Role of Soils

Both the physical and chemical structure of soil influences the ways that plants obtain minerals from it. Soil from an ecological standpoint is that part of the Earth's surface in which plants grow, including—the thin layer on the surface of rocks penetrated by mosses, as well as the rich, black loam underlying Iowa cornfields.

Soils are dynamic and ever-changing. They have a structure that both influences, and in turn is influenced by, the plants that they support. The weathering of rocks over time produces the soils from which plants extract water and all but one of the nutrients they need—carbon being the sole exception. Soils, in addition, provide anchorage for plant roots and thus a means of support for the aerial shoots. Air in the pore spaces of soils provides roots with oxygen for respiration and is a depository for the carbon dioxide released in the process. Though soils change constantly as materials are added and removed, in general they have five components: inorganic mineral particles, decaying organic matter, living organisms, air, and water. These five factors, mixed in almost infinite variety and proportions, give rise to different kinds of soils.

The rocks that produce soil contain mineral elements that, for the most part, are locked in a crystalline matrix and unavailable to plants until physical and chemical weathering loosen the chemical bonds. The nutrients must be released into the soil water before plant roots can absorb them. Those nutrients that become soluble often are leached out of the root zone before roots or soil organisms can pick them up. The positively charged ions ( cations) such as Na+, Ca2+, Mg2+, and K+, on the other hand, are readily adsorbed on the negatively charged surfaces of the soil colloids–and as easily removed. The CO2 released in respiration speeds cation exchange because it forms carbonic acid (H2CO3) in the soil water. The acid, in turn, splits into carbonate (HCO3−) and hydrogen ions (H+). It is the hydrogen ions that dislodge the cations from the colloids and thus make them available for use by plants and microorganisms. Thecation exchange capacity of a soil is an important determinant in how well (or poorly) plants grow.

Most of the negatively charged ions ( anions), e.g. nitrate (NO3−) and sulfate (SO42−), leach out because they are not attached to the colloids. Some anions, e.g. phosphate (H2PO4), adsorb to iron or aluminum compounds in the soil and ultimately become available to plants.