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New Alchemy Quarterly, No. 38

Growing Mulch in Place

No-till broccoli using unconventional cover crops

by Mark Schonbeck and Ralph DeGregorio

Mark Schonbeck, Ph.D., is a New Alchemy researcher working on cover crops and winter greenhouse vegetable production. Ralph DeGregorio, Ph.D. candidate, is New Alchemy's research director.

Many New England vegetable growers use cover crops to protect their soil against erosion, control weeds, replenish soil organic matter and hold nutrients (Schonbeck 1988a, 1988b). Those who cannot afford to take land out of production for an entire season use winter annual crops such as rye (Secale cereale). Rye planted as late as October 15 in southern New England can suppress weeds, prevent erosion and produce ample organic matter by spring. However, since rye is not a legume, it does not have nitrogen-fixing symbiotic bacteria in its roots, and in fact may temporarily tie up soil nitrogen after it is turned under. Also, because of its coarse, tough texture, rye gets tangled in the tines of rototillers, decomposes slowly and may grow back, thus requiring additional tillage and delaying seedbed preparation. Some growers use oats (Avena sativa) because they winterkill and are thus easier to manage in spring. However, oats must be planted by mid August to ensure adequate soil protection.

Most legumes form a symbiosis with Rhizobium bacteria in root nodules which fix atmospheric nitrogen into a form that plants can use. Hairy vetch is a hardy winter annual legume whose rhizobia fix large amounts of nitrogen, and which has overwintered as far north as central Vermont. Hairy vetch and rye grown together may give greater benefits than either alone, including more organic matter, better weed control and support for the vetch vines in the spring (Schonbeck 1988c). Also, the nitrogen-rich vetch may accelerate decomposition of rye when the cover crop is turned under, and a grass-legume combination sometimes maintains soil organic matter levels more effectively than does the legume alone (Gliessman, 1987). According to DeGregorio & Ashley (1985), two other winter annual legumes which may be hardy at least in southern New England are crimson clover (Trifolium incarnatum) and Austrian winter field peas (Pisum sativum ssp. arvense). We conducted a study at New Alchemy Institute to compare the ability of several legume-grass combinations to produce organic matter and nitrogen,' control weeds and provide nutrients to a subsequent vegetable crop.

On August 26-27, 1988, we planted 10'x 10' plots at New Alchemy with rye alone and in combination with each of these three winter annual legumes. We also planted oats+ hairy vetch. Because oats are winterkilled north of Maryland, this combination might be easier to manage than rye+ vetch, but oats offer little support for vetch vines in spring. Treatments were replicated four times so that we could make statistical comparisons. One replicate was eaten by rabbits, leaving us three.

All cover crops grew vigorously, produced over 3000 pounds per acre of dry matter by late May, and effectively suppressed weed growth (Figure 1). Rye + crimson clover produced the largest biomass, significantly more than rye + hairy vetch. Austrian winter peas were partially winterkilled, resulting in lower biomass. However, rye growing with this crop appeared greener and more vigorous than rye alone, suggesting that it may have benefited from symbiotically fixed nitrogen from the peas. Nitrogen analyses showed that rye grown with any of the legumes had significantly higher tissue nitrogen concentrations (1.1-1.4%) than rye grown alone (1.0%).

In a similar experiment planted at Maplewood Farm in Amherst, MA, both crimson clover and winter peas were winterkilled, whereas hairy vetch + rye performed well, producing a combined biomass of 5600 pounds per acre. The crops were planted too late (September 9), and suffered a-12 F freeze without snow protection. It seems worthwhile to try crimson clover and winter peas again in western Massachusetts, planting in mid-late August.

We decided not to till the cover crops in at New Alchemy, but to mow them, leaving the clippings on the surface as a mulch. This has several possible advantages, including soil moisture conservation, weed suppression, and savings on tillage. More importantly, tilling stimulates weed germination, may cause too rapid breakdown of organic matter and may increase erosion. Many farmers use no-till methods to save soil, time and fuel, and to reduce weed pressure, but they often must use toxic herbicides such as paraquat to kill existing vegetation prior to planting their crops. Winter annual cover crops can be killed by mowing after they start to flower, thus making a no-till, no-herbicide system possible.

We tried this system in 1988 for producing tomatoes, and found that the mulch lowered soil temperatures and thus slowed growth and reduced early-season yield in this heat-loving cmp (Schonbeck & Doherty, 1988). However, it also reduced weed growth and conserved soil moisture during the drought, and we felt that the mulch might be advantageous for a cool-season crop such as broccoli.

