Plant remains (including those that have passed through an animal and are excreted as manure) contain organic compounds: sugars, starches, proteins, carbohydrates, lignins, waxes, resins and organic acids. The process of organic matter decay in the soil begins with the decomposition of sugars and starches from carbohydrates which break down easily as saprophytes initially invade the dead plant, whilst the remaining cellulose breaks down more slowly. Proteins decompose into amino acids at a rate depending on Carbon:Nitrogen ratio. Organic acids break down rapidly, whilst fats, waxes, resins and lignins remain relatively unchanged for longer periods of time. The humus that is the end product of this process is thus a mixture of compounds and complex life chemicals of plant, animal or microbial origin which has many functions and benefits in the soil as outlined below;
Humification of leaf litter and formation of clay-humus complexes
Humus which is readily capable of further decomposition is referred to as effective or active humus. It is principally derived from sugars, starches and proteins and consists of simple organic (fulvic) acids. It is an excellent source of plant nutrients, but of little value regarding long term soil structure and tilth. Stable (or passive) humus consisting of humic acids, or humins, on the other hand, are so highly insoluble (or tightly bound to clay particles that they cannot be penetrated by microbes) that they are greatly resistant to further decomposition. Thus they add few readily available nutrients to the soil, but play an essential part in providing it's physical structure. Some very stable humus complexes have survived for thousands of years. Stable humus tends to originate from woodier plant materials, eg, cellulose and lignins.
Humus should not be thought of as 'dead'- rather it is the 'raw matter' of life- the transition stage between one life form and another. It is a part of a constant process of change and organic cycling, thus must be constantly replenished- for when we are removing prunings and crops for the kitchen we are depriving nature's cycle of potential humus. This is why we need to substitute compost and other sources of organic matter to maintain the fertility of our productive land.