Soil restoration: Using plants to fix nitrogen in the ground

Simon Watkins
Tuesday, 16th August 2011

When Simon inherited an unloved garden on his property, it was difficult to imagine being able to grow food in the hard, gravel covered ground. He decided to look at it as a blank canvas and here's how he went about restoring the soil, making it nutrient rich for all his vegetables.

It's a dreach Saturday in early May when I host a session of the "gardening together" group in Coventry – a group which has spluttered into existence like the unconvincing but untimely rain which is now dampening our enthusiasm for doing anything practical outside when there are coffee and muffins to devour and conversation to be had indoors. But the clouds do eventually lighten up enough for us to venture into the back garden and set about the job we've assembled to do.

Fencing and gravel removal

I've inherited a back garden comprised of a rough concrete patio, a few shrubs and a concrete slab path flanked on both sides by plastic sheeting and an impressive amount of gravel. My first job was to put up a fence to stop the neighbours' inconveniently friendly Labrador puppy from clambering through the hedge, fouling everywhere and digging up everything in sight. Now I'm able to tackle the blank canvas which is a gift to any landscape designer, I find myself with a ton of gravel on my hands and nowhere to put it. It seems too good a resource to throw away, so I've begged a hippo sack from a nearby housing co-op and invited the crew around with shovels to help shift it.

Nitrogen fixer vs. compost experiment

The big idea is to try two approaches to restoring the soil, which has been covered by this sheet and gravel mulch for presumably quite a long time (the house was let to students then empty for a while before I bought it). On the one side, I'll be sowing a green manure – Alfalfa – for the purpose of digging in for nitrogen later in the year. The other side of the path, I intend to cover the newly exposed soil with cardboard and greenwaste compost. The experiment, which could be titled "remediate or replace?", will be completed next year, when I'll plant exactly the same crops into both sides, enabling me to compare their success. It'll be specific to this particular garden with this soil, at this aspect and with this prevailing climate, of course, but were the plot bigger, the method used might have wider implications.

We only managed to shovel up one half of the gravel garden before lunch, so that's the half that I immediately earmark for alfalfa. I don't have nearly enough compost to cover the other half anyway, and that can wait for a few weeks... or months! After the others leave, I rake the surface of the newly exposed soil into drills, just deep enough to line out the tiny seeds. "Why not scatter broadcast fashion?" Asks my lodger. I'm not completely sure myself, except that because it's quite a large area it would be useful to be able to step between the rows of alfalfa to get to the border and apple tree beyond.

Within a few days a green patina was beginning to colour the bare ground; A couple of weeks passed and distinct strips of green were running like the grain of a split log down the garden; Two and a half months later, a very hairy carpet of the clover-like stuff has improved the view from the kitchen window immensely

Alfalfa: a nitrogen fixer for yielding crops

I've already taken a slice off the top of the growing plants to feed the compost heap, but also because the developing jungle was in danger of shading out the crops I put in around the edge of the plot. Crops? At this point I have to admit that my experiment's objective and method have slipped and diversified a little...

What is alfalfa?

Alfalfa is an extraodrinary plant. Google "alfalfa experiment" and you'll pull up 4 million, 70 thousand results. Native to the Americas, it's a deep rooted accumulator of minerals and nitrogen fixer. I heard once of a trial in which alfalfa was sown in rows alternating with wheat, spaced at half their usual density. The exchange of nutrients between the two plants was so beneficial that the yield of wheat was as great as when it was planted on its own. So I figured it would be worth throwing a few other things in around the edges of the plot: tomatoes,basil, leeks, cabbages... carrots, garlic... courgettes, beans, sunflowers... chard, lettuce... zinnia... what? Never heard of it, and still don't know what it looks like. It's the one thing which hasn't come up at all (as far as I know). Everything else except the cabbages is at least ok, if not booming. Of course, without the other, non-alfalfa side of the garden (which I still haven't got round to clearing of gravel) to compare growth rates with, I've no way of knowing whether the alfalfa has helped any of these other plants. But then weekends are for changing your mind - in the end it's all about growing food, and in spite of the shady aspect of the garden, the tomatoes are looking pretty robust this year. 

Simon Watkins is a landscape architect based in Coventry, but as a student of permaculture, he is increasingly guided in his work as much by his stomach as his eyes... visit his website to read more about some of the things he gets up to. 

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vnswamy |
Mon, 02/10/2017 - 22:59
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