Biochar and the Mechanisms of Nutrient Retention and Exchange in the Soil

Understanding how water soluble nutrients are retained and exchanged with plants enables you to support the long term fertility of your soil.

Photo by philip junior mail on Unsplash

Preface and Background

This is the second installment in a series of articles on biochar. If you are not familiar with biochar and the practice of co-composting biochar, please be sure to read the first article in this series, A Perspective on Terra Preta and Biochar.

Please pardon the digression from the previously announced article on the right way and the wrong way to do biomass energy. I wanted to address some background knowledge relevant to soil fertility and biochar that is not as widely known as it ought to be — the mechanism of nutrient retention and exchange in soil. I am specifically going to explain cation exchange capacity, CEC (cation is pronounced “cat ion”, not “cashun”) and anion exchange capacity, AEC, in a manner that is accessible to the layperson. CEC and AEC are two capacities which often show up in scientific literature and in news articles about soil science and soil fertility. High CEC and AEC are two of the quantifiable agronomic qualities of a fertile soil. Understanding what these are, why they are important, and how to to improve these capacities will help anyone who works with soil to reason soundly about soil amendments, including biochar, which I discuss toward the end of this article.

What Are Cations and Anions?

This is the layperson’s whirl-wind review of just enough chemistry to understand this article. Skip to the next section if you are familiar with this. Skip to the last paragraph of this section to simply read the definition of cations and anions.

All the stuff in the world is made of atoms, the units of the elements that make up all the matter in the world. Atoms have a nucleus which is positively charged, surrounded by electrons, which are negatively charged. In the case of these electrical charges, opposites attract. The designation of “positive” and “negative” are a reference to the number line in math, where positive numbers added to negative numbers of equal…