Please see the following Power Point No Till and Water, by Mark Scarpitti.
I was recently asked to provide a simplified explanation on the importance of Cation Exchange Capacity (CEC) values. My Response: CEC is the ability of a soil to hold onto plant nutrients. The finer the particle size the higher CEC value, generally speaking. For example sand particles are course and visible to the naked eye, where as clay particles are fine and are not visible to the naked eye. So clay will have a higher CEC value than sand. It is obvious to most of us that sand cannot hold onto water or nutrients as well as a soil with a finer texture. Therefore, soils rich in Clay and Loam size particles are universally recognized as being better for farming - CEC explains this. The numerical value for CEC represents how much nutrition can be held by a given amount of soil. For example one pound of a clay loam soil with a CEC value of 20 will hold 4 times as many nutrients as a sandy soil with a CEC value of 5. 20/4 = 5 . It's all about math, for example Nitrogen in