THE MEANING OF “NUTRIENT AVAILABILITY”

Many different elements in soil are potentially available to plants, and the purpose of soil testing is to estimate them as accurately as possible.

Soil consists of three phases which exist in equilibrium – solid phase, liquid phase and gas phase. The most important equilibrium exists between the soil phase and the liquid phase (soil solution). Plants absorb nutrients from the soil solution. However, the amounts of nutrients in the soil solution are insufficient to sustain plant growth.

When nutrients are depleted in the soil solution, they are replenished from the solid phase of the soil. This occurs through reactions such as dissolution, desorption, oxidation, reduction, hydrolysis or microbial mineralization reactions.

Testing the soil solution alone will underestimate the amount of nutrients really available to the plant, while measuring the total amount of nutrients present in the soil will result in an overestimation of the nutrients available. This is because a large portion of them is strongly bound to soil particles or exists in a form which is not available to plants.

Therefore, to determine the elements that are available for plants in soil, the soil test must quantify a relationship between the solid phase and the soil solution.