All our food comes from the soil . Some of us eat meat, but animals live on plants. If there are no plants, we will have no animals and meat. So the soil is necessary for us to live. The top of ground is usually covered with grass or other plants. Plants grow in soil, which has a dark color. This dark soil is humus. Dead leaves, dead plants and animals' waste make it, but this takes a long time. When the humus has been made, plants can grow well in it. All soil needs food. If we don't give it any, the plants will be weak. Animals' waste is the best food for the soil, but chemical fertilizers   are also very useful. The same crop   should not be grown in the same place every year; it is better to have a different crop. A change of crop and the use of a good fertilizer will keep the land in good condition. When the soil is dry, the wind blows it away. Sometimes heavy rain carries the humus down to a river. People should grow more and more trees and grass to stop wind from carrying the humus  away. It takes hundreds of years to make humus, and so we must save every bit of it. Without soil, where can we grow food? From the text, we know people live on   _  .
animal plant meat soil