Primary and secondary minerals

In weak to moderately weathered soils, a very large pool of nutrients remains locked within primary minerals. These nutrients are unavailable for biological uptake until weathering reactions release the nutrient elements to the soil solution. The elemental and mineralogical composition of igneous rocks is determined by the chemical composition of the magma from which it forms. Igneous rocks are classified based on their chemical composition from granite/rhyolite (acid igneous: silicon-rich, ferromagnesium-depleted) to gabbro/basalt (basic igneous: silicon-depleted, ferromagnesium-rich) (see Figure). With the exception of potassium, rocks of granite/rhyolite composition contain relatively low concentrations of essential plant nutrients. In comparison, rocks of gabbro/basalt composition contain much higher nutrient concentrations of iron, magnesium, calcium, and several other trace elements. Thus, soils formed on basalt are inherently more fertile than a comparable soil formed on granite, with the exception of K fertility.

For sedimentary and metamorphic rocks, the elemental and mineralogical composition is determined by the chemical and mineralogical composition of the sediments and rocks from which they form. Sandstones and shales are composed of preweathered materials (resistant primary minerals and stable secondary minerals) and often contain few weatherable primary minerals capable of releasing appreciable quantities of nutrients. Similarly, limestone is formed from the carbonate-rich shells of organisms and as a result have generally low concentrations of essential plant nutrients other than Ca and Mg. Ultrabasic rocks, such as peridote, dunite, and serpentine, have an especially strong influence on plant nutrition due to their extremely high concentrations of magnesium (30-45% MgO) and trace metals (e.g., Cr, Ni, Mn), and low concentrations of calcium (CaO = 0.5-3%). The high Mg/Ca ratio creates a severe nutrient imbalance that strongly influences the species of plants that can survive on soils formed in these rock types. Thus, the type of rock will have an extremely important affect on the nutrient supplying capacity of the geosphere and the ultimate productivity of a site.