The Geosphere (including the soil)

The geosphere is the ultimate source of at least 14 essential mineral nutrients required by plants to complete their life-cycle. The ability of the geosphere to supply these nutrients depends on the elemental and mineralogical composition of the rock, the release rate of elements by weathering reactions, and the storage capacity of ion exchange and sorption pools. While weathering rates for certain elements may not keep pace with biological requirements on a short-term basis, weathering is an important long-term nutrient source that can provide a sustainable source of several geochemically derived nutrients. As soils become increasingly more weathered and primary minerals are depleted, the ability of the geosphere to provide nutrients is greatly diminished.

Primary minerals are converted to secondary minerals (generally clay minerals less than 2 µm diameter) through the chemical weathering process. The type of secondary minerals formed plays an important role in the physical and chemical properties of a soil, including nutrient storage capacity. Unlike primary minerals, secondary minerals are very reactive due to a greater surface area, charged surfaces, and the presence of hydroxylated surfaces. Secondary clay minerals can be broadly classified based on their structural properties. The general progression of secondary minerals with increasing weathering intensity follows:

Primary minerals ---> 2:1 layer silicates ---> 1:1 layer silicates ---> oxides/hydroxides.

Organic matter is incorporated into the geosphere through the activity of plants and animals. The discussion of the organic matter pool is included in the geosphere; however, organic matter transformations are regulated by processes previously discussed within the biosphere section. Since organic matter originates from biological tissues, it contains all the essential elements necessary for growth. These nutrients are released by the biologically mediated processes encompassing decomposition and mineralization. In addition to providing nutrients upon decomposition, soil organic matter imparts several favorable physical and chemical properties to soils. These properties include increased soil aggregation, water holding capacity, and cation exchange capacity. Depending on the degree of decomposition and the pH, organic matter may impart from 100-400 cmol_c/kg of cation exchange capacity. In highly weathered soils, organic matter may be the dominant source of cation exchange capacity.