AP Environmental Science



Lab: Soil Chemistry Lab Investigation (N-P-K)

We live in an era of increasing concern over the conservation and management of our renewable and non-renewable resources. We seldom think of soil in these terms, and yet improper development or natural erosion can devastate a landscape that nature took centuries to create. Similarly, the mineral elements in the soil that nourish growing plants can be depleted through repeated cycles of cultivation and harvest, resulting in a exhausted soil incapable of supporting healthy plant growth.

For this lab exercise you are taking on the role of a soil scientist. Soil scientists receive samples of soil from individuals in the community and analyze the soil samples to make a recommendation to the individual about the quality of their soil and what they can do to improve the fertility of the soil.

Background:

Soil texture describes the relative amounts of sand, silt, and clay in a mass of soil – it is one of the most important indicators of soil quality. The texture of soil determines how coarse or fine the soil is, its porosity and permeability, and the capacity to store nutrients and bind waste products. Soil is classified into three categories based on their grain size: sand, silt, and clay. (See Figure 1)

Sandy soils have excellent drainage and lots of air spaces, but they do not bind nutrients or support root growth. Sandy soils feel dry and gritty, and nutrients leach out quickly.

Clay soils, on the other hand, consist of microscopic particles that clump together and retain water. Soils with high clay content are easily waterlogged and have a tendency to exclude air and become anaerobic, killing off the living organisms that are a necessary part of healthy soil. Clay has a large surface area, however, and is chemically very active, binding and storing both mineral and organic nutrients. The most productive soils have a balance of sand, silt, and clay and are called loams or loamy soils. (“Rich” soils also contain high concentrations of organic matter.)

The United States Department of Agriculture (USDA) has identified 12 main textural classes of soil based on the percentages of clay, sand, and silt. The textural class is determined using a three-sided graph called the soil texture triangle (Figure 1). Each side of the triangle represents one of the soil separates on a scale from 0 to 100%. The graph is read by following the clay percent line parallel to the triangle base, the sand line parallel to the right side of the triangle, and the silt line parallel to the left side of the triangle. For example, follow the arrows in Figure 2: The asterisk marks soil containing 30% clay, 50% sand, and 20% silt, which is classified as sandy clay loam.

The pH of soil indicates whether the soil is acidic or basic. The pH scale is defined from 0 (very acidic) to 14 (highly basic). Soil pH influences the solubility and availability of soil nutrients, the viability of essential microorganisms, and the movement of toxic heavy metals into groundwater. A pH range between 6 and 7 is ideal for most plants. When soil is too acidic ( ................
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