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