7 - Soil resources

Summary

ON THE NATURE OF SOIL

Soils are formed in situ over long periods of time under the influence of climate and vegetation and they come to have vertical distributions (profiles) characteristic of their genesis. Inorganic materials are the major component of soils. These include partially weathered parent materials, secondary minerals, and dissolved salts. Other components are air, water, organic matter in various stages of decay (with the most reduced form being termed humus), and living organisms including plant roots. Typical agricultural soils have a bulk density (dry mass per unit volume) near 1.3 g cm^-3 (1300 kg m^-3 or 13 x 10⁶ kg ha^-1 m^-1. Organic matter ranges by mass from 1 to 5% in mineral soils to 80% or more in peaty soils. In typical mineral soils, water accounts for 0.1 to 0.4 of the soil volume but some organic and volcanic soils hold much more. Soil is much more than a single mixture of these components, however.

SOIL CHEMISTRY

Soil chemistry is dominated by the abundance of insoluble compounds of aluminum, silicon and calcium, and centers on interactions between solutions and solids. We begin with a review of several basic concepts essential in advanced work with soils and crops. Although we take a simpler approach in this book, familiarity with these concepts is important.

Solutions

Many of the ions in soil solutions are in equilibrium with sparingly soluble minerals and with ion-exchange complexes discussed later.