Agriculture Reference
In-Depth Information
Nutrient-Holding Capacity
While the CEC is one of our main indicators of fertility potential, there are minerals in
the soil that aren't cations. Most notably, phosphorus, sulfur and some forms of nitrogen
are often in the soil as negatively charged minerals called anions (pronounced “ann-eye-
on” with the emphasis on the “ann”). Anions are not held on sand, silt or clay, but they are
held by organic matter. Not only does humus have a spectacular CEC, but it holds onto an-
ions, too. More good news.
Organic matter not only holds onto cations and anions, but it's full of minerals that will
eventually become available when the organic matter decomposes. Leaves, grass, animals,
trees — it's all organic matter that will eventually be broken down. And let's not forget the
microbes. They consume minerals, too, which become available when they die. All of
these, the cation exchange sites, the anion exchange sites, the organic matter, and the mi-
crobes, contribute to the soil's overall nutrient-holding capacity.
It can be seen that while clay is the only rock-derived soil particle that can hold onto
cations, organic matter can hold onto cations and anions, and is actually composed partially
of minerals itself. Organic matter is already looking pretty important and we're only talking
about fertility. That being said, some clay is very helpful because it actually helps keep or-
ganic matter in the soil and stops it from leaching. So we are happy with some clay.
