Green manures, crops grown especially for returning to and improving the soil, have a long, mainly agricultural, history. Such as lupines, usefully a Nitrogen fixing legume, were grown and dug under as far back as Roman orchards. The Medieval fallow continued an ancient break in cropping combined with a green manuring by weeds. Excellent from the weeds point of view as they got to seed and multiply every few years just as the last batch faded away. And maybe also useful for Medieval people as they regarded many of our commonest weeds as excellent fare for adding to pot or sallet rather than a ‘problem’. When agriculture went over to the four course rotation without a fallow some farmers noticed the damage to texture and soil erosion and tried under sowings of such as clover or folding sheep on crops on the land. However most never took to regular green manuring as their fields were rarely bare soil in winter with most crops autumn sown.
Modern green manuring thus comes more from more recent attempts at improving poor soils as from being a part of the annual fertility cycle. The emphasis has been more on the humus creation and texture improvement of highly fibrous residues than on straight increases in fertility even though legumes are recommended for their contributions of Nitrogen. Little research has been done on green manuring for other nutrients, water retention or pest and disease suppression all of which are useful possibilities. But worse, most trials have been done at agricultural colleges, and therein lies a major weakness, which I will deal with further on.
So what are the prime reasons for green manuring? Well the first is to create a mass of succulent growth that can be easily converted, either in situ or sometimes via the compost heap, into soil fertility; nutrients, humus and a boost in micro-organisms. This mass also improves soil texture. Either directly, say by the release of saponins, soap forming compounds which bind the soil into crumbly fragments. Or through the action of organisms, especially worms but also many others big and small, all turning plant material into droppings and dead bodies.
The majority of green manuring is done from autumn till spring when the soil is empty of crops. Winter sunshine, rain and otherwise leached away nutrients are converted into green manures rather than being lost. Then the green manures which take up water during a wet period, once dug in or covered over, retain water in their decaying mass for crops coming later. The winter cover protects the soil from erosion, keeping the temperature and moisture more equable and providing food and habitat to countless small critters. A good green manure will also exclude weeds, either by preventing them germinating or by choking them out at a later stage.
Actually you can grow green manures at any time the soil is bare and available, it’s just most are grown in winter. Short term green manures can be fitted in before or between crops. This means they have to be ended whilst small and although this costs more in seed and effort it is actually the best way to use green manures. You get more benefit from two or three short manurings compared to one left for the same length of time.
Green manures should not be allowed to set seed, reaching the flowering stage is risky but benefits beneficial insects, however if then incorporated promptly little value is lost. All of them break down quicker and contain more goodness if taken earlier than later though! Thus if the soil is warm enough some green manures should be sown thick and incorporated rather sooner not later, even when they have become only a few leaves. Then another batch sown, and another, and so on.
One good reason for growing green manures is they are almost free as you can save your own seed from a few plants for the next year. A handful of seed becomes barrow-loads of compostable residues containing a lot of future fertility. Those small number of plants you need to produce future seed should be grown in a reserved place as their effects on the soil there will not be the advantages of a manuring but will be a reduction in soil nutrients as for other cropping plants.
An important criterion for an effective green manure is that it should break down, if dug or hoed in, within a reasonable space of time. Unfortunately in the cold soil of early spring this may take many weeks for a fibrous long term green manure though it’s quicker for younger ones. During the time of rotting the conditions are not good for many germinating crops though some robust ones such as potato sets can cope with the difficult environment containing actively decomposing plant material. Hungarian grazing rye especially inhibits seed germination for many weeks as it rots. An alternative is to strip the top growth of those green manures whose roots will not re-sprout and compost this elsewhere to return later, or simply strew amongst the stumps as a mulch to plant through. But it’s easiest and quickest to use black plastic sheet or similar to cover the green manure and have it rot down in situ on the surface where it is more rapidly incorporated by the micro-life (because of the warmth) than if dug in or left as a mulch.
