By Roger Higginbottom
Green plants are architects of soil structure. The compounds that they build and the way in which they distribute them largely determine the nature of the remaining soil population. Plant detritus reacts with the soil skeleton (i.e. the rock fragments in the soil. These include, where present, the ultra-fine clay colloids), affecting the physics and chemistry of that skeleton.
The longer a plant cover lives, the more marked is its effect on the rest of the soil. If climate is favourable, associations of deeply rooting grasses and legumes build “black earths” – for example the chernozems of parts of Central Asia. These soils are essentially a mass of dead plants, highly modified as their components pass along a web of detritus-feeding organisms. They are fertile and if the plant cover is destroyed, fragile.
Three components of such a system interact to build fertility. One is carbon fixation by photosynthesis. The second, less obvious, is nitrogen fixation, either by bacteria living in partnership with the green plants or on decomposing compounds in the soil. The last extracts minerals from the soil skeleton and increases the water-holding ability of the soil, whilst also allowing gases to permeate the soil. The decomposition of plant remains drives this last process.
So green plants build fertile soil, given appropriate climate and soil minerals. Some plants are better than others. Some are suited to particular situations. A perfect green manure plant would have these qualities:
1) Efficient photosynthesis, shown by fast production of organic compounds.
2) Deep and widespread rooting.
3) Association with a nitrogen-fixing organism.
4) The plant should be easily established, able to smother weeds and not re-grow when worked into the soil.
5) The seed should be readily available and economically priced.
6) The species should not be vulnerable to adverse weather, especially drought and frost.
7) Ideally, the green manure plant should not belong to a family shared by common crop species, so that it provides a further break in crop rotations.
The species I use
Undersown broad beans on Roger’s allotment. Photo: Colleen Withall
I now briefly review the green manure species that seem to be best adapted to my clay loam overlying limestone in central Derbyshire.
A legume/grass mixture builds a good water-stable crumb structure, given three years. Rye grasses (Lolium spp.) and red clover (Trifolium pratense) work well together. A perennial rye grass, associated with the recently hybridised Aberuby red clover looks a promising combination (see note ) . Mowing stimulates root production.
Cocksfoot (Dactylis glomerata) is another useful grass to mix with red clovers. It is deeply rooting and so not easily affected by drought. The tussocks formed by cocksfoot allow ground beetles, which eat slugs, to overwinter.
Over a two-year period Melilotus alba (or the related species, Melilotus officinalis), sweet clover, seems unbeatable for my situation. This is a straggly, deeply rooting biennial. In the first year growth is exponential, yielding about a metre of foliage by September, from a late March sowing. In the second year each plant throws four or five much thicker, tough stems. These reach about two metres in August. The sweet clover then flowers (bees work these flowers avidly). Only when turning in this remarkable plant does one realise how much organic matter has been synthesised: the roots easily penetrate a metre, often more, and have extensive laterals.
The bacterium associated with sweet clover, which fixes atmospheric nitrogen, differs from the species on common clovers: it is Sinorhizobium meliloti, also found on Lucerne (Medicago sativa) and Trefoil (M. lupulina). A little Lucerne seed (which is available inoculated with S. meliloti) can be mixed with the seed of sweet clover (which doesn’t seem to be inoculated) to establish the nitrogen fixer.
Sweet clover needs an open, unshaded site and a soil where it can push its taproots down. Finely ground rock – either basalt or granite – seems to suit this vigorous pioneer plant, as does a little powdered rock phosphate. Trace elements, taken up by the clover are more readily available to subsequent crops than those present in raw rock particles (see note ). For nitrogen fixation to work, all legume species need a trace of cobalt. This is a component of cyanocobalamin – vitamin B12. The clovers need this compound to synthesize a haemoglobin (see note ). In turn, the haemoglobin sequesters oxygen in the root nodules where the nitrogen-fixing bacteria live. Free oxygen kills these bacteria. I’ve observed that since I’ve used rock dusts, nodulation of legumes is extremely prolific.
Trefoil is useful for undersowing a standing crop, as it tolerates some shade.
Lucerne is a perennial, which does not grow as rapidly as sweet clover in my situation. As Lucerne roots are extremely tough, digging this crop in is like attacking wire netting. Only a carbon steel spade, sharpened to a paper-slicing edge, is of any use. Sweet clover is a more practicable proposition.
Buckwheat (Fagopyron esculentum) is a fast growing annual that responds to drought by producing a mass of fine roots, which open up a clay soil. In a wet season, rooting is much reduced.
Buckwheat is a good weapon against perennial weeds like couch. Simply dig the infested area and sow buckwheat in late March. After six weeks, dig the crop in, and repeat the process throughout the growing season. The couch becomes outgrown and dies. Thus laborious forking is not needed and the weed is returned to the soil, dead, to decompose.
The humus process
What, precisely, happens to the organic matter built by a green plant when it dies remains debatable. Humus – a mixture of two complex organic acids – fulvic and humic acid – seems to be the last organic material to be formed. As quite a large proportion of the green plant material is complex carbohydrate, like cellulose, much of this may be oxidised by soil animals and micro-organisms to carbon dioxide and water. Still, practical observation shows that humus tends to accumulate if one works such material into soil, or feeds the material in, as a mulch. Vigorous soil disturbance accelerates decomposition of organic matter, probably because more oxygen is available to the decomposers.
Some of the plant material consists of lignins. These contain tough components, like phenol groups. Oxidised by basidiomycete fungi, mainly, to quinones, these lignin fragments may unite with amino acids compounds: humus is the result.
A further possible pathway suggests that fungi can build phenols from cellulose or hemicellulose. If this is so, a much larger reservoir of raw material for humus formation exists than if lignin were the main raw material.
Soils that are vigorously and often cultivated, admitting much oxygen, may not permit the production of phenols from complex carbohydrate. How one treats the soil when the green manure is incorporated is therefore important. Minimal cultivation should be considered.
“Green Manure” ed. L. Woodward, P. Burge. Elm Farm Research Centre 1989
“Mineral Nutrition of Higher Plants” (2nd edition) H. Marschner. Academic Press, San Diego 2002
“Principles and Applications of Soil Microbiology” D.M. Sylvia, J.J. Fuhrmann, P.G. Hartle, D.A. Zuberer. Prentice Hall, New Jersey 1999
“Bacterial Biogeochemistry” T. Fenchel, G.M. King, T.H. Blackburn. Academic Press, San Diego 2000
Notes Green manure seeds including the Aberuby red clover can be obtained in kilogram packs from Tuckers Seeds, Brewery Meadow, Stonepark, Ashburton, Devon TQ13 7DG 01364 652233.  Granite dust can be applied at about one kilo per square metre; if the soil under consideration is derived from an igneous rock then addition of granite or basalt dust (which have a similar elemental composition though arranged in different molecular structures) may be pointless.  Leghaemoglobin and haemoglobin both consist of a protein (globin) and an iron bearing group, the haem component. A large number of animal classes use haemoglobin as an oxygen carrier in their blood systems. Leghaemoglobin on the other hand is found in the root nodules of leguminous plants, like peas, clovers, lucerne and sainfoin. Also present in these nodules are bacteria, which have the remarkable nitrogenase enzyme system, allowing them to ‘fix’ atmospheric nitrogen. Molybdenum and sometimes vanadium are essential for this enzyme system to function – another good reason for applying igneous rock dust.
This article appeared in Growing Green International magazine Num 22 (Winter 2008), p28.