What is Soil Made Of?
Soil is made up of a mixture of organic material and minerals. The organic matter comes from dead plants and many of the minerals come from the rocks underground. These rocks, which are part of the lithosphere, are referred to as bedrock. Because the plants grow on top of the soil and the rocks are found underground, soil is made up of layers.
It takes thousands of years for rock to develop into soil, and hundreds of years for rich organic layers to build up. Soil is made of air, water, mineral particles, organic matter, and organisms. Half of soil is pore space. Generally, pores are about half filled with water and half air, though the proportion varies greatly depending on weather, plant water use, and soil texture. Most of the solid portion of soil is mineral particles. Organic matter may make up only 5% to 10% of the volume of soil (less than 5% of the weight), but it is critical in holding soil particles together, storing nutrients, and feeding soil organisms. Mineral particles are divided into three groups based on their size: clay, silt, and sand. Soil texture depends on the proportion of particles from each of these groups. (See the texture triangle below.) For example, a loam has similar proportions of all three classes of particles. A sandy loam is higher in sand; a clay loam is higher in clay. Soil structure, or h
