Global Warming Causes

Visualizing Soil Orders

Figure 7.21 The five factors of soil formation determine how the soil orders are distributed across the United States. Soil profiles of three soil orders from different parts of the country are shown. Each soil profile has soil horizons expressed differently.

Figure 7.21 The five factors of soil formation determine how the soil orders are distributed across the United States. Soil profiles of three soil orders from different parts of the country are shown. Each soil profile has soil horizons expressed differently.

(tl, tr, br) Photo courtesy of USDA Natural Resources Conservation Service, (bkgd)State Soil Geographic Database (STATSGO)/NRCS/USDA

(tl, tr, br) Photo courtesy of USDA Natural Resources Conservation Service, (bkgd)State Soil Geographic Database (STATSGO)/NRCS/USDA

USDA Soil Classification

USDA Soil Classification

100 90 80 70 60 50 40 30 20 10 0 Percent sand

Figure 7.22 A soil textural triangle is used to determine a soil's texture.

100 90 80 70 60 50 40 30 20 10 0 Percent sand

Figure 7.22 A soil textural triangle is used to determine a soil's texture.

Soil Texture

Particles of soil are classified according to size as clay, silt, or sand, with clay being the smallest and sand being the largest. The relative proportions of particle sizes determine a soil's texture, as shown in Figure 7.22. Soil texture affects its capacity to retain moisture and therefore its ability to support plant growth. Soil texture also varies with depth.

Soil Fertility

Soil fertility is the measure of how well a soil can support the growth of plants. Factors that affect soil fertility include the topography, availability of minerals and nutrients, the number of microorganisms present, the amount of precipitation available, and the level of acidity.

Conditions necessary for growth vary with plant species. Farmers use natural and commercially produced fertilizers to replace minerals and maintain soil fertility. Commercial fertilizers add nitrates, potassium, and phosphorus to soil. The planting of legumes, such as beans and clover, allows bacteria to grow on plant roots and replace nitrates in the soil. Pulverized limestone is often added to soil to reduce acidity and enhance crop growth.

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