Farmers use fertilizers to provide nutrients to the crops that are being grown. With the repeated growth of crops on the same land, and not enough time left between the cropping cycles, the soil cannot regain the nutrients that have been utilized naturally.
An excess amount of fertilizer is used by farmers as it is sometimes difficult for the farmers, without expert knowledge, to determine how much of each nutrient is actually required by the plants and how much would actually be available for the plants; some of the fertilizer may flow out of the fields when the crops are watered, etc. If an excess amount is used, it is believed that even if the plants cannot use all the fertilizer applied there would be a sufficient amount for them to get all their required nutrients from.
Using excess fertilizer is not good for the economy as fertilizer production uses a lot of natural resources and fuel. The excess fertilizer applied to the soil is washed off from the fields and ends up polluting water bodies around the fields, leads to the excessive growth of weeds and harms many species of animals that cannot withstand the chemicals used.