From Growing Green International 19
Plants need nitrogen, which constitutes 78% of the air. Among living organisms only bacteria are able to absorb nitrogen from the air and convert it into organic molecules. So what could be better for plants than to enter into symbiosis with the bacteria?
Nitrogen (N2) is a very stable molecule. Industrially it can be split only with high energy expenditure. At 500°C. and 200 bar N2 can be converted into nitrogenous fertilisers. This is one of the main ways in which conventional agriculture contributes to global warming and climate change. Plants have a more elegant way of getting the nitrogen they need - by co-operation with nitrogen-fixing micro-organisms. Legumes (beans, clover etc.) have gone down this road very successfully by living symbiotically with bacteria called rhizobia.
Rhizobia live everywhere in the soil and gather around the root hairs of the plants, attracted by excretions from the plant roots. The plant and the bacteria first sniff at each other to see if they have found the right partner, because each legume needs its appropriate kind of rhizobia. Once they have found each other, the bacteria infect the root hairs, establishing themselves in the cortex (bark) of the root, where they induce an abnormal rate of cell division. The result is root nodules, which can contain millions of rhizobia.
To split the N2 molecule, rhizobia use an enzyme called nitrogenase, which is highly sensitive to ( that is, easily destroyed by ) oxygen. So no fixation of nitrogen happens as long as the rhizobia live freely in the soil, where oxygen is abundant. It can only happen inside the root nodules, the skin of which restricts the entry of oxygen. But, being aerobic bacteria, rhizobia need oxygen to live. To obtain this oxygen supply in the oxygen-poor environment of the nodule, the bacteria produce leg-haemoglobin, a substance which causes the red colour inside the nodules. Like human haemoglobin it can absorb oxygen and transport it to wherever it is needed. So it transports oxygen from the exterior to the bacteria inside the nodules.
In exchange for nitrogen the plant supplies the rhizobia with carbohydrates (about 15% of the plant’s total photosynthetic yield), with energy and with the necessary trace elements. The average fixation rate of soya is 100 kg. of nitrogen per hectare per year, of lupins -150 kg. and of lucerne -250 kg. But these amounts are influenced by several factors. High concentration of nitrate in the soil inhibits the growth of the nodules and hence reduces nitrogen fixation.
Some elements are important for nitrogenase activity, especially phosphorus, molybdenum and iron. A neutral pH and good water and temperature conditions are also necessary to achieve the best fixation rate. Last but not least, there has to be a sufficient availability of rhizobia in the soil. If a legume species has not been grown in a particular place for several decades or it is not native to the area (e.g. soya), the seeds should be inoculated with the appropriate bacteria.
It is generally considered that 25% of forage legumes in an organic crop rotation should supply the system with sufficient nitrogen. The question for vegan organic growers is how these forage legumes can be put to economic use or can be replaced by marketable seed legumes. It is important to note that the latter, depending on variety and species, could cause a negative nitrogen balance, because of the export of the legume seeds from the land. A plant with a particularly positive nitrogen balance is the edible lupin, which has the additional advantage that its seeds have a protein composition that is well balanced for human needs.
In any case clover and lucerne should not be missing from crop rotations, due to their multiple positive effects. In comparison with mixed farms vegan farms only require half the area of clover/lucerne, because with stock rearing only 50% of the fodder nitrogen comes back to the field in the manure. According to circumstances the cut legumes can be used as mulch or they can be composted and the compost brought back onto the field when needed.
EDITOR’s note: while farmers can inoculate their legume crops with rhizobia for increased yields, small growers can use similar tricks.
An advantage of growing your French and runner beans in
the same place for several successive years is that the right rhizobia build
up in the soil and keep increasing fertility; this might be out of step with
the principle of crop rotation but it usually works without problems. When
using rotations, you can inoculate the ground you intend to plant, just take
a small amount of soil from ground where legumes have been growing and
spread it where you intend to plant your next crop of the same variety.