Soils with poor drainage or high water tables usually have a higher organic matter content than those which are well drained, because water excludes air from the soil mass. Since pore spacesare filled with either air or water, the amount of air in a soil at a particular time depends on the amount of water present in the pore spaces. Immediately after a rain, there is more water and less air in the pore spaces. Conversely, in dry periods, a soil contains more air and less water. Increasing organic matter content usually increases water-holding capacity, but adding undecomposed organic material reduces water capacity until the material has partially decomposed.
Many soils have two or more distinct layers or horizons.
When soil horizons are present , they usually differfrom one another in color, texture, consistency, and structure. In addition, there are usually considerable differences in chemical characteristics or composition. The surface and subsurface are usually the coarsest layers. The surface soil contains more organic matter than the other soil layers. Organic matter gives a gray, dark-brown, or black color to the surface horizon, the color imparted depending largely upon the amount of organic matter present.