What is the importance organic matter in soil?

Humus

Humus is the ultimate breakdown product of organic matter. Organic matter is processed into humus by soil organisms. The quantity and quality of humus produced in the soil varies depending on the nature of the raw organic matter (quantity, nutrient content, cellulose content etc), soil conditions (temperature, moisture, pH, etc), and which soil organisms are dominant.

The larger soil organisms shred raw material, and their own droppings and bodies become food for other organisms, in an ecosystem as diverse and interconnected as any tropical rainforest.

Beneficial fungi live in close proximity to roots, and form symbiotic relationships with roots. Bacteria are sixty times more concentrated immediately around roots.

Rhizobium (nitrogen fixing) bacteria and mycorrhizal fungi grow partly inside plant roots. As in above-ground biodiversity, it is not just the number of different species that matters, but the interactions between them.

Long-term humus may exist for thousands of years in the soil. It is concentrated carbon, complexed with many different minerals. This is what is referred to as the carbon sink in ours soils.

Organic growers add nutrients in a natural form, so that they are easily assimilated by soil biology, without the negative effects of water-soluble synthetic fertilisers.

Natural forms include:

• organic matter
• compost
• crushed rocks (rockdust, rock phosphate, granite dust, gypsum etc)
• biochar other additives that have not been acidified.
• Look for certified organic products whenever possible.

Animal manures

Not all animal manures are the same, and fresh manure is a no-go. Only use well-rotted manures. These will not smell and are safe to apply around plants. Liquid manure ‘tea’ is a popular garden fertiliser. If not done well there is a risk of leaves and roots burning though, through an over-supply of nitrogen. The best organic gardeners make their preparations from pre-composted manures to avoid any burning issues.

Seaweed

Seaweed makes an excellent mulch or compost ingredient. Don’t worry about salt unless you have a very salty soil and low rainfall. Seaweed does not have a high nitrogen phosphorus or potassium content, but is an excellent source of a wide range of all other nutrients, and seaweed extracts are a good plant tonic, during periods of stress.

Compost

The best addition to any organic garden is well made compost, and compost will benefit every soil type. Compost works as a fertiliser, a source of soil organisms and as a soil conditioner. The best compost is dark in colour and ‘homogenised’ - the original ingredients are not discernible. Finished compost is safe to dig into soil. Use less well-developed composts only as mulch.

Gypsum

Gypsum is excellent for improving clay soils. Use only natural mined gypsum.

Biochar

Charcoal produced under low oxygen levels, biochar mimics many ancient soils providing a honeycomb structure to improve pore space, water holding capacity, air and micro-organism activity.

Green manures

Green manures are crops grown specifically for turning back into the soil, to provide humus. They also prevent excess nitrates leaching from soil, maintain soil cover, prevent erosion, help to control weeds, and provide an extra rotation crop to control plant diseases.

Many species are suitable for using as green manure, depending on the local conditions. Use any plant that provides a large quantity of humus-forming material (lots of soft leaf and stem), in the required time.

Turn green manures in or slash them onto the soil surface before they reach maturity, preferably while still actively growing.