Fertilizers are added to the soil because they help plants to grow. The environmental problems with fertilizers come if they are washed out of the soil by the rain and go into the ground water, into lakes or streams, or into the lagoon where they contribute to pollution. The fertilizers in the water encourage algae and other plants to grow, and these may become so thick that they start to rot and smell. They may also crowd out or shade out other valuable forms of life like corals. Nitrate fertilizers are also dangerous if they get into drinking water, because they may be turned into nitrites which can cause cancer.
Pesticides are by their very nature poisons for at least some kinds of life, often including people.
Pesticides can hurt the environment or poison people in many ways. They may be used without following the instructions very carefully, so that too much is used, or at the wrong time. They may be absorbed by the people who apply the pesticides, especially in the tropics where people do not know much about pesticides and do not like or even have protective clothing. They may be washed off the crop or field by the rain, or blown into villages or into the forest by the wind. They may be applied too close to harvest time and thus still be on the food when it is harvested and eaten.