Wednesday, 17 December 2014

Plant Led Soil Improvement

We didn't find brilliant soil when we stripped up the concrete. It was pretty heavy clay and pretty well compacted too - especially after being driven over with heavy machinery several times. So in order to fulfill my dreams of a highly productive forest garden, some soil improvement was definitely in order. And me being me, I searched for ways to achieve this using plants.

I found inspiration from two main places. Firstly Martin Crawford's fantastic book 'Creating a Forest Garden', where he talks about dealing with compacted soil by growing deep rooting ground covers. Equipped with the useful information he gave, I hunted down seed mixes on the internet and discovered Cotswold Seeds' Humus Builder. This includes deep rooting and fast growing plants - Cocksfoot grass, red clover and chicory - which can be cut down several times a year in order to add humus to the soil, while their roots work down and through the soil, improving structure. So this got added in the spring of 2013 and has been growing well ever since alongside several species of self seeded plant.

And then I read what has become a very influential book - Teaming With Microbes, by Jeff Lowenfels and Wayne Lewis. This book describes soil food webs in detail and explains how not only are they crucial in providing plants with the nutrients they need, but also how they differ for different types of plant. And then the book describes how you can enhance your own soil food webs or - more relevantly to me - nudge them in the direction they need to be for the types of plant you want to grow. Basically forest soils have different combinations of microorganisms in them to flower beds and it's these specific combinations of microorganisms that provide an appropriately balanced diet for plants in natural perennial systems. Nurture a healthy and appropriate soil food web and your plants will thrive. So if I want to grow a forest garden, my soil doesn't only need more humus and better drainage, it needs a decent set of microorganisms and this means I get to use a favourite technique - Permaculture Principle 10 - use and value diversity.

The way to do this is to think logically - in a forest, the soil food web is fed by falling leaves and branches - this encourages fungi to dominate at the smallest of levels and fungi are great at providing the right kind of food for trees and shrubs. In flower beds, lawns or vegetable gardens, it's a different picture - you get more green materials falling - it's the bacteria that love this and so thrive here, providing nitrogen in a different format to fungi, and one that is appropriate for the less woody plants.

So with this in mind, I need to start dropping more brown and woody materials onto my forest garden area, not just the green humus building plants from my soil improving seed mix.

At the moment, I have two strategies for this -

1. I've planted lots of broom plants - they grow fairly quickly and can be trimmed to provide woody materials to leave lying around on the ground. They also, of course, provide nitrogen, which my trees and shrubs and any other plants going in here will need to grow quickly. So double hit from the broom - just the kind of plant I like.

2. I've also put to use as a mulch the huge mountain of trimmings I got from pruning ivy and virginia creeper from a barn roof this autumn. I've laid it in the specific areas where I intend to plant trees and shrubs. Hopefully they'll act as little nurseries for all the kinds of soil organisms I need in a forest system, helping to nudge things in the direction I need in advance of getting those trees and shrubs planted out this winter.

I would love to record the impact of this and put to use some of the environmental monitoring techniques I learned while at uni. Wouldn't it be great to map how the soil food web changes over time. If I find the time to do this, I will of course keep you updated.


  1. It'll be interesting to see how you get on.

    I'm in pretty much the same situation with compacted heavy clay and have done a few tests with trying to add humus and condition the soil. No doubt all that you have read is perfectly correct but I wonder about the time scale needed to achieve this.

    As an experiment I dug over patch of clay. A year later after growing various vegetables including potatoes I then re-dug it. It had re-compacted and was just as bad as before.

    Another test, I put 2 feet deep of manure on top of un dug clay with the hope that the masses of worms would break down the manure and gradually take it down into the clay. A year isn't long but when I scrapped off the manure the clay soil beneath was identical. I had also grown sweetcorn in the manure but none of the roots had penetrated the clay.

    Another test was to dig a trench in the clay, fill with manure and then place the dug out clay on top. Again, a year after when I dug into it after having grown various vegs in it you could see that the top layer had compacted on top of a nice layer of well rotted manure and then the original bottom of the trench was the same.

    Nothing I have seen or done will break up clay just by growing things in it or placing any amount of mulch on top.

    A very interesting test was to put a layer of manure, then a layer of sand then another layer of manure. Planted directly into the manure. When I dug it a year later the thin layer of sand was still a perfect layer.

    I've ended up double digging, placing 2 to 4 ft of manure on top, waiting while the manure rots (quicker in thick piles) and then re-digging to mix it all.

    Various things may work, such as the ones you are doing, but over what time scale?

    As far as I can tell the best thing I have achieved with piling 30 tonnes of manure on to my field is that the amount of worms is quite spectacular and this brings in moles....moles drain and break up clay wonderfully :)

    Very interested to see if you have better results than myself. A follow up post would be most welcome in due course!

    1. Hi Andy. Thanks for your comment. Sounds like you've been very busy with these soil improvements! I've found similar with trying to get a quick result, that the best method seems to be to painstakingly hand mix the manure or compost in and this can give a really good soil after the first growing season. But my soil is very stoney, so this method isn't going to work for me. I'm looking at long timescales for soil improvement in this area and don't expect to have brilliant soil in anything under a few years on the worst areas. But yeah, I'll definitely post something up about this again.



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