Myths of organic farming
by Dave of Darlington
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.
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 denitrification1.
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.
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.
1 I will consider this in more detail in the next article, which will compare organic and conventional farming in relation to losses of nitrogen to the environment.
More myths of organic farming
(the second of four articles on the nitrogen cycle)
In the previous article in this series1 I drew attention to two of the myths associated with organic farming and gardening. In this one I will refer to two more such myths: firstly, that nitrogen losses in organic farming are necessarily much smaller than in conventional farming and, secondly, that organic farming solves the problem of nitrate leaching by using winter cover crops.
To appreciate the misleading nature of these statements we need to look in detail at why nitrogen losses take place. The main pre-condition for such losses is that the soil should be in a state of so-called nitrogen saturation. This has been defined as a state of the soil in which nitrification 2 exceeds immobilisation3. In other words, the supply of nitrate to the soil water exceeds microbial demand for it. This leaves an excess of inorganic nitrate in the soil and makes nitrogen loss very likely, unless the nitrogen is removed from the soil by some other agent, such as plants. But for plants to do that effectively, they need to have a relatively dense root system and to be actively growing and photosynthesising.
The above-mentioned state of nitrogen saturation hardly ever exists in nature. It is more characteristic of arable farming, where it is deliberately created by the application of fertilisers (chemical or organic) to the soil. The reason for this is to ensure a plentiful supply of nitrate to the crops and hence high yields. So, while in nature (apart from a very low background level of nitrous oxide emission4) there is practically no loss of nitrogen from the soil, in farming and growing (whether conventional or organic) the loss of nitrogen can be massive (up to 30% of the nitrogen applied as fertiliser).
The main causes of nitrogen loss from soil are nitrate leaching5 and nitrous oxide emission. Both processes are favoured by the above-mentioned state of nitrogen saturation, as well as by an ample supply of water. The water is required by the microorganisms that carry out the reactions, so in a completely dry soil the nitrogen losses should theoretically be nil. Water is also necessary (in the case of leaching) to transport the nitrate downwards and (in the case of nitrous oxide production by denitrification6) to fill the soil pores and create the necessary anaerobic conditions for the denitrifying bacteria.
It is clear, from the above considerations, that any agricultural operation that produces nitrogen saturation, in the absence of plants that can take up the excess nitrate, will give rise to relatively large nitrogen losses from the soil. One of the worst operations in this respect is the incorporation into the soil (by ploughing or digging) of cover crops and green manures, particularly when the soil is warm and wet and when the plants are fresh and young. These conditions will give rise to a huge release of nitrate into the soil and hence to a potentially substantial loss of nitrogen through leaching and nitrous oxide emission. The situation is exacerbated by the fact that, at the time of the incorporation, the soil is inevitably bare, so there are no plants to take up the excess nitrate, and there will be no such plants for several weeks – until the following crop has been sown and well established. The incorporation of green manures into the soil in this way is almost universally practised in organic farming and is, in fact, the main method used to fertilise the soil.
In conventional farming, on the other hand, nitrogenous fertiliser is nowadays applied progressively in two or three doses, mostly at times when the crop is growing fast and hence is capable of taking up all the nitrate that it can get access to. Of course there will still be some nitrate that will escape, not being taken up either by crops or microorganisms, but the nitrogen losses may sometimes be considerably less than in the case of organic farming.
Leguminous green manures are potentially an even more serious source of nitrogen losses. They do not even need to be incorporated into the soil to cause nitrate leaching and nitrous oxide emission. Even while the crop is growing, nitrogen loss may be taking place. This is because the soil under a leguminous green manure crop, especially if it is being grown as a pure stand or with a relatively small admixture of grass in it, may be replete with nitrate, ready to be leached or denitrified. (This will only be the case, however, when the bacteria on the plant roots are actively fixing nitrogen, so not in winter.) Also, many nitrogen-fixing bacteria, especially the Rhizobium species, also carry out denitrification when free-living in the soil, i.e. when they are not contained in root nodules on the legume plants. This will increase the nitrous oxide emissions from the soil.
Besides nitrogen saturation and wet conditions, anything that increases the anaerobic character of the soil will promote nitrous oxide production. A common example is compaction of the soil, due to tractor traffic (or even just footsteps in the case of a bare soil). Denitrification can also be stimulated by the cycles of repeated freezing and thawing that often take place in winter in soils in temperate zones. For this reason nitrous oxide emissions can be just as great in winter as in the other seasons of the year. Obviously these factors apply equally to organic and conventional farming.
