By Dave of Darlington. The third of four articles about the nitrogen cycle.

Article 1 Myths of organic farming. Article 2 Not-so-nice nitrogen. Article 4 Saying no to too much nitrogen

In the second article in this series [1] I described how organic (including vegan-organic) farming and growing can cause serious pollution of the environment as a result of nitrogen losses from the soil. In this article I want to discuss some ways in which those losses, which are to some extent unavoidable, can at least be minimised.

A major source of nitrogen loss that I mentioned is leguminous green manure crops. They tend to have a lot of nitrate around their roots, which, especially in wet conditions, is susceptible to leaching and to denitrification to nitrous oxide. These losses may be extremely high when the legume is being grown as a pure stand. In a previous article [2] I stressed the importance, for quite different reasons, of including a non-leguminous plant, such as a grass, in a green manure. Among other benefits the grass will take up any excess nitrate in the soil around the legumes and so will reduce the rate of nitrogen losses. The more grass there is in the mixture, the less the losses will be. It is best to plan for at least 50% grass. If you have no experience of doing this, your seed merchant will probably advise you on what seed rates to use to achieve this sort of mixture of plants.

Another potentially big source of nitrogen loss is the incorporation into the soil of plant material of any kind, including crop residues, green manures and cover crops. This may lead to the release of large quantities of nitrogen compounds from the material and if, as is usually the case, the soil remains bare or only sparsely vegetated for a few weeks after the ploughing or digging, the danger of nitrogen loss is immense. However, the actual amount of nitrogen lost depends very much on the conditions that exist at the time of the incorporation. If the soil is warm and moist, the rate at which nitrate is released into the soil from the incorporated material will be greater than if it is cool and dry. Also, if the ploughing or digging is followed by heavy rain, this will also exacerbate the nitrogen loss. So autumn is clearly a bad time to do this operation. Spring cultivation is preferable, but is also not without its problems, which I will mention shortly.

Sow soon

But first, the post-harvest period is always a critical one, especially for a crop that is harvested late in the year. The plant residues will decay, releasing nitrogen, and, apart from a few weeds, there is no plant cover to take that nitrogen up, so the leaching risk is high. It is therefore important to sow another crop as quickly as possible, which may be either an overwintering crop to be harvested in the following year or simply a cover crop, the sole function of which is to protect the soil and mop up excess nitrogen and other nutrients. This needs to be established and be growing fast before the onset of the autumn/winter rains, so the earlier it is sown the better (preferably in August). In temperate climates October is already too late for effective leaching reduction.

But even with an early sowing there will always be a gap between the harvesting of one crop and the establishment of the next. As already mentioned, during this period the soil is bare or only sparsely vegetated, so there is a significant danger of nitrate leaching. One way to get round this problem is relay sowing, that is, sowing the next crop before the first crop is harvested. A common example of this is the undersowing of a cover crop into a cereal such as wheat. In such a case the cover crop is already growing strongly when the cereal is harvested, so it can make a much earlier start with the task of protecting the soil and taking up excess nutrients.

I mentioned earlier that spring incorporation of green manures, although preferable to autumn, was still not without its problems. Again the main problem is the gap between the ploughing/digging of the soil and the establishment of the following crop. This is not nearly so serious as it is in the autumn, because the soil is colder, so plant residue decay and nitrogen release take place more slowly, but in a warm wet spring (or early summer) there could be a significant leaching risk. Once more it is important to get the following crop sown as quickly as possible after the cultivation.

Another way of reducing the rate of decay of a green manure, and thus limiting the nitrogen supply and the leaching risk, is to allow the green manure to grow on to a point where the stems are quite woody, since woody material decays much more slowly than fresh green material. An even better way to slow down the release of nitrogen into the soil would be to mix the green manure with woody material before incorporating it into the soil, since then the rate of decay could be controlled more accurately. Success with either of these approaches would depend on careful selection of the species and the sowing date of the green manure and also careful selection of the woody material. It might be necessary to experiment a few times to get it right. (I have never tested this in practice.)

Other ways to reduce losses

All these nitrogen losses can be greatly reduced if, instead of incorporating the green manure or cover crop into the soil, we simply cut it and mulch it on the soil surface. In this case there will be negligible nitrate leaching or nitrous oxide emissions. However, there will still be some loss of nitrogen in the form of ammonia volatilisation [3]. The rate of ammonia loss will be greater, the higher the temperature, moisture and nitrogen content of the mulch material, but will normally not amount to more than about 5% of the total nitrogen in the mulch.

The decay of a mulch is much slower than that of plant material that has been turned into the soil, which gives the following crop more time to develop to a point where it can start taking up the nitrogen that is released into the soil from the plant residues. However, in dry conditions the decay of the mulch could be too slow, thus reducing the supply of nitrogen to the following crop. A possible solution to this problem (which again I have never actually tried out in practice) is, from time to time, to break up the mulch layer between the crop rows and mix it with the surface soil a bit, using a hoe or other suitable implement. This would speed up the decay of the mulch a little and increase the nutrient supply to the crop.

Ultimately the best answer to the problem of nitrogen loss in organic farming and growing may be bi-cropping, in which the crop and the legume are grown together in a mixture. In this case some of the nitrogen fixed by the legume is made directly available to the accompanying crop. No incorporation of the legume into the soil is necessary, as, after the harvest of the crop, the legume (normally a perennial) is allowed to grow on until the following spring. Then, after it has been mown to reduce its competitiveness (at which stage there is a small risk of nitrogen loss by leaching), the following crop can be sown or planted into it. (This means that, contrary to what I recommended previously, the legume would, for part of the year, be growing in a pure stand, but this would be in the winter, when there is practically no nitrogen fixation going on in the root nodules of the clover and hence the danger of nitrogen loss is much less.) Bi-cropping is still a relatively new technique and, while it has been used successfully for cereals and brassicas, it may not work for every crop. There is some evidence that, like mulching, it can provoke serious slug damage, especially in small gardens and allotments.

Notes

[1] Not-so-nice nitrogen

[2] The article entitled Excuse Me! There’s a Grass in my Legume, in issue no. 16 of this magazine

[3] Ammonia (NH3) is continuously emitted in small amounts from all plant material, whether living or dead. It is not a greenhouse gas, but still causes some environmental damage in the form of acidification and eutrophication of soil and water.

This article appeared in Growing Green International magazine Num 24 (Winter 2009), p40.