Agriculture Reference
In-Depth Information
Soil Exchange Capacity
To determine a soil's productive potential, start by determining the
total exchange capacity. That is the first thing we need to know. Then,
after we know the capacity of the soil to hold plant nutrients, there is an-
other portion of the test that goes hand-in-hand — the base saturation
percent. The reason it comes second is because you can never establish
the base saturations unless you know the exchange capacity. Base satura-
tion teaches us that in each soil there is a specific percentage of nutrients
that grows crops best, and that it is not the soil that receives the most
pounds per acres that always delivers the best crop.
The longer a farmer works with this program, the more this will be-
come plain. Anatomically, you use the pounds to get the percentages, and
percentages tell how a soil is going to perform. Yield and quality are de-
termined by the percentages, not the pounds. Thus our bottom line: base
saturation percentage tells us what the soil is composed of in terms of
cations — calcium, magnesium, potassium and sodium. It also tells us
that the availability of these nutrients to plants generally increases with
their percent saturation.
Magnesium and manganese are exceptions. A higher percent satura-
tion of magnesium in a soil does not necessarily mean that this nutrient is
more available. It is possible to get to the point where the percent of
magnesium — as it goes up — actually makes less magnesium available
to the plant. Here is the optimum percentage base saturation of cations
generally for most soils. The cation calcium should be 60 to 70% of the
saturation of the soil. In other words, 60% of the minerals attached to the
colloids should be calcium. That is on a light, sandy soil. On a heavy
clay soil, 70% would be optimum. The correct number for magnesium
should be between 10 and 20%. On a heavy clay soil, it would be better
at 10%. The ideal is for calcium and magnesium to total 80%. In high
clay soil, 70 + 10 = 80, and in a light sand, 60 + 20 = 80.
— Neal Kinsey and Charles Walters in Hands-On Agronomy
