Soil has rock pieces made small by wind and rain and sun and snow. The rocks are made of minerals and some of the minerals dissolve in water. Some of the minerals dissolved in water can be used as food by plants. Soil also has dead and living things (organic matter) in it. When a plant dies, it is eaten by animals including bacteria in the soil. When the bacteria are done eating, what is left behind is called humus. When the bacteria die the plant food (minerals) goes back into the soil. A food for plants is called a "plant nutrient". There are many kinds of plant nutrients. Soil has many empty spaces. Half of the soil is space. The spaces are filled with water and air. Water can enter the spaces in the soil. Plants drink the water and the minerals in the water. Plant roots need air to live. If plant roots do not have air, the roots will die. If the plant roots die, the plant will die. A plant that dies is eaten by bacteria and becomes plant nutrients again.