Organic matter and humus

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Organic matter and humus

It is very rare, if not impossible nowadays, to be able to purchase well rotted manure. Although farmyard manure will often look rotted it has probably just been well trodden by beasts. So if you can store your own it will pay dividends in the long run. However, if you do store it remember to cover it over. Piles of manure left to the elements are one of the biggest polluters emitting ammonia and other noxious gases. You will also lose a great deal of the nutrient content from an uncovered heap, both by the loss of ammonia (a valuable source of nitrogen) into the air and when nutrients are leached out by rain.

If you have to use fresh manure in the autumn, which is perfectly acceptable, do not forget to compensate for the loss of nitrogen from the soil, nitrogen taken up by the fungi that rot down the straw and fibrous material in the manure. Add, in the spring, sulphate of ammonia or, if you have an acid soil, nitro-chalk. The latter is particularly good for brassicas because of its calcium content. Nitro-chalk is also often better on a clay soil as sulphate of ammonia makes many clays greasy.

And manure can be used prior to sowing root crops such as carrots. Is the idea that it cannot, or should not, be used before sowing roots yet another myth handed down the generations?

Roots will fork if they meet any resistance, such as a stone or a lump of cold airless manure. I get round this problem by rotavating the ground well. Root forking is then very rare, even though I have used fresh manure. So, too, is carrot fly. Is there any connection? Cabbage root fly and carrot fly are prevalent in land that has not been manured, I think there is a connection, particularly with their predators.

Bulky organic manure by itself has no magic properties and farmyard manure can be over 80% water. So the NPK content is often negligible - in fact as a food farm yard manure is best ignored. But what it breaks down into is magic, that dark black stuff we call humus.

Humus, a sweet smelling material that is the end product of a long decomposition process, is the final form of soil organic matter. Gardeners often assume that the final material in their compost heap is humus, but this is not strictly true, for the organic matter, made up from plant and animal debris, can still break down further. In appearance, humus is a black material composed of tiny remains of dead plants and soil organisms that have completely lost their structure.

Humus is very important in maintaining soil structure and in providing a nutrient reservoir for plants and soil organisms. Its colloidal nature improves the texture of the soil and, a point which is increasingly important with the dry summers that we are experiencing, its water retaining capacity. It is humus that is the principle substance enabling soil to hold moisture. But nothing is ever entirely simple - despite its manifold benefits humus in excessive quantities can actually lock up trace elements.

How often did old gardeners say "You must add lime to release the goodness"? Too much humus makes the soil too acid and lime will be needed -although few of us will suffer from too much humus in the soil! I have at the moment access to limitless supplies of 20 year old horse manure so I do not have to find room on my allotment to store it. However, if you are not so fortunate, then don't think storing it is wasting good growing ground, it isn't. As the saying goes 'The answer lies in the soil".

This article originally appeared in the Members Bulletin, the journal of the National Vegetable Society, which is sent quarterly to members. You can Join the National Vegetable Society here

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