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The signs that promise 'Well-rotted manure' are generally a little economical with the truth. The manure was probably donated only a short time before. So the manure is left under cover, for three to six months, and then applied to the land. If unrotted manure is applied, then plants are likely to be harmed by the release of ammonia.
I follow this system, and allow the manure to stand for three to six months before applying it. It occurred to me, though, that manure, like so many other things, could have more functions than the ones usually credited to it - that is, making a very valuable contribution to the humus level of the soil and making a more modest contribution to the supply of plant nutrients. Harming plants, releasing toxic ammonia, could also be valuable. Now, I like to obtain manure that is as unrotted as possible, and to apply it as soon as possible, so that it can cause maximum harm. Since lime reacts with manure, releasing gases which include ammonia, it's recommended that application of manure and of lime should be widely spaced. (So, if manure is applied in autumn, lime should be applied in spring.) All the more reason to apply manure and lime at the same time, again to cause as much harm as possible - to weeds, that is.
There are two situations where this deliberate injury to weeds can be beneficial -
Whatever use is made of the manure, it rots underneath the fabric just as it rots in the usual manure heap. When the fabric is taken away, then we have an area clear of weeds, or with weeds which have been very much weakened (if, that is, the test beds provide evidence for this) and with manure in place which will contribute very significantly to humus levels and to soil fertility: an area which is ready for planting.
The system I use is designed to keep the concentration of ammonia underneath the black fabric as high as possible. The edges of the black fabric are in close contact with the soil, since the boards, supported by stakes, are pushed down fully. The page Bed and Board gives full information about this method of supporting the boards which surround the beds.
There may well be benefits in disease control as well as weed control. At the end of the growing season, when a greenhouse has been cleared of its crops, it's common for gardeners to burn sulphur candles inside the greenhouse, to kill pathogens. Ammonia released by fresh manure acts, it may be, as a fumigant, making disease less likely in the following season.
There may well be further benefits in conservation of nutrients. In the established system of storing manure under cover - corrugated iron or a plastic sheet - there are unavoidable losses of nitrogen to the air. In this regard, established methods could be described as wasteful: the avoidable loss of nitrogen may well need to be compensated by buying in fertlizer (in inorganic systems) or, in organic systems, a product such as dried blood (which may contain 10% nitrogen), which is expensive and not cost-effective. The system I propose is likely to minimize loss of nitrogen. Even if, after investigation, it's found that manure isn't of significant use as a weed-killer, it may well be that the technique of placing fresh manure under fabric has great advantages for conservation of nitrogen. Manure becomes, in effect, a natural counterpart of the inorganic products described as 'weed and feed.' If it's found that the system is effective in some or all of these ways - although it's certainly its effectiveness in eliminating weeds which interests me particularly - then elaborate experiments will be needed to answer further questions, such as these:
I don't give here my predictions as to the likely effects of ammonia on different kinds of weeds: annual, biennial and perennial. It can be predicted that, like the synthetic herbicide paraquat/diquat, ammonia will kill top growth, but ammonia solution, produced if water levels underneath the fabric are adequate, is likely to damage the roots of weeds as well, and may well make some contribution to the control of perennial weeds - although to weaken them, not kill them entirely.
The non-selective herbicide glufosinate-ammonium causes disruption of ammonium metabolism within the plant. The enzyme glutamine synthetase is inhibited and the detoxification of ammonia is prevented. There is an accumulation of ammonia, 'a powerful cellular poison,' in the words of the information from the manufacturer, Bayer. Simultaneously, photosynthesis is strongly inhibited, so that the plants die in a matter of days. In the technique proposed here there is, of course, no inhibition of glutamine synthetase, which can continue to detoxify ammonia, but the known toxic effects of ammonia in the absence of glufosinate-ammonium lead me to believe that an increased concentration of ammonia in the environment will have similar effects to that of glufosinate-ammonium - and that these will include inhibition of photosynthesis.
This page is more speculative than other pages in this site. I know that the equipment and the techniques I've described in other pages work well, because I've tested them thoroughly. The techniques I describe here - not just the 'new use' for manure, but suggestions for improving its established applications - haven't been tested to the same extent at all. It would be impossible for me to test them adequately with the resources available to me. I don't have available an extensive area which is weed-covered. I have available some instrumentation - for measuring temperature, humidity and pH, but not the analytical apparatus for measuring ammonia concentrations. There are numerous variables which could potentially make the technique successful or not and to investigate them all is impossible.
Large scale use of this system
If this technique is found to be effective on the small scale - the garden, allotment and smallholder scale - then further work will be needed to make it as effective as possible on the market gardening and agricultural scale.
From the very useful and very interesting 'Organic Weed Control: a Practical Guide' by Charles A Merfield (www.gardenorganic.org.uk/organicweeds/weed_management) "Sheeted materials...are usually laid by machine. Machinery has been developed that will raise the soil into beds and lay the plastic mulch, securing it at the edges.
"Biodegradable film mulches have been developed that adjust their degradation according to the soil and weather conditions. The polymers are strong enough to withstand mechanical laying. 'Biolene' a biodegradable black weed control mulch is available in several grades to provide a range of breakdown rates to suit all soil types and crop needs. One biodegradable polymer, referred to as 'P2' breaks down fully in the field to non-toxic residues under aerobic and anaerobic conditions. It can be composted at temperatures over 60C. In the field, UV in sunlight breaks it down gradually and makes it increasingly soluble through the growing season."
As regards depositing the manure, then of course, mechanical methods are very efficient, but 'in situ manuring' would also need to be examined - livestock being fenced in the areas to be cleared of weeds before putting down fabric. Some farm animals are much more useful than others in this regard. However, supplementary manure would almost always need to be applied. The integration of livestock and horticultural activity may be possible in some circumstances, not possible in others. Transportation of manure for use in the system would be cost effective in some circumstances but not in others.
Some consumers who buy organic produce may imagine that organic farming is almost effortless - after all, they believe, it's simply 'working with nature.' In fact, organic farmers can only succeed with a huge knowledge of the most diverse areas - from extensive mechanical knowledge to the subtle knowledge of pollinators, natural predators much else. Their sense of timing, their flair, their instincts, their versatility may well be tested to the limit. There are the obvious constraints of natural setbacks - the weather - and artificial setbacks - the market, legislation, finance. Effective weed control is not the least of their problems - in fact, it poses great problems for organic systems. I hope that this proposed system will be able to make a real contribution.
The use of urine in organic growing - less widespread and less significant than the use of manure, of course - does raise related issues. On the small scale, it's most often applied as an activator to the compost heap. The product tactfully described as 'Household Liquid Activator' consists of approximately 1 part urine to 3 parts water. (Lawrence D Hills, 'Organic Gardening,' Page 24. If urine is applied undiluted to beds, the plants will be scorched. However, as in the case of manure, this is an advantage, not a disadvantage, in killing weeds.
In the future, I think it will be useful to carry out investigations into large-scale separation of urine and faecal matter. The urine, so much less hazardous than faecal matter, is such a valuable source of nutrients - and so potentially useful as a herbicide - that to waste it is very inadvisable, when the pressure on resources is so great. Conservation should include conservation of materials which can contribute to soil fertility and which have other useful functions in growing.
