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Garden with Insight v1.0 Help: cation exchange capacity

In the soil, tiny charged particles called micelles usually have many areas of negative charge (called sites) on their surfaces. Positively charged ions (cations) are drawn to these negative charge sites and stick to the clay particles (are adsorbed). In most soils, 99% of soil cations can be found attached to micelles (clay particles and organic matter) and 1% can be found in solution. Mineral cations in the soil (mainly Ca2+, Mg2+, K+ and Na+) maintain an equilibrium between adsorption to the negative sites and solution in the soil water. This equilibrium produces exchanges -- when one cation detaches from a site (leaving it free), another cation attaches to it. Therefore the negatively charged sites are called cation exchange sites.

The number of these sites per unit weight of dry soil is called the cation exchange capacity, or the capacity of the soil to hold cations. Because any cations loose in the soil solution are vulnerable to leaching as water flows out of the soil, a high cation exchange capacity is always desirable. Cation exchange sites act as a sort of mineral buffer for the soil, storing minerals important to plant and animal growth for long periods of time.

The attraction of cations to cation exchange sites is strongest for H+ ions (which make the soil acidic) and for polyvalent ions such as Ca2+ and Al3+. The weakest attraction is for monovalent ions such as K+. When ammonium nitrate fertilizers are added to the soil, the ammonium ions (NH4+) are strongly attracted to cation exchange sites because of their high valence (4). The ammonium ions displace many other cations which are then leached out of the soil and lost to plants. Some of the ammonium ions are converted to nitrate during nitrification (by aerobic soil bacteria); the process produces excess H+ ions which acidify the soil (causing earthworms and other soil organisms to die or desert the area). (For an excellent description of cation exchange capacity, see Widdowson's Towards Holistic Agriculture: A Scientific Approach.)

How it works:
plant nutrient uptake
auto pH control
increase in acidity due to fertilizer

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Updated: May 4, 1998. Questions/comments on site to
Copyright © 1998 Paul D. Fernhout & Cynthia F. Kurtz.