Tillage and Green Manures

By Eliot Coleman

Given the recent condemnations of organic farming for its use of tillage, I was pleased to read the following passage from a blog by Andrew McGuire, an agronomist at Washington State University. It parallels our experience with, and faith in, the benefit of tilling in large biomass green manures. They have been integral to creating and maintaining the fertile soils on Four Season Farm.

Green manuring is the lesser-used option in cover cropping. Most cover crops are killed and left on the soil surface, but green manures are tilled into the soil. That is where they have their unique effects. They feed soil microbes and larger organisms and thus change the community composition to benefit crops that follow. This enlarged soil microbe community then produces more stable aggregates and better soil structure for overall increased soil function. However, for the benefits of green manures to outweigh the tillage required by the practice, a large amount of biomass must be grown.

Tillage breaks down aggregates, disturbs soil microbial communities, and quickens breakdown of organic matter. Biomass does the opposite. The more plant biomass incorporated into the soil, the more steps forward the soil takes to overcome the backward steps of tillage.

Big biomass also improves green manure’s effects on soilborne pests. Both suppression of pests by the chemicals in green manure crops (as with mustard) and general suppression of pests by the feeding of the existing microbe population will be increased by greater amounts of biomass.

So, yes, I still use my rototiller as needed. I have seen no detrimental effect from rotary tillage if plenty of organic matter is incorporated, either from green manures grown in place or from what the Europeans call “cut and carry” green manures grown elsewhere.

We plant approximately a third of our vegetable land every fall in mid-October to a combination of winter rye and hairy vetch following harvest of root cellar storage crops like carrots, beets, celeriac, cabbage, turnips, potatoes, etc. That rye/vetch will remain in place for one winter, one growing season, and a second winter.       

We sow by broadcasting the rye/vetch seeds with a chest seeder and press them into the soil with a cultipacker. Our fall rains are dependable, but we could irrigate if necessary. We let the rye/vetch grow to maturity the following summer. On or about August 10th when the rye is about seven feet tall and well headed out and the well-podded vetch vines are climbing the rye straw, I back through the rye/vetch field with the rear cover on my tractor flail mower raised (and the depth roller removed) to shred the standing straw right down to soil level. The result is a two-inch deep mulch of chopped rye and vetch straw over an enormous number of rye and vetch seeds. Rain or irrigation germinates all of them and the area soon looks like a golf fairway.

That begins the second winter of rye/vetch root mass. (Some authorities suggest that root growth may be more valuable than top growth for adding long lasting organic matter to the soil.) I leave that resown rye/vetch until early April the following year when there is enough fresh green growth to aid the final decomposition of the straw and I till it all in. The young rye plants incorporated at that stage of growth do not become weeds as they sometimes can, probably because they have been stressed by such close spacing (about 20 times the standard seeding rate.) Four weeks later the area is ready to start transplanting that year’s vegetable crops. Two winters and one summer of rye/vetch creates big biomass, and the results in soil fertility are impressive. This process rotates to a different third of the vegetable land each year.

The fields on the other two thirds of our vegetable land are resown, as soon as all the crops planned for them have been harvested, to either annual alfalfa, buckwheat, forage radish or a combination of peas and oats, all which winter kill in our climate. The dead matter can be raked aside for sowing the following spring. The choice of one green manure over another is determined depending on time of year, expected weed pressure, and which vegetable crop will be grown next in the rotation on that area. With the crops which leave a good bit of post-harvest biomass, like cabbage and sweet corn for example, (a not inconsiderable amount of organic matter) we till in the shredded residues shallowly before seeding the green manures.

I call our rye/vetch program biomass till. It has helped us to create and maintain a very productive soil.