The term agricultural chemical refers to any substance involved in the growth or utilization of any plant or animal of economic importance to humans. An agricultural chemical may be a natural product, such as urea, or a synthetic chemical, such as DDT. The agricultural chemicals now in use include fertilizers, pesticides, growth regulators, animal feed supplements, and raw materials for use in chemical processes.

In the broadest sense, agricultural chemicals can be divided into two large categories, those that promote the growth of a plant or animal and those that protect plants or animals. To the first group belong plant fertilizers and animal food supplements, and to the latter group belong pesticides, herbicides, animal vaccines, and antibiotics.

In order to stay healthy and grow normally, crops require a number of nutrients, some in relatively large quantities called macronutrients, and others in relatively small quantities called micronutrients...