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| A type of cover crop, crimson clover, Trifolium incarnatum. Source |
Digger Spade, MG Plant Detective
The client didn’t have a plant problem. She had just read “Dirt” by David Montgomery which described how tilling led to erosion and that to the downfall of civilizations. So she wanted to know how to switch from tilling to no tilling in her vegetable garden. She just happened to ask the question last Fall when Digger had decided to do the same.Tilling can be hard work. The extreme for the gardener is probably double digging. Rototillers reduce the workload but can in heavy soils create an almost impervious layer where the tines beat against the clay at the bottom of their stroke.
While Digger’s use of raised beds and tilling with a broad fork reduces the work and doesn’t alter the soil profile as much as “turning it over,” it seems that no-till systems produce even better results.
Organic matter has been shown to be most beneficial when it is concentrated in the top 2” of soil. That is where water and air penetrate and most of the biological activity goes on. Mixing composts into the soil can sometimes create a competition for nutrients between the compost as it continues to decompose and the plants that it is intended to benefit. But applied to the surface that doesn’t happen.
No-till gardening is actually quite common in Whatcom county. The perennial and shrub gardens covered in shredded bark frequently deserve that definition except when a layer of weed barrier keeps the mulch away from the underlying soil. The idea behind no-till gardening is that earthworms and other denizens of the living soil will integrate mulch or cover crop residues into the soil and over time will convert even heavy clay into friable organic-rich highly productive soil. As Darwin showed earthworms can in a year bring 0.2” of soil to the surface and in the process carry humus and nutrients into the soil.
The client however was going to start with soil that had been cultivated for the last three years. It was naturally fairly heavy but was quite productive. For her the conversion should be easy. There are options. One is to add a hefty layer of compost during the winter so that the plot would be ready for planting in the spring. She could use layers of cardboard, paper mulch or paper on top of the soil to inhibit weed growth and put several inches of compost on top of that. Or just use compost or shredded plant material such as straw, or better yet alfalfa. At planting time pull aside the mulch and, if an under-layer is still intact, cut holes in it to put in transplants or seeds. Each year, an inch or so of mulch is added and the soil organisms slowly integrate it into lower layers of soil. Many households don’t produce enough food and garden waste to generate enough compost to start a no-till garden so at least in the first year commercial compost may be needed.
Another method is to plant a cover crop in the fall and kill it in the spring by covering it with plastic for a week or two or by hoeing it flat, separating the tops from the roots and doing it before it has gone to seed. Then the dead plants are pulled aside to plant a crop. In this method the cover crop roots serve to “till” the soil and tops and roots serve to enrich the soil and provide nutrition to both plants and soil organisms. And summer cover crops can be planted adjacent to the food crop. But care must be taken with this approach since different cover crops favor pathogenic soil organisms. Winter rye and wheat support wire worms but inhibit pathogenic nematodes, and the clovers favor root knot nematodes and some pathogenic fungi. So, best to rotate cover crops, for example oats/hairy vetch/brassica/buckwheat. Weeders Digest articles have in the past described the cover crop strategy in detail.
A great deal of research has been done to support no-till agriculture; for example a link below is to a 230 page book on managing cover crops in no-till systems. But the basic strategy is to maintain the soil profile, support a healthy soil biome, improve water retention, improve tilth, control erosion and, in the garden, to reduce weeds and work while increasing yields.
Resources
Concentrate Organic Matter At Surface To Improve Soils (WSU)
http://irrigatedag.wsu.edu/concentrate-organic-matter/
Managing Cover Crops Profitably (SARE)
http://www.sare.org/Learning-Center/Books/Managing-Cover-Crops-Profitably-3rd-Edition
