The Organic Grower's Dilemma: How to Manage Weeds Effectively Without Compromising Soil Quality

Organic farmers and gardeners seek to sustain high crop yields and quality by building and maintaining a healthy, living soil rich in nutrients and organic matter. To do so, they return on-farm organic residues to the land and use compost, organic mulches, and cover crops to “feed the soil.” At the same time, organic producers strive to prevent soil losses and damage to soil structure and soil life in day-to-day farm operations. Indeed, effective practices to conserve soil and to achieve and maintain high soil quality are required under the USDA’s National Organic Program, and these practices must be documented in the organic certification process.

Because they do not use synthetic herbicides, organic farmers rely more heavily than conventional farmers on timely tillage and cultivation, as well as preventive measures and cultural practices for weed control. This presents a challenge, because tillage can hurt soil quality, as demonstrated during the Dust Bowl of the 1930s, when the wind blew topsoil off millions of intensively-tilled acres in the Great Plains. Even when effective measures are taken to prevent soil losses, the turning, fragmenting, and pulverizing actions of various tillage implements can degrade soil quality by:

• Stimulating soil microorganisms to consume soil organic matter more rapidly
• Burying the most biologically active layer to a depth at which partially anaerobic conditions kill off some of the soil life (deep inversion tillage, such as moldboard plowing)
• Cisrupting the filamentous networks of plant-symbiotic mycorrhizae and other beneficial soil fungi that help maintain soil structure (tilth) and enhance crop nutrition
• Killing beneficial insects, earthworms, and other soil macrofauna, and disrupting their burrows
• Physically breaking down stable soil aggregates (crumb structure)
• Leaving the soil surface more prone to crusting after rainfall, which can interfere with aeration, water infiltration, and crop seedling emergence

While the stimulatory effects of tillage can release plant-available nutrients and speed the breakdown of fresh organic residues, intensive tillage will sooner or later exhaust most soils. As soil organic matter levels and biological activity decline, crop yields and quality suffer. Thus, many organic farmers grapple daily with this dilemma: "How can I control the weeds without tilling the soil to death?"

Since the early 1970s, increasing awareness of the adverse effects of tillage on soil, combined with the widespread availability of herbicides, has led to the development and adoption of conservation-tillage and no-tillage systems. When combined with sufficient annual return of plant residues and other organic materials to the soil, no-till practices can actually build soil organic matter and soil quality during annual crop production. Researchers and farmers across the United States have attempted to adapt no-till systems to organic production, utilizing mowed or otherwise mechanically-killed or winter-killed cover crops to provide a weed-suppressive mulch for no-till vegetable or row crop planting. Results have been mixed, and perennial weeds can become a problem when continuous no-till is attempted without herbicides. However, various organic reduced-tillage strategies, with some cover crops managed by no-tillage, or by strip tillage or ridge tillage, have shown some promise.

Tillage practices can affect nutrient availability and nutrient flows. For example, reduced tillage can delay nitrogen mineralization and cause nitrogen limitation in certain crop, especially in cooler climates. Therefore, organic growers need to assess current and future crop nutrient needs in selecting tillage practices.

The tillage dilemma has led to concerns that pure organic methods for annual vegetables and row crops may not be truly sustainable. Yet USDA researchers, working in a humid temperate region, found that an organic reduced tillage system with cover crops and other organic inputs outperformed conventional continuous no-till in terms of soil quality (see sidebar).

The key to resolving the organic grower’s dilemma is that sustainable organic weed control does not simply substitute steel for synthetic herbicides. Rather, multiple practices—“many little hammers”—are employed in a coordinated strategy to reduce weed pressure on crops (Liebman and Gallandt, 1997). If steel becomes the organic “big hammer” to replace the herbicide “big hammer” of conventional systems, it can "hammer" the soil to death before weeds are adequately controlled. However, if tillage and cultivation are utilized in conjunction with other practices (crop rotation, optimizing cash crop growth, cover cropping, intercropping and relay cropping, mulching, mowing, flame weeding, and grazing), the farmer will realize more weed control with less tillage. Note that most of these other practices do not compromise soil quality, and many can enhance it, allowing the organic grower to accomplish sustainable weed and soil management simultaneously (Hatfield et al., 1998). Furthermore, the nonuse of herbicides gives the organic farmer greater latitude to implement these practices (see sidebar).

