ABSTRACT

At the end of Chapter 2, humic matter was addressed as an integral part of soil humus. It was the purpose then to explain it as the alkali-soluble fraction of soil humus, and resolve the controversy that the extracted substances were identical to soil humus, which was the general idea held in the past. This chapter will discuss the concepts of humic matter as a chemical compound or as a mixture of several types of humic substances with different chemical compositions. The intention is to focus now on the idea or general understanding of what, in fact, humic matter is all about, or what it is when one is talking about humic acid. Since it was isolated more than a century ago for the first time from soil humus and named humic acid, its identity and concept were surrounded by a cloud of controversy and debates, which have not abated even with our present knowledge in humic acid science. Although the understanding of this organic substance has changed with time, slowly at the start and somewhat abruptly toward the end of the twentieth century, no satisfactory agreement has been reached about a concept that is not controversial and truly representative of the compound called humic acid, or better humic matter. The change from one concept to another was not in a sense of a sequential evolutionary process; in other words, the new concept is not built from the older one, but it was created more because of the lingering opposing views. Today, several concepts on humic matter are available and it depends on the particular concept what one is talking about when referring to “humic acid.” Traditionally, humic substances have been considered as either natural or artificial compounds produced during extraction (Aiken et al., 1985; Stevenson, 1994; Frimmel et al., 2002; Tan, 2003a).