Soil

SOIL

Who knew that soil had much to do with geology? Soil is a lot more than dirt, as you will soon see. It is a mixture of minerals, water, air, and organic stuff forming where the geosphere meets the biosphere. Once weathering takes place, the bedrock becomes sediment and as long as the sediment persists, organisms dig into the mineral and make things out of atmospheric gases, water, and sedimentary minerals. This becomes organic soil.

Soil is a place where plants, animals, and microbes live and thrive. Humus is the organic part of soil; it is very rich in bioavailable nitrogen animals and plants can use. The main component of soil used by organisms is nitrogen. Animals and plants cannot use the nitrogen in the air but nitrogen-fixing bacteria will easily take nitrogen gas and make it bioavailable for us to use.

Interestingly, while nitrogen fixing bacteria are used to make ammonium, amino acids, and other biomolecules in living things, you also get ammonium from decomposing plants. This ammonia gets processed by nitrifying bacteria to make nitrites and nitrates, which are ions with nitrogen and oxygen in them. Finally, denitrifying bacteria takes nitrates and turn them into nitrogen gas again. This is one giant cycle called the nitrogen cycle.

Productive soils are those that have a lot of water and nutrients in them. These include the andosols, which are volcanic soils, and soils high in clay content. Soils with little nitrogen in them are not able to grow crops as easily.

How do we characterize soils? There are five major components we will look at. These are the minerology, the weathering, the topography, the climate, and the microorganisms found in the soil. Erosion will erode more rapidly on some soil types and not others. The content of any soil will also determine what will grow best in it or if anything much will grow at all.

Most of the well-formed soils of the world will have layers called horizons. If you ever see areas where the earth is deeply dug, you will see these horizons. There are names

and letters associated with each of these important layers. Figure 36 will help you see them visually:

Figure 36.

The O horizon is the organic layer. This is thin and contains plant parts that might be decaying, leaves, and twigs. These will decay into humus.

Next is the A horizon, which is topsoil. This is humus that has been mixed with minerals as sediment. This is rained on and has some leaching of soluble minerals and other