Fertilizer is nutrients for plants - like vitamins and minerals that humans eat to become strong and healthy. Fertilizer can come from livestock like cows, pigs, sheep or poultry in the form of manure, which farmers apply to the land in amounts appropriate for that particular field. It can also come in granular or liquid form, containing nitrogen, potassium and phosphorous, and specially formulated to meet the needs of the crops. Farmers often have their soil analyzed by soil scientists to determine what nutrients their crops need and to ensure that they apply the right type and amount of fertilizers for the crops that they are growing.