The United States Department of Agriculture commissioned their Soil Survey Staff to develop a detailed worldwide taxonomy of the Earth's soils in the late 1960s. In 1975, they presented Soil Taxonomy, the second edition printed in 1999. This taxonomy classification system divides the earth's soils into 12 orders, and from there into multiple suborders which are constantly under scrutiny and debate. The four soil orders most able to sustain plant life are histosols, spodisols, oxisols and entisols.
Histosols are defined as soils composed of between 20 and 30 percent organic matter. They are common in marshes and wetlands where decaying plant and animal matter compress to form materials we most closely associate with peat. They have low weight-bearing capacity, provide fertile ground for wetland plants and are often dried and burned as fuels. Histosols are divided into folist, wassist, fibrist, saprist and hemist suborders.
Spodosols have high acidity with abnormal subsurface deposits of humus, a mix of minerals characterized by large amounts of aluminum and iron. This type soil has poor nutrient content and so typically hosts coniferous forests, which are able to survive under such conditions. Spodosols are divided into aquod, gelod, cryod, humod and orthod suborders.
Oxisols sorely lack nutrients, though they possess high quantities of oxidized iron. This would encompass soils native to subtropical regions throughout the world. They are productive because of the introduction of phosphates and lime by the excrement and decaying bodies of native wildlife. Oxisols are divided into aquox, torrox, ustox, perox and udox suborders.
Entisols have undergone almost no change from their projected origin. Their content is no different than it was when the earth cooled. This order is also used as a catch all for those discovered soils that do not fit the criteria of the 11 other established orders, consequently making up almost 12.3 percent of the earth's surface land. Entisols are found in rocky, volcanically stable areas such as the valleys of the Rocky Mountain range. They are divided into wassent, aquent, arent, psamment, fluvent and orthent suborders.