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Cover Cropping & Green Manures

By Ellen Polishuk

After 22 years of farming, my farm’s soil is markedly more fertile and productive. It has been a wonderful journey learning what works and how to continue to improve long-term productivity while harvesting bountiful crops.

There are several methods that deserve credit for this increase in soil quality: the use of compost, the use of a balanced mineral fertilizer and a serious commitment to cover cropping.

For this article I want to focus on the growing of cover crops and green manures. When I became the farm manager at Potomac Vegetable Farms – West in 1992, I sought advice on how to transition the farm into organic production.

The land had been growing mostly sweet corn, pumpkins and green beans with commercial chemical fertilizers and herbicides. The soil was biologically inactive and nutrients were missing. I can clearly remember the words from a fertility consultant, “I can sell you something in a bag, and I can sell you something in a bucket, but what you really need to do is to make compost and grow cover crops.” And thus began my journey into both compost-making and cover cropping.

Why take on both of those jobs rather than just one? Well, they go hand in hand, each leveraging the benefits of the other to move a soil more quickly toward health and resilience.

The consultant warned me that just growing huge biomass cover crops might overwhelm this inactive soil — without a good population of microbes present it would be difficult for the soil to digest so much raw organic material. That’s where the addition of compost helps — by inoculating the soil with a diverse mix of microbes.

flail mower
Flail mowing summer Sudangrass/cowpea green manure at Potomac Vegetable Farms – West.

The addition of a well-developed compost is always good, but it’s expensive and isn’t much of an organic matter addition at the rate of 5 tons per acre. Compost alone is too costly and at high rates will create nutrient imbalances, but putting both practices together is a winning combination.

I think it’s important to make a distinction between the term “cover crop” and the term “green manure.” What these practices have in common is that a crop is grown that will not be harvested — everything is given back to the soil. The difference between the two practices is when this gift crop is grown. A cover crop is grown between cash crops — for those of us in Zones 3-7 that means over the winter. A green manure crop is grown instead of a cash crop — during the main growing season. The soil benefits of each practice are virtually the same, but the erosion prevention aspect of a cover crop is more relevant and important as it is grown in the winter when many farmers don’t plant anything at all.

In case you have forgotten the myriad benefits of a green manure/cover crop, here’s a reminder:

  • Organic matter extraordinaire – both roots and tops of plants can add up to 9 tons per acre of dry matter.
  • Biological stimulant – the growing plants’ exudates feed the soil microbes.
  • Erosion control – there is nothing better than a healthy crop to keep the soil in place.
  • Biodiversity – many cover crop families are different than those typically grown on a mixed vegetable farm and provide a habitat for different insects, some of which will be beneficial. Also these different plant families will feed the soil microbes a different ration of exudates.
  • Soil aggregation – always having plant roots growing helps to form and maintain soil aggregates, which leave the soil more aerated.
  • Weed management – healthy timely cover crops reduce weed pressure in future years.

While many growers commit to growing cover crops over the winter, few practice green manuring on a regular basis. Why? Most perceive it as too costly in terms of time, effort, seed money and losses from forgoing the opportunity to grow a cash crop.

By growing a green manure crop in the main season, there is usually little time left to grow most cash crops.

There are a few exceptions to this rule — either with very short season cash crops, like salad mix species or radishes, or with very short season green manure crops like buckwheat. I have found that the benefits gained through a well thought out and executed green manure program outweigh the costs. How did I come to this conclusion?

I used to believe I could not afford to implement a green manure system. What was missing for me was an aggressive and exacting rotation plan and the determination to follow through on actually planting the green manure crops. I heard the Nordells of Beech Grove Farm in Trout Run, Pennsylvania, give a talk about crop rotation at a conference. Finally it all came together for me and I understood how to approach the management of the land via a rotation that includes green manure crops.

The Nordells created their rotation with weed management as one of the primary goals. In its simplest definition, my farm is neatly divided in half; at any given time at least half of the acreage is “on vacation,” growing cover crops or green manure crops. The other half is “working,” growing cash crops. Another way to say it is that half of the farm is making money and the other half is getting ready to make money next season. The cash crop years alternate between early and late crops. This system mixes up the times of year that tillage takes place, thus helping to combat weeds that result from always tilling in April/May and September/October.

My farm has been on this vacation rotation plan for several years and the results are gratifying. The soil continues to improve and the crops are more vigorous and productive.

We are also making a dent in the weed pressure on the farm. There is nothing that makes me smile more than driving by a gorgeous field of Sudangrass and cowpeas in the middle of summer, knowing that all those soil benefits will make the following season easier and more lucrative.

Cover Cropping: What do you need to do to get started?

First, I would recommend reading SARE’s Managing Cover Crops Profitably. This manual tells you all you need to know about which species to choose, when to plant, seeding rates and the attributes of each species.

Second, get serious about your rotation. Figure out how to set aside up to 50 percent of your acreage each season for soil improvement. If that sounds outrageous, start small with 15-25 percent. Get organized and make a plan that you can enact.

Third, make sure you have the implements you need to make the rotation work. How will you plant the cover crop/green manure? Spin seeder or drill? How will you manage the top growth? Bushhog or flail mower? How will you work that dry matter into the soil? Plow/disk or rotovator or spading machine? Fourth, get your seeds before you get too busy with your cash crops. Have everything ready to go so that when the time is right you can get those cover crops/green manures planted.

I encourage you to make any form of progress possible in incorporating cover crops and green manures into your regular farm schedule. Once you begin to see the benefits on your land, you’ll be convinced to improve and increase their use. Don’t worry too much about the fine points of the perfect polyculture right off the bat — just get started with planting one or two species at a time. I promise you will love watching those “gift” crops grow.

This article appeared in the December 2014 issue of Acres U.S.A.