Soil, the living skin of the land

Soil is a mixture. A handful has tiny pieces of rock and minerals, bits of dead plants and animals, air and water. Rock is broken into smaller and smaller pieces by weathering, such as by heat and cold, water and roots. Erosion carries the pieces away and mixes them with others. These tiny rock pieces are the nonliving, or inorganic, part. The organic part is the remains of dead plants and animals. When they decay, they become dark humus, which gives plants nutrients and helps soil hold water. In a healthy garden soil about half is solid (mostly rock pieces and a little humus) and about half is spaces called pores, filled with air or water. Soil forms very slowly, so it is a resource we must protect. Humus is not just dirt: it is a valuable part of soil.