Farmers now need to use ten times as much nitrogen fertilizer as they did 40 years ago. Often only big businesses can afford to pay for all this - which is why, in the USA at least, just a few big agribusinesses farm so much land. In the grain Cereals such as wheat, rice, maize, barley, oats and rye are the world's major source of food. They are grasses, and we usually eat their seeds, or grain, leaving the stalks and leaves to rot into animal feed called silage. There are more than 30 kinds altogether.
Spring wheat is planted in spring and harvested in early autumn.
Winter wheat is planted in autumn and harvested the next summer. In Asia, most people live on rice grown in flooded fields called paddies, where they can grow up to three crops a year. Maize is an important food for animals as well as humans. The farming year For crops to grow well, they must be planted and harvested at the right time of year. So the farmers follow seasons closely.