In 1989, we mowed the cover crops in late May, when the rye had just shed its pollen and all legumes were beginning to flower. On June 3, we transplanted seedlings of 'Emperor' broccoli through the mulch, providing each with a heaping teaspoon of bonemeal and a cup of water to get started. The seedlings grew vigorously, but rabbits, slugs, earwigs and climbing cutworms soon came to the banquet table, destroying half the crop. New Alchemy intern Albert Wurzberger noted many spiders throughout the experiment, a potential predator of some pests. We spent many frantic hours erecting a fence, replacing devastated plants with seedlings purchased from a nearby nursery, picking nocturnal pests by flashlight, and applying natural pest control sprays and dusts.

By the time we had all the varmints under control, we noticed that broccoli plants in rye plots were far smaller and less green than plants in the other four cover crops. On July 3, we gave each plant in all treatments a heaping teaspoon of an organic fertilizer, but the size difference between the rye-only and the legume treatments just kept on increasing. The bone meal and fertilizer together supplied only 32 pounds of nitrogen, 23 pounds of phosphorus and 11 pounds of potassium per acre, and the experiment was conducted on a soil testing low to medium in all three nutrients. Leaf tissue tests showed that broccoli after grass + legume cover crops had significantly higher foliar nitrogen and phosphorus concentration than broccoli after rye alone. The legume supplied nitrogen to the broccoli, and at least indirectly improved phosphorous nutrition.

Main heads of broccoli were ready for harvest between July 22 and July 31. Broccoli heads produced in the four legume treatments were substantially larger and heavier than broccoli heads from rye plots (Table 1).

Weeds were never pulled or hoed during the experiment, and no additional mulch was brought in. On August 3, after broccoli harvest was finished, the center square meter of each plot was weeded and weed dry weight was determined. Plots mulched with a rye-only cover crop had by far the most weeds, and broadleaves were largely responsible for the difference (Figure 2). Rye plots contained far more horseweed (Conyza canadensis), a serious weed of no-till agriculture, than did grass+ legume plots. Weeds in the latter were so sparse that they cast essentially no shade on the broccoli. Furthermore, very little cover crop regrowth occurred in any treatment. This was accomplished without weeding or herbicides, and since nitrogen deficiency and weeds are normally the organic farmer's biggest headaches, mow-killed rye+ legume cover crops may be an important strategy.

Although oats died in winter and oats+ hairy vetch weighed less in May than other cover crops, they still suppressed weeds well. However, tree leaves blown across the plots from nearby compost piles accumulated in the oats + vetch plots in amounts exceeding 3000 pounds of dry matter per acre, whereas other treatments collected only 100-900 pounds per acre of leaves. The erect oat plants, unlike the prostrate rye and legumes, trapped leaves effectively, a useful trait for growers near urban areas who want to spread municipal leaves in the fall. The leaves increased the total amount of dry matter mulching the oats + vetch plots to over 7000 pounds per acre, higher than other treatments. Without this "subsidy," the oats + vetch may not have suppressed weeds any better than rye alone.

In 1990, we are repeating this experiment on rye, hairy vetch, rye + vetch, and vetch + oats. Cover crops will be turned under in some plots, and mow-killed in others, to determine how the two management systems influence weeds, crop nutrition, soil temperature and moisture, and certain pests and beneficial organisms in broccoli grown after the cover crops.

References DeGregorio, Ralph E. and Richard A. Ashley, 1985. Screening living mulches and cover crops for weed suppression in no-till sweet corn. Proceedings of the Northeast Weed Science Society. 39: 80-84.

Gliessman, Stephen R., 1987. Species interactions and community ecology in low external-input agriculture. American Journal of Alternative Agriculture. 2: 160-165. Schonbeck, Mark. 1988a. Cover crop research at New Alchemy Institute: why, what and how. New Alchemy Quarterly 33 (Fall): 3-4.

-. 1988b. Cover Cropping and Green Manuring on Small Farms in New England and New York: an Informal Survey. New Alchemy Institute Research Report No. 10.

- 1988c. Hairy vetch and winter rye: mutual support. New Alchemy Quarterly 33 (Fall): 11. Schonbeck, Mark and Wendy Doherty, 1989. Cover crops for Northeast vegetable farms: a report on research at New Alchemy Institute. The Natural Farmer (Spring): 1213.

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Original Article from:
New Alchemy Quarterly
Winter 1989-90, No. 38
© 1990, New Alchemy Institute, Inc.
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