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The very top layer consists of partially decayed organic debris like leaves. Beneath this is the topsoil. This horizon is usually dark in color because of humus- partially decayed organic matter - which has been incorporated in it from above. Humus gives the soil a loose texture that holds water and allows air to diffuse through it. Oxygen is essential to permit cellular respiration in plant roots, decay organisms, and other inhabitants of the soil.
The subsoil is usually lighter in color that topsoil and often contains an accumulation of inorganic nutrients.
Weathered parent material
This represents the first steps in the chemical breakdown of rock into soil. Often the weathered parent material is underlain by the parent material itself, although in some places it has been carried from another location by wind, water, or glaciers.
The chemical nature of the parent material, whether granite, limestone, or sandstone, for example, has a great influence on the fertility of the soil derived from it.
The Effect of Water on Soil
The Tropical Rain Forest
The lushness of the jungle biome is somewhat illusory. While productivity is high, the soils themselves tend to be of very poor quality. Because of the high rainfall, nutrients are quickly washed out of the topsoil unless they are incorporated in the forest plants. As plant and animal debris falls to the ground, it is quickly decomposed because of the warmth and moisture there. Thus minerals are found mainly in the forest plants, not in the soil. When the plants are removed and cultivation attempted, the soils quickly lose fertility. The situation is made worse by the lack of humus (the topsoil may be no thicker than 2 in. [= 5 cm]) and the high iron and aluminum content of most of these soils. Once exposed to the sun, these lateritic soils soon bake into a bricklike material that cannot be cultivated.
The most ancient (some might say primitive) way of working these soils is still the best:
- clearing a small area of jungle
- growing crops for only a year or two, and then
- abandoning the area to jungle once again.