One approach is to grow a perennial legume, such as red clover or alfalfa, around the edges of the garden or between rows, says Marianne Sarrantonio, Ph.D., a professor of sustainable agriculture at the University of Maine at Orono. Cut back the foliage and add to the compost pile when it becomes hard to wade through or is getting bothersome in general. With extensive root systems that “fix” nitrogen (capture nitrogen from the air and stockpile it in root nodules), these legumes can help balance the carbon-to-nitrogen ratio in the compost pile as well as encourage beneficial soil microbes. It’s easy to harvest the clover or alfalfa with a lawn mower but best if done by hand or with a sickle. “It’s extra work, but you’re getting lots of benefit both in place and then on the compost pile,” Sarrantonio says.
In beds that would otherwise be bare through the winter, try planting a winter annual, such as cereal rye, as a cover crop after harvest. The cereal rye will protect the soil over the cold season, and its tremendous root system will grab nutrients that remain in the soil after the growing season. Before planting the following spring, harvest the aboveground, carbon-rich parts of the rye for the compost pile, leaving the nutrient-laden root system in your beds. “Normally I would recommend just tilling a winter cover crop down,” Sarrantonio says, “but don’t turn a grass like cereal rye under, because it will tie up some nitrogen, won’t decompose quickly, and will be in the way of early crops.”
Some compost crops feed both the gardener and the compost pile. Most legumes that are cultivated for their edible beans don’t retain much nitrogen in their leaves and stems after the seeds have fully matured—they have concentrated most of the nitrogen in their seeds. But cowpeas, a subtropical legume species that includes black-eyed peas and red cowpeas, are mostly indeterminate; the plants will flower continuously and leaves will stay green until killed by frost in northern climates. Rather than tilling the plants into the soil in place, the tops can be removed and added to the compost pile, Sarrantonio says. The seeds of another legume, the soybean, can be harvested while green for edamame, while its foliage is still green and full of nitrogen. Throw the soybean plants on the compost pile after harvest.
When planning your garden this spring, make room for plants with a double benefit—a hardworking root system or edible seeds in addition to plentiful, carbon- or nitrogen-rich foliage to feed your compost pile.
Originally published in Organic Gardening Magazine, April/May 2012
Photos: Bill Johnson