About a month ago I trimmed our mulberry trees to encourage new growth and extra fruit production. It seemed such a waste to throw the branches in the compost pile so I looked for a suitable place to plant 10” pieces of the branches to grow some new mulberry trees. I found two neglected parts of our garden where I could push the branches into the ground by hand. I was very surprised at how easily the mulberry sticks went into the ground with just hand pressure. This soil was compacted clay subsoil almost as hard as rammed earth two and a half years ago!!!! Now it’s turned into about 12” of topsoil. It almost seems fake so I decided to document what happened. Basically, nature has already devised the most amazing soil building methods. This is another reminder to work with nature and also get out of the way at times.
I made a close examination of the areas that turned to topsoil very quickly. There are only two places like this in the garden and both have similar characteristics:
1. A weedy 16′ long section of fish net trellis for climbing plants. This area became partially overgrown with weeds because it was too difficult to trim with a weed wacker.
2. Metal edging along the fence in the back of the property acts like a check dam to hold everything in place. Nearby areas with just weeds show little improvement, although I believe weeds are part of the solution. The most important factor seems to be slowing the runoff of water, sediment and organic material.
So here’s what I think happened. The barriers (fish net, overgrown weeds and metal edging) slowed runoff and trapped sediment and organic material such as chopped weeds, and also straw and manure that we put around the fruit trees. This created a layer of protective mulch on top of the soil to help retain moisture. Water is also slowed down by the barriers, and the mulch makes it easier for the water to penetrate the soil more deeply. The full grown weeds help break up the soil more than surrounding weeds that get cut regularly. The tangle of weeds shaded the earth to better support microorganisms in the soil. In addition, the nearby leguminous tree and fruit trees all help break up the soil as well.
Put all these things together and you can turn hardpan clay into 12″ of decent topsoil in just a few years. The most exciting part is being aware of this process and realizing it could be repeated throughout a garden or farm of any size. You see, we’re not talking about creating a thick accumulation of sediment behind a barrier. (Although that might happen in some situations.) In our case about 1”-2″ of organic material and sediment built up. The miracle happens when there’s a combination of all the things listed above (a barrier to slow runoff, mulch, water, clod busting weeds, shade and nearby trees). That’s when nature takes over and builds soil through biological processes. Evidence of this process can be found in other gardens such as Paul Gautschi’s Back to Eden garden, who started with gravelly and hardpan soils. In his garden you can now push your arm up to the elbow into loose rich soil.
Additional notes: You could put these mini barriers in between typical berms and swales to more evenly slow runoff. Think of it like mini terraforming. The barriers could also include the use of keyline cultivation and vetiver hedge rows. Covering the areas between fruit trees with wood chips is also recommended if you can get them (we can’t).