4 - Soil phases: the organic solid phase

Summary

Organic matter is quantitatively a minor fraction of soil. It typically comprises 0.1 to 10% of the soil mass, and up to 40% in the case of Histosols. However, soil organic matter is quantitatively very important at the scale of the planet. The global soil carbon pool of 2500 Pg (1Pg = 10¹⁵g) includes about 1550 Pg of soil organic carbon (SOC) and 950 Pg of soil inorganic carbon (SIC). The soil C pool is 3.3 times the size of the atmospheric pool (760 Pg) and 4.5 times the size of the biotic pool (560 Pg) (Lal, 2004). Hence, the soil C reservoir could be used to control the composition of the atmosphere.

Despite its low relative concentrations in soil, organic matter has major qualitative importance because of its functions. Soil organic matter (SOM) contributes to soil properties and functions. The major, and long-recognized role of SOM is its contribution to soil chemical fertility. SOM is a reserve of nutrients such as N, P, S, which are its constitutive elements and are released with mineralization. Furthermore, SOM is charged and has a high cation exchange capacity (60 to 400 cmol(+) kg⁻¹); it thereby retains nutrient cations such as K⁺, Mg⁺⁺, Ca⁺⁺, Fe⁺⁺⁺ on its negative charges. Thus SOM ensures most of plant nutrition in natural ecosystems as well as in organic farming and low input cultivation systems. This role of SOM is less important in the case of intensive cultivation systems relying on mineral fertilizers.