Soil contains air, water, and minerals as well as plant and animal matter, both living and dead. These soil components fall into two categories. In the first category are biotic factors—all the living and once-living things in soil, such as plants and insects. The second category consists of abiotic factors, which include all nonliving things—for example, minerals, water, and air. The most common minerals found in soil that support plant growth are phosphorus, and potassium and also, nitrogen gas. Other, less common minerals include calcium, magnesium, and sulfur. The biotic and abiotic factors in the soil are what make up the soil’s composition.

Soil composition is a mix of soil ingredients that varies from place to place. The Natural Resources Conservation Service (NRCS)—part of the U.S. Department of Agriculture—has compiled soil maps and data for 95 percent of the United States. The NRCS has found that each state has a “state soil” with a unique soil “recipe” that is specific to that state. These differing soils are the reason why there is such a wide variety of crops grown in the United States.

Consider the soils of three states: Hawai'i, Iowa, and Maine. Hawai'i’s deep, well-drained state soil contains volcanic ash that makes it perfect for growing sugar cane, as well as ginger roots, papaya, and macadamia nuts. Iowa, which is in Midwest region of the United States, has a state soil that is good for farming because it is made up of a thick layer of organic matter from the decomposition of prairie grasses. Corn and soybeans are the primary crops grown in these soils. The state soil of Maine, located in the northeastern part of the country, is made from materials left behind after local glaciers melted. This soil is perfect for growing trees—specifically, red spruce and balsam fir. Many of the trees being grown today in Maine are harvested for timber or for making paper.

Soil scientists conduct various tests on soils to learn about their composition. Soil testing can identify the amounts of biotic and abiotic factors in the soil. The results of these tests can also reveal if the soil has too much of a specific mineral or if it needs more nutrients to support plants. Scientists also measure other factors, such as the amount of water in the soil and how it varies over time—for instance, is the soil unusually wet or dry? The tests can also identify contaminants and heavy metal in the soil and determine the soil’s nitrogen content and pH level (acidity or alkalinity). All of these measurements can be used to determine the soil’s health.