**3. Fermentation in soil systems**

#### **3.1 The soil system**

The soil systems comprise of organic (5%), inorganic (45%), water (25%) and air (25%) components. These components are always in equilibrium. However, the soil serves as repository for all forms of waste, be it solid, organic and inorganic. The organic matter content of the soil is the portion that contains plant tissues and animal remains occurring at different decomposition stages. Majority of agriculturally productive soils contain about 5% content of organic matter.

Organic matter comprises of components in three major categories:


The first category comprises of microorganisms which break down residues of plants and animal remains as well as detritus. Humus represents the stable portion that is resistant to further degradation, hence, it is the final product of decomposition.

The first two categories of organic matter play a significant role in determining the fertility status of soil. This is because their breakdown account for the mineralized nutrients (such as NO3 − , PO4 2−, Ca2+, Mg2+ etc.) available for the nutrition of crop-plants. The humus component has little contribution to the fertility status of soil, and it is called "stable organic matter". However, it is still very relevant to soil fertility management because it enhances the structure and tilth of soil as well as providing surfaces for cation exchange. Humus is responsible for the soil's dark coloration. The most important way of adding organic matter to the soil system is by the process called composting.

#### **3.2 Composting**

The term "composting" is used worldwide with differing meanings. It was defined narrowly by some textbooks as aerobic form of decomposition mainly by aerobic or facultative microbes.

There are two categories of composting, consequent upon the mode of decomposition.

These categories include anaerobic composting, often referred to as *dry fermentation* or anaerobic digestion [46] and aerobic composting, often referred to as *aerobic fermentation* [47].

In anaerobic composting (also known as anaerobic digestion), decomposition occurs where oxygen is absent or in limited supply. During this process, anaerobic micro-organisms (mostly bacteria) play prominent roles in the breakdown of substrates resulting in production of intermediate by-products such as organic acids, methane, and other gases. As the bacteria "work," they generate biogas. Generally, different materials exhibit different digestibility and the more digestible the organic matter is, the more biogas is produced. In the absence of oxygen, the intermediate compounds accumulate and are not further metabolized. These (intermediate byproducts that are not fully oxidized) present some phytotoxic properties and very pungent odors. Anaerobic composting occurs under a low-temperature condition; hence, the process does not eliminate pathogens and seeds of weeds. The process also occurs at a slower pace than aerobic composting. These drawbacks offset the advantages of the process, such as: low energy requirement and no loss of nutrients during the process.

Aerobic fermentation or composting, on the other hand, occurs where oxygen is available. The facultative or aerobic bacteria are involved in the breakdown of substrates to release some plant nutrients, heat, biogases, and stable materials (humus). Although intermediate by-products are also produced during aerobic composting, these by-products are further oxidized to yield some useful ions or nutrients for plant growth with little or no danger of phytotoxicity, free of odor and leaving materials resistant to decomposition such as lignified materials (cellulose and hemi-cellulose). The resultant end-product is regarded as compost. The heat generated (due to high temperature regime) facilitates decomposition process within a very short time frame. Moreover, this process destroys many micro-organisms that can cause diseases to crop-plants and humans, as well as weed seeds, owing to the sufficiently high temperature. There are tendencies for loss of nutrients under this process, but it remains an efficient and a more useful method of composting than the anaerobic process for agricultural production and productivity.

Composting of waste is a form of *aerobic fermentation* mode of decomposing solid wastes under controlled conditions of pH, moisture contents, particle size, C/N ratios, etc. [47]. The process resulted in the formation of humus, usually regarded as compost, which serves as a source of nutrients to crop-plants. It involves the accumulation of organic waste in a form of heap. Usually, the waste materials can be shredded to manage the particle size. The accumulation is a multi-layered heap of organic wastes in a windrow, subjected to regular turning to ensure good aeration; and addition of water in order to regulate temperature (heat). Facultative bacteria and fungi feed on the substrate to ensure decomposition and release of ammonium ions and other nutrients required by crop-plants for growth and development.
