Day by day, there are more and more people to be fed. Farming produces most of the food that is needed. In many parts of the world, such as the open grasslands of the United States of America, Russia, Canada, and Australia, huge areas are plowed and planted with crops, year after year. This kind of farming is called intensive agriculture.

Intensive agriculture produces huge harvests for a few years. Farmers grow the same crop every year. The crops need substances called nutrients found in the soil to grow. In time, the growing plants use up the soil’s nutrients. Fertilizers are added, but the soil still becomes worn out. Trees and hedgerows that divided the fields thin out and die. The soil is then more easily swept away by strong winds and rain.

Some pests feed on one particular crop. If the same crop is planted each year, the pests will thrive. They often carry diseases to the new crop. If different crops are planted each year, the pests are not able to feed, and the diseases begin to die out.