Chapter 6

Cover Crops for Organic Farms

Establishment of Cover Crops

In theory cover crops can protect soil from erosion, improve soil tilth, supply nitrogen, reduce weeds, provide nectar and pollen for beneficial insects, increase the winter survival of mycorrhyza, help to manage soil-borne diseases, and provide other benefits. In practice, it’s often difficult to establish cover crops in organic farming systems well enough to to take advantage of these benefits. Success is dependent on several factors: seed selection and quality, seedbed preparation, planting date, seeding depth, seeding methods, and fertilization.

Cover Crop Seed Selection and Quality

Cover crop establishment will be only as good as the seeds that are planted. Growers are encouraged to consider seed selection carefully because understanding the components of seed quality and selecting high-quality seeds can save time and money. Beyond this, the quality of the cover crop itself will have a direct impact on the overall organic farming system.

National Organic Program (NOP)

National Organic Program (NOP) requires the use of organically grown seeds for cover crops. There are several exceptions for using non-organically grown plant materials or those treated with prohibited substances.

Seed Quality

The law requires that each seed lot offered for sale must be truthfully labeled, which is regulated by the Federal Seed Act as well as state seed laws. In addition, all state certification agencies comply with the minimum requirements and standards of the Association of Official Seed Certification Agencies (AOSCA) to insure uniform testing methods and minimum standards of seed quality. The format of seed labels may be variable, but all labels will have some semblance of the following as required by the Federal Seed Act for seed in interstate commerce.

Seedbed Preparation

Seedbed conditions are critical to ensure proper germination and plant development. There are two basic methods of seedbed preparation.

Conventional Seeding

Conventional seedbeds are prepared in two phases, first with plows, disks, chisels, or sweeps, commonly referred to as primary tillage. After the primary tillage is completed, final seedbed preparation is performed to smooth and firm the seedbed. Roller harrows, culti-packers, packers or spike tooth and spring tooth harrows are used to firm and smooth the final seedbed. Two essential requirements for good stand establishment are a firm, clean seedbed (relatively free of residue) and a smooth, uniform surface. Packing during seeding and afterwards will yield good soil contact with the seed.

No-till Seeding

No-till seeding involves directly seeding into most weed-free crop stubble on coarse to medium textured soils. Stubble that is free of volunteer crop and weeds provides a firm seedbed and a favorable microclimate for seedling establishment. Crop stubble should be removed from the field or shredded and uniformly scattered.

Planting Times

Cover crops can be planted in the fall, spring, or even the summer. The timing depends on the purpose for implementing the practice as well as equipment and the cover crop species chosen.

Pre- or Post-season Seeding

Cover crops can be planted early in the growing season before late-season vegetables or field crops (e.g., dry bean) or after cash crop harvest. Example: Cereal rye following potato harvest. 

Winter Annual or Cool Season Cover Crops. Most of the winter cover crops are planted in the fall after the cash crop is harvested, and provide cover over the winter months. In general, cover crops that are not winter hardy need to grow at least four to six weeks in the fall before a killing freeze in order to achieve reasonable benefit from them.

Summer Annual Cover Crops. Summer annuals need to be planted in late spring or early summer when soil temperatures are warmer. With warm temperatures and enough moisture, many of these will grow quickly and produce lots of biomass in a short time.

Frost Seeding

Frost seeding involves broadcasting appropriate species just after snowmelt in late winter/early spring. The freeze-thaw action of the soil works the seed into the soil cracks and germinates when the temperature rises in the spring. Frost seeding should be done early in the morning when frost is still in the soil. Seed early enough allow for several freeze-thaw cycles.

Dormant Seeding

Dormant seeding involves broadcasting appropriate species in the early winter just after the field has frozen and the air temperature is low enough to prevent germination. Ideally this is just prior to snowfall. Seeds remain dormant under the snow through the winter and emerge in the spring.

Interseeding

Interseeding in this guide refers to the process of broadcasting a cover crop into a growing cash crop. Also, known as undersowing or oversowing. Interseeding a cover crop into a standing cash crop is a way get a jump on the fall/winter cover crop season and can help protect soil between rows from erosion and compaction. Careful consideration needs to be made regarding the stage of the cash crop, cover crop species selection, and environmental conditions.

Seeding Depth

Cover crop species vary from 0.25 to 1.5 inches in their optimum seeding depth. Small-seeded legumes such as true clovers, alfalfa and sweetclovers, have very small seeds and should not be planted too deep – ¼ to ½ inches is adequate. These small seeds can be “frost-seeded” – broadcast in late winter and worked into the ground naturally through the expansion and contraction of freezing and thawing soil.

Seed Drill Configurations

Many drills have a large and a small seed box. The large box, sometimes called the grain box, is optimized for large-seeded crops and the drop tubes from the large box direct seeds to the deepest point in the furrow created by the disk openers. The small box, sometimes called the legume or grass seed box, is optimized for small-seeded crops. Seed boxes should also be equipped with agitators to keep fluffy seed flowing smoothly out of the boxes. The drop tubes from the small seed box can be directed to drop seed at the rear of the disk openers, resulting in a shallower seed placement. The drop tubes from the small box can also be left hanging straight down to dribble seed on the soil surface.

Seeding Methods

Cover crops in organic farming systems are usually planted with a drill or broadcast on the soil surface, but several alternate methods can be used. Good soil-seed contact is required for germination and emergence. The benefits of a cover crop will be greatest when a good stand is established with as little soil disturbance as possible.

Broadcast Seeding

Broadcast seeding is not very precise and is the least successful seeding method. Seed is broadcast onto a prepared, roughened seedbed, followed by raking, dragging or rolling to cover as much of the seed as possible with about 1/4 to 3/8 inch (0.6 to 1cm) of soil. The seed must be covered though care must be taken not to bury fine seed too deeply. It is usually useful to take a light roll over after harrowing to consolidate the seedbed and ensure good soil/seed contact.

Seed Drilling

Drilling cover crops (either conventional or no-till) generally speeds germination and early establishment and creates an even stand. Seed drills place the seed with much more precision than broadcast seeders. Seed drills can be calibrated to apply seed at precise rates and depths. For expensive or small seed, seed drills are often the best approach.

No-Till Drilling

Another option in seeding cover crops with no-till drills, which offers the greatest long-term potential benefits in erosion control, soil quality and productivity, water conservation, energy efficiency, and production efficiency.

Harrow Seeding

Rotary harrows, coulter harrow type vertical tillage tools or similar tools can be used to aid in fluffing or cutting residue to allow improved contact with the seed and soil. Air delivery seeders can be mounted to these tools to deliver the seed to the soil as the residue is lifted or cut. The implement shall be set to run no deeper than one inch and not be designed to invert the soil or to bury the crop residue.

Aerial Inter-Crop Seeding

Broadcast via a plane, helicopter or high clearance spreader into existing vegetation or residues. This method relies on rain, freeze/thaw cycles, or snow to incorporate the seed. Timing in the fall should be just prior to leaf drop or crop maturity for most cover crops. Some shade tolerant species may be adapted to earlier seeding.

Fertilization

Cover crops usually follow heavily fertilized crops and do not require fertilization. Fertilizer and lime recommendations are generally based on future cash crops. In some situations where a goal is to produce a lot of biomass and build organic matter with a crop like sorghum sudangrass, or produce a large root to reduce soil compaction with a crop like forage radish, additional nitrogen in the form of manure of compost may be necessary to achieve maximum growth.

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