Using Green Manure

Try this planting combination around your crops to protect your vegetables and the soil around them.

Young squash

While the young squash plants are getting established, the oat/pea green manure keeps right on growing.

Photo by Will Bonsall

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With more than forty years of experience redefining gardening's boundaries, author Will Bonsall shows how readers can eliminate the use of off-farm inputs like fertilizers, minerals, and animal manures by practicing a purely veganic, or plant-based, agriculture-not for strictly moral or philosophical reasons, but because it is more ecologically efficient and makes good business sense.

In Will Bonsall’s Essential Guide to Radical, Self-Reliant Gardening, (Chelsea Green Publishing, 2015) he offers readers in-depth information on growing, harvesting, and processing an incredibly diverse variety of food crops. The following excerpt is from Chapter 2, “Green Manures.”

You can purchase this book from the MOTHER EARTH NEWS STORE: Will Bonsall’s Essential Guide to Radical Self-Reliant Gardening

Creative Combinations

I want to share an exciting idea with you, as it combines features of both green manuring and companion cropping; moreover it builds up the soil at the same time that a food crop is growing there. Start by considering squash or pumpkin: a sprawling heavy feeder that cannot be planted much before Memorial Day in my area. When the soil is warm and frost is but a memory. Between snow-go and squash planting time, we have seven or eight weeks during which the ground is bare and doing little or nothing, except maybe sprouting some early weeds. Why not put it to better use? say I. Here’s how I do it. As soon as I see some snow-rid ground, albeit half-frozen mud, I seed it down thickly to oats and field peas (you know, the sort you use as pea soup, only not split). Then I rake or tread the seed in as conditions allow. Seed depth is not important; I merely need to hide the seed from newly arriving birds until it sprouts. As April slides into May the growth rate accelerates to form an ankle-deep carpet. A week or so before planting squash, I need to chop in the vegetation, but only where the actual squash will be planted; that’s because too much decaying matter can actually cause a nitrogen deficiency, as the decay process pulls nitrogen from the soil. Once the decomposition has commenced apace, it will of course release lots of usable nitrogen for the squash plants.

Since I mean to plant the squash in hills, I only chop 2-foot (0.6 m) circles every 6 to 8 inches (15.2–20.3 cm), leaving the oats/peas in between to continue growing; they’re not in my way yet. I generally use an Italian hoe—the poor man’s rototiller—to chop stuff in. I may or may not fertilize those circles for the squash; I typically plant it on lean land just to utilize the benefits of this method. Although the oat/pea mix will soon become a source of fertility for the squash, it may not be soon enough. I use either fully composted humanure or regular compost that is not yet thoroughly decayed (I have so much need for finished compost elsewhere, whereas squash is not so picky about the state of the compost). I don’t turn in the compost; I merely top dress each ring, using as much as I can spare.

I used to plant squash seed directly until I wearied of feeding the voles; I now start seedlings indoors in 4-inch (10.2 cm) peat pots (thinning to three plants each), at least two weeks earlier. That head start allows me to be in less of a rush to set them out: I can afford to let the chopped oat areas break down for a few days longer than I had previously. The rest of the area—oats and peas—continues to put on lush growth until the squash plants are sitting in wells of light with walls of oats/peas surrounding them.

Up to now the compact squash plants have been demurely occupied with sending roots down and out into the decaying stuff, but by the Fourth of July they are really ready to sprawl. The squash vines would easily scramble up and over the knee-high oats/peas, but I do not make them do that.

So far both crops have shared the space, but now the squash needs it all to itself, so I help it out. I could mow the oats/peas or till them in and that would be the end of it, but I demand further service. I take a 4 × 4-foot (1.2 × 1.2 m) sheet of plywood and stand it on edge next to the row of squash hills, where I flop it down and tread on it, flattening out the green manure while careful not to damage the vines (should we say I don’t want to crop the squash or squash the crop?). Timing matters here. I continue flip-flopping the panel and treading it until the whole piece is flattened and the squash looks to be totally in control. This lays the oat/pea mulch in a neat tightly thatched layer that few weeds can penetrate and that will now decay to nourish the out-reaching squash roots.

It looks quite elegant, but for one problem. The oat/pea mulch isn’t really dead; the stems are merely crimped, and a few days’ sunshine will make it all try to straighten up again. The trick is to exclude sunshine. Black plastic sheeting would probably work well enough, but it doesn’t grow on my land, and besides, it’s made of ticky-tacky (yes, I know many people consider it organic; some people think Elvis is still alive). I do have tree leaves, lots of them; not the nice dry ones I shredded up back in October, but old piles of matted leaves that sat under the snow all winter. They’ll do just fine. I spread those in a layer over the mulch; it needn’t be particularly thick, just solid enough to exclude any sunlight. The only problem now is that those whole leaves will dry out and tend to blow away, so I add a thin third layer of trashy old hay or chipped twigs—not thick, just enough to pin down the leaves, sort of like a hairnet.

