Improving Your Soil Under the Snow
By Harriet Behar

This article was first printed in the September - October 2006 issue of the Organic Broadcaster, published by the Midwest Organic and Sustainable Education Service.

While green manures can be planted any time of the year, many benefits are gained by planting a crop that will be in place during the winter months.  When you plant a crop that is not meant for harvesting, but instead for soil improvement, it can have at least one of three functions, as a catch crop, a cover crop or a green manure.  When planting a fall crop, think about what functions you may want the fall cover crop to perform and what type of cash crop had been grown in that field this year, as well as what you want to grow next year. 

Thinking through what the specific field’s issues are will help you decide what to grow and why. Things like soil type (lack of organic matter), weed problems, pest problems and beneficial insect habitat for adjoining crops next spring can all be impacted with fall plantings.  Always make sure that the soil building crop you plant does not harbor disease or insect problems for the subsequent cash crop you want to grow.  Also, make sure that your green manure crop will not come back the next spring from non-germinated seed and will then be a weed that is difficult to remove.  For instance, don’t grow a small spring planted grain such as oats or barley after last fall’s buckwheat. Growing a row crop that may have some buckwheat the next spring would be okay, since buckwheat does till out more easily than something like quackgrass.

A catch crop is planted to pull up the excess nitrogen from the soil. The plants “catch” or hold the nitrogen until it is tilled in as a “green manure”.  “Excess” nitrogen can be present in the soil from nitrogen nodules of leguminous crops, from your own applications of livestock manure or compost, or from legume plowdowns.  Since nitrogen tends to leach or wash down through the soil, and we want to keep it for our plant roots and not in our ground water, planting a winter small grain as a “catch crop” in the fall can be a source of good fertility the next spring when it is incorporated.  Catch crops can also be planted in midsummer and tilled in as well.  If you plan to apply manure in the fall, especially for those of us who are vegetable growers, applying the manure thinly on an existing catch crop, or planting a fall grain after the application of this fall’s manure, will help you to hold onto the nutrients for next spring.  Research has shown that the nitrogen from the spring incorporation of these crops is available within 4-6 weeks.

A cover crop is one that protects the soil from erosion and compaction.  These type of crops can also be used as forage for livestock, as long as they are not overgrazed.  Soil borne pathogens, weed and disease cycles are also disturbed by the planting of these crops and they can provide a mulch through the winter.  We all know the limitations of the land we work with, and some wetter ground may be difficult to till early in the season. This would be a place where planting a crop that winter kills into a mulch could be used.  Many folks plant oats or whatever spring grain they may have in storage early in September.  These oats will stay green until sometime in November or later, but will eventually turn brown and mat onto your soil.  Your field is then protected from wind and water erosion from winter storms and freezing and thawing cycles.   I have actually planted oats in September and then dug a furrow and planted garlic into the growing oats, letting the oats then winter kill over my garlic and act as a mulch.  While this was not a thick enough mulch to prevent weed growth the next spring, it did protect the garlic through the winter and was easily incorporated when we cultivated our garlic in early April.

Green manures are grown specifically as fertilizer for the subsequent crops and to improve soil structure.  While the timing of a good green manure crop is a little bit more difficult to manage than for cover crops that may winter kill or catch crops that will be tilled in early in the spring, they are well worth it.  The breakdown of organic matter by microorganisms causes various compounds to be formed in the soil, and they help the smaller soil particles bind together as a granule or aggregate.  Well aggregated soils till more easily, are well aerated and have good water infiltration and retention.  Weed control is much more effective when the soil rolls into the row at cultivation, rather than throwing clods of dirt that do not crumble.  The challenge with fall planted green manures can be your window of opportunity to till these in the following spring.  While you may gain a lot of organic matter from 6 foot tall rye, it takes the soil microbes much longer to breakdown this fibrous plant.  Instead of converting nutrients into food for your plants, they are spending their time breaking down this mass of matter, and it does take them some time, making it more difficult to prepare a smooth seed bed for your planting.  When you till in your green manures when they are still green, 8-12 inches tall, they incorporate more readily and planting of your cash crop can occur sooner.

I have seen farmers grow strips of buckwheat or hairy vetch and use these as beneficial insect habitat for adjoining crops, cutting the cover crop when needed and driving the beneficials onto the cash crop.  If you don’t have a cover through the winter, and then have an early spring, a planting of oats 3-4 weeks before the cash crop is planted that is tilled in builds organic matter. I have seen many soybean growers do this with good results.  For vegetable growers especially, if you have quackgrass problems and can take a half a season or even a full season for incorporation of green manures and include buckwheat as one of your crops, you will find your weed control and crop health improved.  These green manure crops are a low cost and effective way to improve organic matter.

Planting of legumes such as clovers and vetch to last through the winter and then incorporated the next spring can give you a good start for your cash crop’s nitrogen needs.  A mixture of a grain with a legume is also a good idea, such as oats and peas, rye and vetch or even soybeans with buckwheat.  Experiment with what works well for you, as well as the percentages of the grain to the legume in your mix.  Nature hates bare ground and will cover the soil with weeds, so why not work with the natural process and plant crops that can work for you?  Building organic matter can be fun!

Harriet Behar is the MOSES Outreach Specialist. She was an organic inspector and inspector trainer for many years and has an organic bedding plant and vegetable operation with her husband in Southwest WI.