What are the main organic inputs?

Soil organic matter covers all the living and dead organisms contained within the soil, of which dead plant material is the commonest.

 

What are plants made from?

When plant and animal residues are reduced to simple chemicals, the term ‘mineralization’ is used’.

Cellulose and hemicellulose

Most plant tissue consists of cellulose (15-50%) and hemicellulose (10-30%). Plant walls are made from a combination of cellulose fibres that are encrusted with hemicelluloses. Both compounds are made up of sugar molecules.

Proteins and amino acids

Proteins (5%) and amino acids (5%) consist of carbon and nitrogen compounds. Nitrogen is essential to all living organisms. Plant material that is rich in protein therefore offers a valuable food source for soil micro-organisms.

Lignin

This is the most resistant compound found in plant material. Plant material is 15% lignin; whereas woody plant tissues are about 25-30% lignin. This is broken down by fungi.

Fats and waxes

The dry-matter weight of fats and waxes in deciduous plant material is 1-5%; whereas coniferous plant litter may be 20-25%.
 

How does organic matter begin to accumulate in soil?

Rock fragments alone lack the ability to supply adequate quantities of water and nutrients. However, certain bacteria, fungi and plant species have evolved to live in these conditions and are called ‘primary colonizers’. Lichen can fix nitrogen from the atmosphere. Lichens are a mutually beneficial association between algae and fungi. The algae obtain carbon and in some cases nitrogen from the atmosphere using a combination of photosynthesis and nitrogen-fixation. Once their own carbon and nitrogen requirements have been met, surplus nutrients are then passed to the fungi. The fungi attack the rock with organic acids. This releases minerals for the algae. When these organisms die, their tissues become combined with the mineral material, so forming the first organic-matter additions to the soil. As the amount of soil organic matter increases it becomes possible for other plants ( which extract their nitrogen from soil, rather than atmospheric sources) to colonize the site.