The results of this treatment are several: Counting all three layers, I’ve added a huge amount of soil-building matter to the area while dooming any weeds to suffocation. Squash is a water hog, but the heavy mulch takes care of that, allowing rain to percolate through it but not evaporate out again.

Perhaps the greatest benefit is one I never would have expected: The squash plants have been completely freed of striped cucumber beetles (though not the squash bug). Don’t ask me why, but for several years we’ve seen few if any of the flittery stripers, though they do still pester the cukes and melons, which are not mulched that way. Someone suggested that the stripers can’t find the plants in their oat-walled canyons. Perhaps, but even after the mulch is flattened the beetles are conspicuous by their absence. Maybe it’s something about the oats, because one year I used red clover instead and did not notice the phenomenon.

Complex Mixtures

I avoid using most of the perennial pasture grasses as green manure, even though they are potentially more eco-efficient than the annual grasses. To realize their potential requires a longer lay, or rotation cycle, usually two years and ideally more. Given a longer time period they establish a dense root system that sequesters phosphorus and lots of other goodies, which are then released for the ensuing crop. For shorter season lays the succulent annuals like oats and peas seem more practical to me.

A few times I have experimented with more complex mixtures and observed them over a full two-year lay. I used a combination of oats, barley, peas, herd’s-grass or timothy, red clover, white or ladino clover, alsike clover, hairy vetch, and alfalfa. I broadcast each separately, one after the other, because if I mixed them beforehand they tended to settle out slightly in the mixing bowl, grasses floating to the top, and I wanted a consistent stand overall. I also spread wood ash on the piece. As expected the annual oats, barley, and peas leaped ahead, and for several weeks the entire patch looked like a nearly pure stand of those three alone. When it grew too rank I scythed it and carried away all the top growth to compost—even though the main object was to build up this piece. The oats, barley, and peas were nearly finished off by the low mowing, and that was fine with me. I had intended them to serve as a nurse crop until the other species got off and running.

Within a few weeks the whole area was dominated by the red clover. The other species were all there, scattered throughout the area, each biding its time until conditions might favor it. A second mowing sometime in late July or August (if memory serves) gave the ladino and alfalfa their chance to shine. Both love hot weather and tolerate drought better than, say, red clover, which now kept a low-key presence.

The vetch was always a bit player in this show; it thrives in situations where lack of humus, minerals, and water keeps everyone else off the stage, and it then gets the leading role, which it plays admirably. In this mixed environment it pushes no one aside, but does a creditable job of filling in wherever others are weak. Appreciate vetch for what it does well, not for what others can do better.

The second year started off without the annuals of course—they had died at the end of last season, and were by now food for the long-termers. The red clover resumed the lead in the cool, wet spring weather. Again alsike lurked among it, thriving only where aeration was an issue. (Mind you, all the legumes are highly aerobic—for them air is food; however, alsike seems to manage with less than the others.) After mowing, ladino had an expanded presence, but as summer brought hotter and drier conditions, it was the alfalfa that really took charge. The timothy that had in the first year showed just a scattering of fine spears now became better established, which helped keep the various legumes from sprawling.

I believe if I had continued the experiment a third year, timothy would have been the main actor, with everyone else continuing in a supporting role. Instead I turned it all in and followed it a couple of weeks later with a glorious crop of russet potatoes, even though four or more cuttings had been exported from that area for use elsewhere. I’ve used this procedure on a few occasions, with comparable results each time.

As this experiment with a complex mixture of crops clearly showed, an important part of eco-efficiency is adaptability. A crop that is highly eco-efficient in one context may be surpassed by other species when some factor is altered. Temperature, moisture, humus, shade, day length, pH, compaction—any number of subtle factors could favor one over another. Maybe it’s some obscure trace element like pandemonium. (You’ve never heard of it? It sounds no sillier than californium, and no one challenges that one.) The point is, the world is full of niches, and every niche has some organism or group of organisms that is ideally adapted for that place (and time). Well, no, not ideally—nature is not an ideal; it is a dynamic living reality—but rather something that is optimally adapted for those particular circumstances. Of course an organism cannot adapt unless it is present, which is a point in favor of complex seeding mixtures—I could have added many more. This is the principle behind fallowing, an ancient practice whereby a plot is left for a season to grow up to whatever it will (as in Leviticus 25:2–7). It assumes that whatever plants volunteer there—come up on their own—will automatically be the best adapted, better than whatever we might have decided to plant there. In fact what comes up on its own depends mainly on what seed happens to be already dormant in the soil and only secondarily on the conditions in the soil. Therefore the fallow population has been self-selected not for soil-building potential so much as for seed dormancy. That may be useful for a particular species, but less so for the soil community.


Reprinted with permission from Will Bonsall’s Essential Guide to Radical, Self-Reliant Gardening  by Will Bonsall and published by Chelsea Green Publishing, 2015. Buy this book from our store: Will Bonsall’s Essential Guide to Radical, Self-Reliant Gardening.