Interestingly some green manures give off compounds that harm or promote particular micro-organisms. Generally these changes are beneficial and particular green manures have been used, again agriculturally, to suppress soil borne diseases. Mustard is reckoned especially useful as it can encourage brassica clubroot spores to start into life and then if the plants are dug in early the clubroot has nothing to live on and so is reduced in virulence. And likewise a green manure may be used to reduce pests in the soil. Many have found Tagetes mariglds, especially T. minuta, will rid their soil of eelworms, and suppress herbaceous weeds at the same time. (Two drawbacks- T. minuta can only be grown in summer and minuta refers to the flowers not the height as this plant gets massive.)
However as noted above- there is one major problem with almost all of the green manures offered to gardeners. Because they have come from agriculture most are well suited to being ploughed under by a two hundred horsepower tractor, but sadly are very hard work to incorporate by a one person powered spade. Secondly in farming any re-growth that might occur would have been soon eliminated in the herbicide regime that would have been the norm at that time. A regime rarely copied by gardeners then and especially not now.
So green manuring has great potential for improving our soil’s fertility; specific and general, texture and moisture holding capacity –but because of the ones recommended they are ill matched, hard work, and therefore under-employed. We need more research to find better green manures; then we should never need to buy fertilizer or soil improvers again.
Green manures generally recommended-
- Hungarian grazing rye; inhibits seed germination for weeks as it rots, does, as claimed, produce a lot of fibrous bulk. Sure and you need a sharp spade and a lot of oomph to deal with it. And it is very prone to re-grow.
-Alfalfa, Lucerne; leguminous and deep rooting so sounds good but is too slow to establish and bulk up, too hard to dig in and too prone to re-grow. Better for long term leys and orchard herbage.
-Tares and vetches; again in theory brilliant as leguminous, hardy etc. But again too tough and too prone to re-growth. Most useful if taken young so prevented from getting too well established.
-Mustard; related to the cabbage family so not necessarily easy to fit in a rotation. Mustard should only ever be used as a short term manure and incorporated whilst small and succulent. Oil seed rape and other closely related plants are similar.
-Winter or field beans; related to the crops but not commonly causing rotational problems. One of the legumes and so adding nitrogen, but a lot of seed needed for a dense enough cover to do the job properly protecting the soil, and they are poor at suppressing weeds.
-Phacelia tanacetiifolia; a pretty flower attracting beneficial insects so if this is let go this is a good companion plant, but not hardy enough for winter use so more useful for summer use when it then needs sowing often and taking small.
-Buckwheat; another good companion but rather ineffective green manure. Unless sown really densely this loses out to the weeds.
-Lupins; usually field varieties not the garden ones. Leguminous of course and so sounds good but slow to develop and to bulk, and too tough to incorporate unless taken small.
-Clovers; leguminous so do enrich the soil, but again tough stuff to dig in and too prone to re-grow. Only good at suppressing weeds in damp soil and given a good start. However this seems to be one of the more effective traditional green manures.
Alternative green manures found in my trials to be more adapted to the needs of gardeners-
-Limnanthes douglassi, Poached egg plant, this well known companion plant is a very good winter cover choking out almost all weeds. Very easy to remove or dig with web like roots that bind the soil nicely if the tops are pulled and composted elsewhere, and quick to rot down under a cover. If some are left on a margin to flower they are excellent for beneficial insects.
-Claytonia /Montia perfoliata, Miner’s lettuce. This is a self seeding salad plant that covers the ground densely growing throughout winter but tends not to grow in hotter drier weather disappearing and reappearing spontaneously. Very easy to remove or incorporate leaving a fine web of roots binding the soil which do not re-grow. And most importantly this is dual purpose as providing winter salad leaves or hen food too.
-Valerianella, Corn salad; less rampant than claytonia but another dual purpose green manure and salad, not quite so effective at smothering weeds unless densely sown. Easy to dig in or remove for composting. A good bee plant when flowering in spring.
-Spinach; high in saponins this is good for improving the soil texture and quick and easy to grow. However not hardy enough for winter use but handy for fitting in between crops. And of course edible.
-Borage, a fluke find this. I had been experimenting with borage as an alternative to comfrey for liquid feeds. Borage makes a feed richer than comfrey and with more Magnesium balancing the high Potassium. Not very hardy however so borage often dies out in harder winters. Where I had some beds of it I noticed it not only suppressed weeds well whilst still in the green but then after death also. The soil appears well enriched after a borage green manure and I think this has great potential. Try it out yourself and let me know your experiences please.