17.9: Nutrients cycle from soil to organisms and back again.

Soil is an almost perpetual source of nutrients that are critical to a plant’s health and survival. But how do the nutrients get there? To answer that, we need to realize that soil is not just a homogeneous mound of dirt. Soil is a complex mixture of four distinct components (FIGURE 17-22): (1) minerals: inorganic particles, usually from the breakdown of rock; (2) organic materials: carbon-containing matter, usually from the decomposition of dead plants and animals; (3) water; and (4) air.

Minerals About half of the total volume of soil is inorganic materials. These materials come primarily from the weathering of rocks, which decay physically and chemically over time, releasing minerals. The inorganic particles are grouped into three sizes: sand, silt, and clay. As you may know from feeling sand at the beach, there’s plenty of air between the particles, so sand doesn’t hold water very well. That’s why it is so easy to grab handfuls of it and let the grains fall through your fingers. Silt particles are smaller, as you would feel if you grabbed a handful of soil from the bottom of a riverbed. Clay particles, the smallest of all, pack together very densely. Because minerals passing through the soil cling to clay, it is a valuable component of soil; clay holds the essential minerals in place for plants to absorb as needed. However, too much densely packed clay in soil can limit the supply of air available for absorption by plant roots, leading to poor plant growth.

The best soils have about equal amounts of each type of particle. The minerals present in soil determine the pH of the soil and can affect how well plants grow; they can even influence the color of flowers (FIGURE 17-23).

Organic Materials Leaves fall and animals die, but decomposition returns their chemicals to the soil, making reuptake by plant roots possible. Animal droppings are very important to plants, too, serving as highly concentrated sources of nitrogen, perhaps the most important of the essential plant nutrients. Called humus (pronounced HYOO-muhss), all these organic decay products make up about 1% to 5% of the soil (FIGURE 17-24). The humus also absorbs water and nutrients easily and can release them as needed. With too little humus, the soil may be deficient in nutrients. With too much, it may retain too much water.

Most of the decomposition of organic matter is carried out by huge populations of bacteria, fungi, insects, and earthworms that live in soil. Aristotle called earthworms “the intestines of the earth.” Passing soil through their guts as they feed on organic matter, earthworms increase tremendously the amount of nitrogen available for plants.

A very practical application of the decomposition of organic matter can be seen in the process of composting, used to decompose organic wastes—such as kitchen garbage, manure, yard clippings, and even sewage. In composting, bacteria metabolize the material and, in doing so, convert it into fertilizer (FIGURE 17-25). Compost heaps can speed organic breakdown and greatly reduce the volume of waste that cities must process and dispose of, while generating a product that can be distributed in gardens and croplands to help return important plant nutrients to the soil.

The growing populations of bacteria in compost generate large amounts of heat—a compost heap can reach temperatures above 60° C (140° F), which is why so much steam is coming out of the overturned earth pictured in Figure 17-25. The heat generated by compost heaps can kill many of the original composting organisms, which are replaced by more heat-tolerant species that are not harmful to humans. For many centuries, and up until the middle of the 20th century, the use of human feces, called “night soil,” as fertilizer was a common practice. In much of rural Japan and China, the purchase, collection, and distribution of night soil was a large industry. The night soil from wealthier villages even sold for higher prices; because these villagers had better diets, there were more nutrients (particularly nitrogen from protein) in their waste products. For sanitary reasons (and as a consequence of the increased availability of chemical fertilizers), however, the use of night soil has become significantly less common. Because human waste may carry pathogens, its use has been strongly discouraged in order to reduce the public health risk.

Water and Air Water and air, the final components of soil, fill the spaces between the particles of inorganic and organic matter. They account for about half of the total volume of soil.