Agriculture Reference
In-Depth Information
the garden or get a good rain storm. Clay particles generally have a negative (-)
charge, so they attract and hold positively (+) charged nutrients and non-nutrients.
Soil organic matter (SOM or just OM) has both positive and negative charges, so
it can hold on to both cations and anions.
Both the clay particles and the organic matter have negatively charged sites that
attract and hold positively charged particles. Cation Exchange Capacity is the
measure of how many negatively-charged sites are available in your soil.
The Cation Exchange Capacity of your soil could be likened to a bucket: some
soils are like a big bucket (high CEC), some are like a small bucket (low CEC).
Generally speaking, a sandy soil with little organic matter will have a very low
CEC while a clay soil with a lot of organic matter (as humus) will have a high CEC.
Organic matter (as humus) always has a high CEC; with clay soils, CEC depends
on the type of clay.
Base Saturation %
From the 1920s to the late 1940s, a great and largely un-sung hero of agriculture,
Dr. WilliamAlbrecht, did a lot of experimenting with different ratios of nutrient
cations, the Calcium, Magnesium, Potassium and Sodium mentioned above. He
and his associates, working at the University of MissouriAgricultural Experiment
Station, came to the conclusion that the strongest, healthiest, and most nutritious
crops were grown in a soil where the soil's CEC was saturated to about 65%
Calcium, 15% Magnesium, 4% Potassium, and 1% to 5% Sodium. (No, they don't
add to 100%; we'll get to that.) This ratio not only provided luxury levels of these
nutrients to the crop and to the soil life, but also strongly affected the soil texture
and pH.
The percentage of the CEC that a particular cation occupies is also known as the
base saturation percentage, or percent of base saturation, so another way of
describingAlbrecht's ideal ratio is that you want 65% base saturation of Calcium,
15% base saturation of Magnesium etc. Don't get too hung up on these
percentages; they are general guidelines and can vary quite a bit depending on
soil texture and other factors.
It's still a little-known fact that the Calcium to Magnesium ratio determines how
tight or loose a soil is. The more Calcium a soil has, the looser it is; the more
Magnesium, the tighter it is, up to a point. Other things being equal, a high
Calcium soil will have more Oxygen, drain more freely, and support more aerobic
breakdown of organic matter, while a high Magnesium soil will have less Oxygen,
tend to drain slowly, and organic matter will break down poorly if at all. In a soil
with Magnesium higher than Calcium, organic matter may ferment and produce
alcohol and even formaldehyde, both of which are preservatives. If you till up last
years corn stalks and they are still shiny and green, you may have a soil with an
inverted Calcium/Magnesium ratio. On the other hand, if you get the Calcium level
