Soil Organic Matter: The Key To Enhanced Soil Chemical Properties

LEANDRO O. VIERIRA II AND SAULO A.Q. DE CASTRO

BATON ROUGE, LOUSIANA

Soil organic matter improves soil chemical properties, which includes the increase of nutrient status, cation exchange capacity (CEC), and anion exchange capacity. Soil organic matter is also known for the slow release of nutrients to the plants, protects nutrients in available forms to the plants, and reduces nutrient leaching. 

Starting with the improvement of nutrient status, soil organic matter is a source of plant nutrients such as nitrogen (N), phosphorus (P), sulfur (S), and micronutrients. Nitrogen is the primary nutrient in soil organic matter, presenting about 1,000 lbs per 1% of soil organic matter in one acre, although most of it (~95%) is not immediately available to plants without microbial activity. Through mineralization, 2 to 3.5% of this N becomes available annually, providing 20 to 35 lbs of N per acre each year (Figure 1). Furthermore, soil organic matter boosts microbial activity, enhancing biological N fixation by free- living bacteria. Organic compounds within soil organic matter can increase phosphorus availability through mechanisms such as forming more soluble organophosphate complexes, displacing P retention sites with organic anions, and enhancing the mineralization of organic P into plant-available forms. Additionally, increased soil organic matter positively impacts S status by retaining it in a form that minimizes leaching while keeping it accessible for plant uptake. 

Another critical aspect is the increase in cation and anion exchange capacity. A significant portion of the soil's CEC is attributed to soil organic matter, which can hold positively charged elements (cations) and reduce their leaching. The relatively weak interactions between soil organic matter and cations ensure that these nutrients remain available to plants. Examples of such cations include nitrogen (in ammonium form, NH4+), potassium (K+), calcium (Ca2+), magnesium (Mg2+), zinc (Zn2+), and manganese (Mn2+). Despite soil organic matter being predominantly negatively charged, it can also retain negatively charged elements (anions). Anion exchange capacity is not measured in regular soil testing, but it refers to the ability of a soil to retain anions, reducing their leaching while keeping them accessible to plants. This is crucial because not all plant nutrients are positively charged, and soils typically have limited anion exchange capacity. Examples of anions include nitrogen (in nitrate form, NO3-) and sulfur (in sulfate form, SO42-). 

While most nutrients in soil organic matter are not immediately accessible to plants, soil organisms decompose organic matter, breaking down organic nutrient forms into simpler inorganic forms that plants can absorb. This mineralization process supplies a considerable amount of the N, P, S, and micronutrients required by plants. Moreover, some micronutrients, such as iron, zinc, and manganese, are often present in unavailable forms in certain soils. However, through chelation, these micronutrients remain in forms that can be absorbed by roots. Chelates are byproducts of organic matter decomposition or root exudates, which bind nutrients to multiple parts of their organic molecules. 

Last but not least, cation and anion exchange capacity of soil organic matter, along with the presence of chelates, reduces nutrient leaching. This is particularly important for enhancing fertilizer use efficiency and preventing water body contamination (eutrophication). By retaining nutrients in the soil, soil organic matter helps ensure that plants have a steady supply of essential elements, promoting healthier and more productive crops.  ∆

LEANDRO O. VIERIRA II AND SAULO A.Q. DE CASTRO: LSU AgCenter