and can result in deaths

Since the chemical fertilizers adversely affect soil fertility, biofertilizers were brought into use. These are substances that contain living or latent cells, and even micro-organisms. They provide the soil with the necessary nutrients and microbes for the growth of the plants. They help the soil to retain its fertility. They are environment-friendly and also destroy pathogenic components responsible for causing disease in plants. Acetobacter and Rhizobium are widely used biofertilizers.