Soils are formed in situ over long periods under the influences of climate and vegetation during which they develop characteristic vertical profiles. 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 called 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 × 106 kg (m depth)−1 ha−1]. Organic matter ranges by mass from 1 to 5% in mineral soils, and can be 80% or more in peaty soils. In typical mineral soils, water content at drained capacity accounts for 0.1 to 0.4 times the soil volume but some organic and volcanic soils hold much more. Understanding the physical, chemical, and biological properties of soils as media for plant growth provides insight into plant adaptations to soil conditions and crop management practices to overcome soil-related constraints.

Soil chemistry

We begin with a review of several concepts important to the study of soils and crops. Familiarity with these concepts is fundamental to crop ecology. Soil composition is dominated by an abundance of insoluble compounds of aluminum, silicon, and calcium, and soil chemistry centers on interactions between those solids and the water phase, called the soil solution.