Soil Organic Carbon and Nutrient Retention

Soil organic carbon is the carbon contained in soil organic matter, including plant residues, microorganisms and decomposed material. It improves nutrient retention mainly through the properties of this organic matter, especially humus, rather than through carbon acting as a fertiliser by itself. These properties help soils hold nutrients, recycle them biologically and reduce their loss with eroded soil.

Holding nutrient ions on exchange sites

Decomposed organic matter has negatively charged surfaces that attract positively charged nutrient ions. This contributes to the soil's cation exchange capacity, meaning its ability to hold and exchange these ions.

• Exchange sites retain nutrients such as calcium, magnesium, potassium and ammonium, reducing their immediate removal by water moving through the soil.
• These nutrients are held in an exchangeable form, so they can replenish the soil solution as roots absorb nutrients.
• Organic matter can be especially important in sandy soils, which generally have less clay and a lower capacity to retain nutrient cations.

Storing and recycling nutrients biologically

Soil organic matter is also a reservoir of nutrients, particularly nitrogen, phosphorus and sulphur. Microorganisms regulate their movement between organic forms and forms that plants can absorb.

• Through mineralisation, decomposition releases nutrients from organic matter into inorganic forms that can become available to crops.
• Through immobilisation, microorganisms temporarily incorporate available nutrients into their biomass, retaining them in a biological pool rather than leaving them immediately exposed to loss.
• Residues with a high carbon-to-nitrogen ratio can temporarily reduce nitrogen availability because decomposer organisms draw nitrogen from the soil.

Protecting nutrient-rich soil and improving field management

Organic matter promotes soil aggregation, helping bind particles into stable units. Better structure and surface protection can reduce erosion and the associated removal of nutrient-rich topsoil.

• Organic matter also improves water-holding capacity, supporting root activity and nutrient uptake, particularly in coarse-textured soils.
• Retention is selective: nitrate is negatively charged and is not held by the negatively charged exchange sites that retain nutrient cations, so it remains vulnerable to leaching.
• Residue recycling, suitable organic manures, cover crops and erosion control can help maintain soil organic carbon, but they must accompany balanced nutrient application.
• More organic carbon does not automatically mean greater immediate nutrient availability; decomposition, soil moisture, temperature and crop demand determine when retained nutrients become available.