If you’re thinking of farming, GFM writer Ellen Polishuk’s new book Start Your Farm is a great place to start. The excerpt below is a good explanation of the chemical properties of the soil for new and experienced growers alike.
Cation exchange capacity
The best measure of a soil’s ability to hold nutrients is the cation exchange capacity (CEC). A CEC determination is part of any basic report from a soil testing lab. In the soil, many plant nutrients take the form of cations—positively charged ions (e.g., calcium, magnesium, and potassium). Silt, clay, and humus particles, conversely, are mostly negatively charged, which attract and hold those useful cations. Anions—negatively charged ions—are either not held at all (as in the case of some forms of nitrogen, boron, and sulfur) or held through complex chemical mechanisms (as in the case of phosphorus). These anions, which are quite mobile, are nutrients that should be added on an annual basis.