When cultivation becomes necessary, the organic farmer can choose from a large and ever-growing cultivation toolbox to get the most weed control for the least soil damage. Rotary hoes, torsion weeders, vibrating tines, rolling baskets, and finger weeders work a fraction of an inch into the soil to remove millions of just-emerging weed seedlings without disturbing most of the soil profile. For larger weeds, springtooth harrows, rolling cultivators, and sweeps can be adjusted to work just deep enough to knock out the weeds at hand. Different implements can be deployed on a tractor toolbar for precision within-row and between-row weeding in one pass. Skillful selection and use of these implements can result in effective and fairly soil-friendly weed control, even in challenging crops like carrots and onions. Thus, cultivation technology has come a long way since the days when cultivation meant repeatedly pulverizing the soil between crop rows, and hoping that the crop can somehow beat the within-row weeds (Bowman, 1997).

Following are some tips to help new organic farmers knock out weeds effectively while protecting soil quality.

• Consider alternatives to tillage and cultivation such as flame weeding, mulching, interseeding cover crops, and livestock grazing (avoid livestock grazing within 120 days before harvest of vegetables or other food crops to protect food safety and comply with USDA organic standards.)
• Cultivate when weed seedlings are small and can be removed with very shallow, light stirring of the soil.
• Select the best tool for the weeds present and the growth stages of crops and weeds, so that the operation is most effective, and fewer passes will be needed.
• Consider mowing rather than tilling to prevent seed production after crop harvest or during other fallow periods (effective for certain annual weeds).
• Avoid cultivation for aesthetic reasons only (that is, removing weeds that do not threaten current or future crop production).
• Compensate for tillage impacts with cover crops, organic mulches, and other measures that restore soil health while reducing weed pressure.
• Prevent weeds from forming and dispersing viable seeds by mowing, grazing, or tilling weeds prior to seed formation, or by capturing weed seeds during small grain harvest.

Even on the best-managed farms, weed pressures sometimes necessitate vigorous tillage to permit successful organic vegetable production. For example, a newly-broken sod containing perennial weeds may require several passes with a chisel plow, disk, rotary tiller, or rotary spader. Invasive weeds like the deep-rooted perennial Canada thistle (Circium arvense) may arrive on the farm despite diligent efforts at sanitation, or untimely rains may delay weed control operations until weeds are too large to be controlled by light cultivation. When aggressive tillage is needed, follow it with a fast-growing, competitive, high-biomass cover crop or cash crop to replenish organic matter, and use high quality compost or compost tea to restore soil life and promote humus formation.

The moldboard plow, often shunned by organic growers as a “worst” implement for soil quality, may be the right tool in certain cases—for example, to knock out certain perennial weeds, or to break sod after several years in a perennial cover crop. Soil inversion can also eliminate the short-lived, surface-germinating seeds of certain weeds like galinsoga (Galinsoga spp.) in the event of a heavy “seed rain". However, if the field contains longer-lived weed seeds buried in the soil profile, plowing can backfire by bringing them back to the surface where they may germinate and grow rapidly. In any case, care must be taken to avoid plowing wet soil, or plowing so deeply as to to bury the biologically active topsoil under a layer of subsoil. When done appropriately and followed by cover cropping and other restorative measures, a single pass with the moldboard plow may control certain weeds better, and damage the soil less, than efforts to deal with the weeds through repeated disking, rototilling, or cultivation.

References and Citations

• Bowman, G. (ed.) 1997. Steel in the field: A farmer’s guide to weed management tools. Sustainable Agriculture Network Handbook Series Book 2. National Agricultural Laboratory, Beltsville, MD. (Available online at: http://www.sare.org/publications/steel/index.htm.) (verified 23 March 2010).
• Comis, D. 2007. No shortcuts in checking soil health. Agricultural Research. 55: 4–5. (Available online at: http://www.ars.usda.gov/is/AR/archive/jul07/soil0707.htm) (verified 23 March 2010).
• Hatfield, J. L., D. D. Buhler, and B. A. Stewart (ed.) 1998. Integrated weed and soil management. Ann Arbor Press, Chelsea, MI.
• Liebman, M., and E. R. Gallandt. 1997. Many little hammers: Ecological approaches for management of crop–weed interactions. p. 291–343. In L. E. Jackson (ed.) Ecology in agriculture. Academic Press, New York.