Mark Schonbeck, Virginia Association for Biological Farming
Ronald D. Morse, Virginia Tech (emeritus)
Continuous no-till is difficult to achieve in organic production, but “rotational no-till” may offer a more feasible approach as part of an organic reduced-tillage system. Using this approach, fields are tilled after harvest and before planting a high-biomass cover crop, but both termination of the cover crop and the subsequent cash-crop planting are handled using no-till methods. This strategy provides weed management while offering some of the other benefits of growing a no-till cover crop, notably soil health improvements. The cover crop is usually killed by roll-crimping, flail mowing or winter kill, followed by no-till planting of transplanted or large-seeded vegetable or row crops. Examples of this strategy:
- transplanting tomatoes and peppers into roll-crimped winter rye and hairy vetch
- planting fall brassicas into flail-mowed summer foxtail millet and soybeans
- planting early spring vegetables into winter-killed oats and peas
Whereas a reduced-tillage system such as this can enhance soil quality, reduce annual weeds and give good yields, organic conservation tillage systems are not recommended for fields in which the population of weed seeds (weed seedbank) is extremely high and/or when perennial weed species such as Canada thistle, yellow nutsedge and Johnsongrass dominate the weed flora . When they emerge from rootstocks, tubers or rhizomes, these weeds can grow through even a heavy cover crop mulch and compete severely with a no-till planted crop. Before attempting no-till cover crop management, first attempt to bring existing weed problems under control, as discussed in the section, “Remedial Practices for Improving Weed Management and Soil Health.”
Other factors can influence the likelihood of success with no-till planting into a cover crop mulch.To generate an adequate weed-suppressing mulch: at termination, ensure the cover crop is mature (at heading/flowering with pollen shed); is nearly weed free (with less than 5 percent of aboveground biomass consisting of weeds); and has developed at least three tons of dry weight biomass. This level of biomass is usually achieved when the stand is solid and 3–4 feet tall, the ground cannot be seen when viewed from above, and thoroughly air-dried clippings from 1 square yard weigh about 1.5 pounds. Include cover crop species that provide a persistent mulch, such as a cereal grain or other grasses. Buckwheat, crucifer (e.g., radish or mustard) or all-legume cover crops tend to break down too rapidly to provide weed suppression.
New developments in non-chemical weed control tools and tactics can make rotational no-till more practical in organically managed fields with moderate weed pressure. Examples include high-residue cultivators, tractor-drawn weed pullers and thermal weed control based on hot water or steam rather than flaming, which can be a fire hazard in the presence of dry cover crop residues.
Also consider historic and current weed and pest issues in a field that may receive mechanical cover crop termination. If the field has been converted from sod to annual production within the previous year, bits of sod may be present that can regenerate and become perennial weeds without an herbicide to control them. Additionally, use caution if slugs, squash bugs and other pests that typically thrive in organic mulch have recently been a problem.
Considerations beyond weed management play a role in deciding whether to use organic no-till management. The type of soil will influence success with a crop following no-till cover crop termination, including whether the soil is heavy or clayey and slow to drain or warm up, or is light to medium in texture, well drained and quick to warm up. In addition, soil health plays an important role in successful organic no-till, as these systems rely on a vigorous and diverse soil biota (or soil food web) to release nitrogen (N) and other nutrients from cover crop residues. A no-till cover crop residue keeps the soil cooler and leaves more residue on the soil surface, which leads to a slower rate of N mineralization compared to incorporating the cover crop, or green manuring. So, a crop such as spring spinach or broccoli that requires a lot of N in a short amount of time early in the season when soil temperatures are lower may experience an N deficiency under no-till cover crop management, especially in heavier soils. Due to the slower start that a no-till residue gives to subsequent crops, it may not be ideal when trying to capture an early spring market for a particular crop such as tomatoes.