As for cover crops, it needs to be pointed out first of all that they are by no means a monopoly of organic farming. In fact, farmers of all kinds use cover crops, as anyone who travels round the English countryside in winter will see. However, they are not as much in evidence as they used to be in the old days, when it was normal practice for farmers to leave the stubble over winter, which provided a feeding paradise for seed-eating birds like tree sparrows and yellow hammers, now unfortunately both in serious decline. It was usual then to grow a dual-purpose fodder/cover crop – commonly the so-called stubble turnips. Nowadays farmers in England often plough the stubble straight after harvest and immediately sow an overwintering crop. So the function of a cover crop is served by next year’s cereal or rape crop.
The second point to understand about cover crops is that they do not completely prevent nitrate leaching. They just reduce it by, on average, about half. Their effectiveness depends when and how they are grown and what is done with them afterwards. The point at which the crop is dug or ploughed into the soil is, as explained above, crucial for the conservation or loss of soil nitrate. The period between crops is also very important from this point of view. The watchword of cover-crop growers should be “Mind the gap!”, because it is precisely in the gaps between harvest and the establishment of a cover crop and between the incorporation of the cover crop and the establishment of the following crop that the soil is most liable to leaching.
All this demonstrates that organic (including vegan-organic) farming and growing can be just as bad as, if not worse than, conventional, where nitrogen losses to the wider environment are concerned. Of course this does not mean that organic farming is inferior to conventional farming. There are many ways in which organic farming is clearly superior to conventional, not least in its much lower consumption of fossil fuels and consequent lower carbon dioxide emissions. It just happens that nitrogen loss is often a weak point in the organic system. In the next article in this series I will discuss some ways in which such losses can be minimised.
1. The article entitled Myths of Organic Farming, in issue no. 20 of this magazine
2. Nitrification is the process by which bacteria oxidise ammonia (NH3) to nitrate (NO3–). The process takes place in a series of stages, in one of which nitrous oxide (N2O) is produced as a by-product and is emitted from the soil to the atmosphere. The bacteria involved are aerobic, that is, they need to breathe air.
3. Immobilisation of nitrogen is the process by which soil microorganisms take up inorganic nitrogen from the soil (in the form of ammonia or nitrate) and incorporate it into their own bodies as organic nitrogen compounds such as proteins.
4. The term nitrous oxide emission refers to the production of the gas nitrous oxide (N2O) in the soil and its upward percolation through the soil into the atmosphere (where it acts as a powerful agent of global warming, 240 times stronger than CO2). In well-oxygenated soils nitrous oxide comes mainly from the process of nitrification, as described in note 2 above, but denitrification is also a major source of nitrous oxide (note 6 below).
5. Nitrate leaching takes place when water (from rain or irrigation) washes nitrate out of the topsoil into the subsoil, from where it can either percolate down into the groundwater or enter the field drains, which will carry it into ditches, ponds and water courses.
6. Denitrification is the bacterial reduction of nitrate to nitrous oxide (N2O) and nitrogen (N2) gases. The bacteria in this case are anaerobic – they do not need air, because they get the oxygen they need from the nitrate itself (NO3–). There are small airless pockets in the soil at all times, so, even in a well aerated soil, there is a constant stream of nitrous oxide being produced by denitrification as well as that from nitrification, but denitrification greatly increases, and becomes the main source of nitrous oxide, when the soil is predominantly anoxic, for example, when it is waterlogged after heavy rain.
Some ways to minimise nitrogen losses in organic farming
In the second article in this series1 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 article2 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.
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.)
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 volatilisation3. 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.
1. The article entitled More Myths of Organic Farming, in issue no. 21 of this magazine
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.
Too much nitrogen
(the fourth & final article in the series on the nitrogen cycle)
When I was a child, people often used to say “You can have too much of a good thing.” I have never heard this saying used in recent years, perhaps because it does not fit the hedonistic culture that is prevalent in England today. But you really can have too much of good thing, particularly when the good thing is plant-available nitrogen1 in the soil.
It may seem surprising to say that there can be too much available nitrogen in the soil, when it is well known that supplying more nitrogen to the crops increases their yields. But unfortunately the increase in yield is not proportionate to the increase in nitrogenous fertiliser applied. Doubling the nitrogen supply does not double the yield. The law of diminishing returns applies here. All other factors being equal, as we apply more and more fertiliser (inorganic or organic), the proportion of it taken up by the crop decreases. And at the same time the proportion that is lost from the soil to the wider environment increases. It is this lost nitrogen that is of particular concern. Some of it goes into the atmosphere as nitrogen-containing gases and some of it dissolves in water and is leached down into the subsoil, from where it may carried by the field-drains into ditches, ponds and watercourses. In ponds and other areas of still water the nitrogen can accumulate and lead to an over-feeding (or eutrophication) of algae and other small organisms in the water, which eventually damages the ecosystem of the pond.
