Cation Exchange Capacity
The role of Cation Exchange Capacity in Vineyard Soil
The Cation Exchange Capacity (CEC) of soil is its capability to hold nutrients. Generalizations can be made about soil types, but each type of soil will have a unique CEC. Just like anything in nature, a soil’s CEC is influenced by a multitude of variables, many of which are difficult if not impossible to measure.
In many ways, the quality of soil is indicated by its capacity to exchange cations. It is difficult to adjust this value and is indicative of the limitations of a particular soil. The CEC should be critically evaluated by a viticulturalist before planting vines.
The two component particles in soil are clay and humus. Together, they are called Organic Matter (OM); and the CEC of a soil is dependent the amount of clay or humus present.
Essentially, the clay and humus components of soil act as the nutrient reservoirs of a soil. For example, sandy soils have a very low CEC because they have very little organic matter. Contrary to this are heavy clay soils, which have a high OM content and thus a high Cation Exchange Capacity.
The mechanism of cation exchange is relatively simple; in the roots, one hydrogen ion (H+) is exchanged for a cation (ie: Calcium: Ca2+). If this looks foreign, don’t worry; understanding the mechanism is not as important as realizing the importance of cations to the vine and how the CEC of soil affects nutrient availability.
The degree to which cations are available for exchange is dependent on their valence charge, the soil’s pH, their tendency to become hydrated and the presence of other ions. Other factors include the presence of nitrogen fixing organisms, water content of the soil, soil drainage, ability of move in the soil and viticultural intervention.
A practical example of a cation exchange is the uptake of iron (Fe) from soil. Iron typically exists in the Fe3+ state in soil. However, roots cannot absorb iron in this form. Because the soil surrounding the roots is relatively acidic, Fe3+ is converted to Fe2+, which is readily assimilated into the root system.
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