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

Article 2 Not-so-nice nitrogen. Article 3 Hang on to your nitrogen. Article 4 Saying no to too much nitrogen

A few years ago Anthony Trewavas, Professor of Cell and Molecular Biology at Edinburgh University, published, in the prestigious scientific journal Nature, an article entitled Urban Myths of Organic Farming, in which he tried to discredit organic farming on the basis that, according to him, it did not promote any more biodiversity than conventional farming, that it used just as much energy as conventional farming and, most extraordinary of all, that trace quantities of toxic pesticides in our food were actually good for us!

Most people will probably regard these claims as absurd. But at the same time it has to be acknowledged that there really are some myths associated with organic farming, albeit different ones from those Prof. Trewavas suggested. One that is widely believed is that, to carry on organic farming, you have to keep animals and use their manure as a fertiliser. Readers of this magazine will not need any convincing of the falsehood of that one. But there are some supposed tenets of organic farming, the mythical quality of which is perhaps not quite so obvious. They are: that in organic farming we grow the crops in nature’s way, that green manures, unlike chemical fertilisers, are harmless to the environment and that in organic farming we feed the soil, from which the plants then get their nutrients, whereas in conventional farming they feed the plants directly. All these claims are very misleading and can fairly be described as myths.

Feeding the plants

Let us look first at the question of feeding the plants. To simplify the argument let us confine ourselves to considering only one plant nutrient, namely nitrogen, but much the same considerations apply to phosphorus too (although not to mineral nutrients like potassium and calcium.) In nature the cycling of nitrogen (and other nutrients) is very tightly controlled and each plant experiences strong competition from adjacent plants, as well as from soil microorganisms like bacteria and fungi, for the small amount of water-soluble nitrogen compounds that are available. In most cases the restricted nitrogen supply limits plant growth, which is one reason why plants grow more slowly in nature than in agricultural systems.

Because of the shortage of inorganic nitrogen in natural soils, plants in natural ecosystems are almost entirely dependent on the small amount of nitrogen that is supplied by the slow microbial decomposition of the organic matter stored in the soil (the humus). In this sense they can be said to feed from the soil. This is not the case with plants in agricultural systems (whether conventional or organic), which still get some nitrogen in that way from the soil organic matter, but draw a lot of their nitrogen from the relatively large quantity of inorganic nitrogen that is dissolved in the soil water and which originates directly from chemical fertilisers and green manures respectively.

For the sake of brevity I will use the term fertiliser here to denote both chemical fertilisers and organic ones. Then roughly what happens in an arable soil under a crop of some kind is that the applied fertiliser greatly increases the nitrogen content of the soil water. This excess of dissolved nitrogen, mostly in inorganic form (ammonia and nitrate), can do one of three things. Some of it (between 0 and 30%, depending on the weather and soil) is lost to the environment. It is either leached down into the sub-soil or it is emitted into the atmosphere as nitrous oxide and nitrogen gases as a result of bacterial nitrification and denitrification [1].

Mighty microbes

Microorganisms like this pea mildew can be pests but also breakdown plant material. Drawing: Chris Mackey

The nitrogen remaining in the soil water is split two ways. Some of it is consumed by the soil microorganisms and some is taken up by the roots of the crops. The quantitative balance between these two depends on many factors, including the root development of the crop plants, the amount of microbial activity in the soil and the availability of organic matter for the microorganisms to consume. This last point is an important one – the microorganisms cannot use nitrogen unless there is a corresponding source of carbon to complement it. An appropriate source of carbon might be, for example, a stubble or other form of residue from a previous crop. Given the availability of such a carbon source, the microorganisms will take up nitrogen and incorporate it into their own tissues. When they die, a little of this nitrogen will be absorbed into the soil’s store of organic matter (humus), but most of it will be released back into the soil water again as ammonia.

Besides the part consumed by the soil microorganisms, a substantial part of the dissolved nitrogen is absorbed by the plant roots. However, the crops do not get all the nitrogen they need in this way. The rest comes from the soil. There are other species of soil microorganisms that are continually feeding on the humus in the soil and, since this normally contains more nitrogen than they need, they release the surplus nitrogen into the soil water and it becomes available to the crop plants, which use it to make up the balance of their nitrogen requirements. So ultimately the crops get part of their nitrogen from the fertiliser and part from the humus. Thus there is a continual flow of nitrogen into, as well as out of, the humus. Much of the additional nitrogen going into the humus as microbial remains comes originally from the fertiliser, of whatever kind it may be.

‘Natural’ or not?

The processes I have described happen in all agricultural systems, whether conventional or organic. It makes no difference in principle whether the fertiliser is chemical or organic. So in both conventional and organic farming we are feeding both the plants and the soil. This is not to say that there is no difference at all between conventional and organic farming in respect of the nitrogen cycle, but the difference is only one of degree. Generally the nitrogen from organic fertilisers, such as green manures, contributes proportionately less to the needs of the crops and more to those of the soil microorganisms, compared with chemical fertilisers.

So it is difficult to claim that organic crops are growing in a natural way that is qualitatively different from conventional crops. We could, of course, force agricultural crops to mimic nature and feed entirely from the soil, by hardly providing them with any external nitrogen supply of any kind. The result of this, however, would be a serious decline in yield, which is a price that most of us would be unwilling to pay.

I need to end with the usual word of caution – that all the above is an over-simplification of the truth, which is actually far more complicated than this. For example, organic nitrogen compounds, such as amino-acids, can also be taken up by plants and microorganisms. But in agricultural systems nitrate is by far the most important form in which nitrogen is consumed.

Footnote

[1] I will consider this in more detail in the next article Not-so-nice nitrogen which will compare organic and conventional farming in relation to losses of nitrogen to the environment.

This article appeared in Growing Green International magazine Num 21 (Summer 2008), p38.