But it is not just a few ponds that are being eutrophied. In a sense the whole world is eutrophied. Everywhere there is an excess of nitrogen in the soil and in the water. Nitrogen enters the atmosphere mainly in centres of human population and intensive agriculture, but it is carried by winds all over the world. Some of the nitrogen-containing gases, like ammonia and nitrogen dioxide, are soluble in water and therefore dissolve in rain drops and are deposited on the soil when it rains. So even the remotest ecosystems, like the vast coniferous forests of the northernmost parts of Asia, Europe and America, are getting fertilised with nitrogen to an extent that they never did before human intervention. The global nitrogen cycle has been augmented to an enormous extent. In the last two hundred years, largely through the production of nitrogenous fertilisers and the growing of leguminous crops, the amount of atmospheric nitrogen being fixed2 has more than doubled. A significant part of that extra fixed nitrogen gets carried into natural ecosystems.
It might not seem a bad thing that plants in nature are being fertilised. It should make them grow faster, you might think. But it has to be borne in mind, as I mentioned in a previous article, that most natural plants are highly adapted to grow in a very restricted nitrogen supply, because in natural ecosystems nitrogen is very tightly controlled. So for such plants suddenly to get extra nitrogen is not necessarily helpful. On the contrary, it could be a source of stress for them. There is evidence, too, that the excess of nitrogen suppresses the growth of mycorrhizal fungi3, on which most plants are highly dependent for their nutrient supply, especially for phosphorus. So, while the plant gets more nitrogen, it could suffer from deprivation of other important nutrients.
As well as harming individual plants, the extra nitrogen could also harm the ecosystem as a whole, because there are some species of wild plants that, like cultivated crops, can thrive on nitrogen. In a nitrogen-saturated ecosystem the growth of these nitrogen-loving species would be favoured at the expense of the rest and the whole balance of the ecosystem would be upset. The beginnings of this sort of ecological damage has already been detected in some of the colder parts of the world, where natural nitrogen cycling is slowest.
Then there is the effect of inorganic nitrogen compounds on the pH of the soil. They tend to make the soil more acid and, if they accumulate in the soil, the acidity could reach a point where it seriously impairs plant growth. Below a certain pH the cycling of nutrients in the soil practically ceases, partly because most microorganisms will not function in very acid conditions. Aluminium and other metals that are toxic to plants can be mobilised at low pH, while non-metals that the plants need, like phosphorus, can become immobilised.
Finally, there is evidence that excess nitrogen in the soil de-activates the bacteria that oxidise methane. These bacteria play an important part in limiting the atmospheric concentration of methane, which is a powerful greenhouse gas. Any inhibition of the activity of the bacteria will increase that concentration.
So, all in all, too much nitrogen in the soil can do serious harm to the natural ecology of this planet. There is therefore a pressing need to reduce the quantity of inorganic nitrogen compounds in the environment (quite apart from the global warming effect of atmospheric nitrous oxide). Industry and motor transport are important sources of these nitrogen compounds, but agriculture plays a major part in the problem too, principally as a result of the emission of oxides of nitrogen from the soil. The only way to cut that down is to reduce the inorganic nitrogen concentration in the soil water by applying less nitrogenous fertilisers (including green manures) to the soil. However, that would cause a reduction in crop yields, which would not be popular, either with farmers who depend on high yields to make a living or with people who are concerned about world hunger.
In the short term the answer to the latter problem is vegan-organic growing, since this would immediately make much more farm land available for feeding people, which would compensate for the lower crop yields. But in the longer term serious consideration has be given to reducing the demand for food by limiting or even reversing the growth in the world’s human population so that human demand for the earth’s resources is brought down to as sustainable level.
1. The term plant-available nitrogen here denotes not only inorganic nitrogen (nitrate and ammonia) but also organic nitrogen that is readily mineralised into inorganic forms.
2. Fixation of nitrogen means taking nitrogen from the air and combining it with other elements to form compounds such as ammonia or nitric oxide. The principal means by which this takes place in nature is through lightning or through the action of nitrogen-fixing bacteria, some of which are free-living in the soil and others living symbiotically on the roots of plants.
3. Mycorrhizal fungi are those fungi that live in very close association (symbiosis) with plant roots, getting their carbohydrate from the plant while supplying the plant with phosphorus and minerals that they get from the soil. Most plants need the help of these fungi to survive, especially in poor soils, because fungi can scavenge scarce nutrients from the soil much more effectively than plant roots can and they can also digest more recalcitrant nutrient sources like woody residues and rock particles, which plants cannot do.