Organic gardening requires a deep interest and an optimistic outlook, but the biggest need is healthy soil. This time of year, with many fewer crops waiting to be planted, opportunities to improve our garden soil open up daily. Three of the simplest soil-improvement methods — composting, cover cropping and mulching — give the best returns when they are undertaken in the fall, perhaps because this closely follows nature’s blueprint. With no human help, fallen leaves and spent plants form a patchy winter mulch, with cold-hardy weeds popping up in open spaces.
Off-season soil improvement methods make the most of these natural seasonal processes. Mulches and cover crops support the soil’s determination to cover itself with something, for example a fuzz of winter weeds. Soil does not like to be bare in any season, and bare soil will always attempt to heal its open “wounds” with weeds. Mulches and cover crops do much more by increasing the soil’s organic matter content and its microbial diversity, which in turn enhance its ability to store nutrients and water. A garden that is neatly tucked in for winter is also a reassuring sight on cold, dreary days.
To get access to your soil, start by pulling up spent plants and weeds by the roots, and cut them into smaller pieces before throwing them on your compost pile or into an enclosed composter. Several pest insects and common diseases can overwinter in plant tissues, but they don’t have a chance in the hot chaos of a compost pile. Pulling plants so the roots rot in compost rather than in garden beds also can help prevent root-rot diseases, because soil microorganisms that depend on dying roots are deprived of an easy food supply.
With withered plants out of the way, consider spreading an inch or two of homemade compost over the beds, even if it is coarse and chunky. Compost made from summer’s garden waste is not especially rich in plant nutrients, but it is a treasure trove for the soil’s food web of bacteria and fungi. Spreading compost over resting beds boosts soil biodiversity, and creates a pleasing habitat for crickets and other voracious eaters of weed seeds.
Cover crops that are sown in fall and either pulled up or turned under in spring hold the soil with their fine network of roots, help the soil handle excess water, and even help hold nutrients in the soil. Studies from Iowa have shown that rye and other grassy cover crops can reduce winter soil erosion and nutrient losses by more than half.
Cereal rye, which usually can be purchased as “rye berries” in health-food stores, will germinate and grow in cool soil, so seeds can be planted well into fall in most climates. Hard winter wheat “berries” can be planted the same way, but some of the hardest-working winter cover crops — nitrogen-fixing vetches and clovers — need to be sown in September, while the soil is still warm.
A special relationship appears to exist between hairy vetch, grown as a winter cover crop, followed by tomatoes planted in late spring. When the vetch plants are cut off at the surface with a garden knife, and a week or so later tomatoes are planted in openings made in the dried vetch mulch, they grow into bigger, healthier plants. The tomatoes make use of nitrogen stored in root nodules by the vetch, so they need much less fertilizer, and the mulch reduces early blight by keeping low tomato leaves clean.
Numerous other cover crops are useful, including whole, unhulled oats, a cover crop and winter mulch rolled into one. Oat seeds sown in early fall grow into a lush, knee-high stand of thick grass by the time the leaves fall, and then winter temperatures in the teens kill the plants, roots and all. Left alone, the oat foliage collapses into a soil-saving surface mulch, while the roots rot into threads of organic matter spread through the top two feet of soil. When the oat hay is raked off in spring, the soil below is soft, friable and ready to plant.
If you opt not to plant cover crops, plan to mulch over your beds in winter with grass clippings, chopped leaves, pine needles, or hay if you can get some from fields that were not treated with herbicides. Any organic mulch will protect the soil from compaction by cushioning the surface from the force of pounding rain or heavy snow cover. Winter mulches also provide habitat for ground beetles, crickets, ants and other creatures that eat weed seeds, along with earthworms and other consumers of decaying organic matter. When protected by a thick mulch, these garden helpers often stay active through winter, even in cold climates.
The soil improvement practices you implement now will show noticeable returns in spring, both in terms of the soil’s suitability for planting and how much time is needed to get beds ready for planting. It’s also likely that simply working with soil gives us a probiotic boost. Australian researchers cultured 350 strains of bacteria from a soil sample, which you can’t help but touch and breathe when you work in the garden. Perhaps this is part of nature’s plan, too.
Fall cultivation does little good for the garden or the gardener, and has been called “recreational tillage” by researchers at Purdue University in Indiana. Each time you dig your garden soil, millions of microscopic fungal filaments are broken, nutrients are released as gases, and habitat is devastated for earthworms and beneficial insects that live or hunt near the soil’s surface. It is best to limit cultivation to times when digging is necessary, for example when preparing to plant a new crop. In fall, cultivate only enough to sow cover crop seeds, or let the soil rest undisturbed beneath a protective mulch.
Local farm-supply stores sell cover crop seeds, or you can order them from these excellent mail-order seed companies:
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