Thursday, April 25, 2013

Keith Stewart: Spring Cover Crops and Green Manures on an Organic Farm

Oats and bell beans. Photograph by Keith Stewart

One of the big differences between an organic vegetable farm and a conventional one is that organic farms don’t use synthetic petrochemical fertilizer. I’m talking about the various combinations of NPK (nitrogen, phosphorous, potassium) you’ll find in any farm supply store. This stuff does provide plants with the three major nutrients they need, but over the long term it is harmful to soil health, and the making of it uses up a lot of fossil fuel.

On an organic farm applying compost and animal manures to the soil is one way to maintain and boost fertility. Another way is by growing cover crops and green manures, and that’s what I’d like to talk about now.

First, what’s the difference between a cover crop and a green manure? Well, often not much. At the most basic level, with a cover crop the emphasis is on maintaining enough vegetative cover on the land to prevent erosion. With a green manure the focus is on adding organic matter to the soil and, when a legume is chosen, atmospheric nitrogen as well. But an established green manure can also be a good shield against erosion, and all cover crops contribute organic matter to the soil in varying amounts. So the crops you choose and the words you use to describe them will depend on what your primary objective is. If in doubt, the term cover crop will suffice. It’s often used as a stand-in for green manure anyway.

On our farm this April, as soon as the soil had dried out enough to use a tractor and disc, we planted two different combinations of cover and green manure crops in fields that will not be needed for cash crops until July or August. One of these combinations was oats and bell beans (a.k.a. fava beans). The oats germinate and grow rapidly in cool weather. They offer quick erosion control and choke out weeds. They also provide a good environment for the leguminous bell beans to get started. Like all other legumes, bell beans are able to convert atmospheric nitrogen into a form that both they and other plants can use. They do this by working symbiotically with soil bacteria called rhizobia.

Oats and bell beans. Photograph by Keith Stewart

These two crops can reach a height of 4 feet within 40 to 60 days. When grown individually, in a healthy soil oats can contribute as much as 8,000 pounds of dry biomass per acre, while bell beans can add up to 7,000 pounds. These are big numbers and achievable only under ideal growing conditions. When grown together, the combined biomass production can be impressive but is likely to be well below the total of these two numbers.

The bell beans also bring to the table an additional, and very valuable, 70 to 200 pounds of nitrogen per acre. Bell beans have large taproots that are good for penetrating heavy soils. Oat have a more fibrous and spreading root system that resists surface erosion.

Before these crops produce viable seed (usually in about 60 days), they should be mowed, then incorporated into the soil, where they will improve soil structure and water retention and provide food for the vast array of organisms that live under our feet.

When we plant oats and bell beans together, we use 70 pounds of each seed per acre (about 2 pounds per 1,000 square feet). We always inoculate the legume seed with rhizobia bacteria, just in case there are none present in the soil. There are different species of rhizobia, and it’s important to select the right one for the species of legume you are planting. For bell beans a pea/vetch inoculant will work best. We sow our oats and bell beans with an over-the-shoulder broadcast seeder, then use a tractor and disc to cover them with a couple of inches of soil.

Another green manure/cover crop combination we used in early April of this year was oats and field peas. Like bell beans, field peas are a fast-growing annual legume that does well under cool and moist conditions. Because their stems are weaker and more succulent than the stems of bell beans and because they have a vining growth habit, field peas benefit greatly from being grown with an upright crop such as oats, which provide them with support. The field peas and oats combination usually gives us similar yields of biomass and nitrogen. The seeding rate we use for field peas is 80 pounds per acre and for oats 60 pounds per acre. We use the same pea-vetch inoculant for the field peas.

These are just two possible cover crop combinations that a farmer or gardener might use in the spring on ground that is scheduled to receive late-season crops such as kale, cabbages, or broccoli. They fit well in an organic system and confer many benefits.

Keith Stewart is a NOFA-NY certified organic vegetable grower in Westtown, New York, who has been selling to the NYC Union Square Greenmarket since it began. Keith’s garlic has been called “the most soulful garlic on earth” by Time Out New York. The New York Times said, “Keith’s farm grows garlic from another planet compared with the stuff in supermarkets.” He is the author of Storey’s Guide to Growing Organic Vegetables & Herbs for Market, It’s a Long Road to a Tomato: Tales of an Organic Farmer Who Quit the Big City for the (Not So) Simple Life. His essays appear in The Valley Table, “the Hudson Valley’s only magazine devoted to regional farms, food, and cuisine.” 

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