Naturally occurring organic matter in soils
In natural landscapes untouched by cultivation soil the organic matter in soils is governed by the carbon cycle. Here plants photosynthesize and add organic material to the soil though their roots, fallen leaves and eventually when they die. This then rots and becomes part of the soil organic matter. The soil organic matter in turn rots and releases carbon dioxide that in turn is used by plants in photosynthesis. Animal matter too is returned to the soil - all animals ultimately feed on plants. To learn more about what is meant by organic matter, have a read of our page: Organic matter: what is it?
Under natural conditions in cool climates organic matter accumulates in natural soils to fairly high level where gain balances losses, but upon cultivation soil organic matter falls as man made vegetation usually supplies less organic residues then natural vegetation, and the effects of tillage (digging, raking, hoeing) lead to a higher rate of soil organic matter loss. Ultimately the soil organic matter can fall to levels where the soil function is impaired. Most soils in southern England have less than 2% organic matter, but in the rest of the British Isles 2-6% may be found.
Organic matter in soils binds other soil particles into aggregates that can be penetrated by roots and hold moisture, provides binding sites for nutrients so they are not washed out of the soil by rainfall and as it decays releases plant nutrients to feed plants. Typically this soil organic matter decays very slowly.
Soil organic matter is also present as free organic matter derived from plant or animal remains or from recently added manures and composts. This decays quickly but some of it will be converted to longer lasting forms bound to soil minerals.
Enriching garden soils with organic matter
Gardeners (and farmers) aim to manage the levels of soil organic matter to get acceptable plant growth, which will typically mean that organic matter levels should be 3-6%. The main way they do this is to add organic matter. Because the effects of organic matter are so pronounced it is feasible to add sufficient organic matter to improve soils in a way that would be very difficult to add, say clay to a sandy soil and vice versa. Because clay and sand are not very effective in small quantities enormous amounts would have to be mixed into the soil to make a real difference.
Natural soils contained 30-40% more organic matter than they now contain under cultivation. Good garden plants can be grown at lower levels of organic matter than found in virgin soils, but once organic matter levels fall to below 2% the impact can be severe. A fall in soil organic matter of 0.5% can reduce, for example, nutrient holding capacity by 4% of even fertile soils.
Happily there is no need to test soils as it is relatively easy to judge whether a soil is manageable and add more organic matter if you encounter difficulties in make seed beds, planting or if the soil dries out in summer. With experience it is quite feasible to maintain enough soil organic matter to garden, and there is no need to add more than the minimum required to do this. A soil that is light coloured and sets solid when wet or dry is likely to require organic matter.