Spring is really not that far off. Next month I’ll be planting starts and getting ready, and then the soil will warm and the fall planted covers will put on all their growth. When they are ready, I’ll cut them and till them into the top three inches of the soil to fertilize summer crops. As I reflect on this spring’s work, I thought I would post some thoughts on using green manures.

Green manures can be cut and left on the soil surface as a mulch or incorporated into the soil by tillage. If left as a mulch, the green manure will help with weed suppression and water conservation, but decomposition will be slower, as will the nutrient release to the cash crop. To incorporate or not depends on the goals the grower has for the cover crop.

2.JPEG Tomatoes planted into a tilled-in rye, vetch, and red clover green manure, May 21. Photo: Lee Rinehart, NCAT

3.jpg Tomato crop 50 days after transplanting, July 10. Photo: Lee Rinehart, NCAT

Incorporating cover crops provides a faster release of nutrients but does not provide weed suppression. With mowing and tillage, you get better contact between organic matter and the microbes that will feed on it. Tilling in cover crops also tends to allow soil to warm faster in the late spring, but the tillage also releases carbon dioxide into the air.

Timing is critical for incorporating cover crops into the soil. Soil moisture and temperature are important for soil microbes to wake up and begin the decomposition process. Soil moisture should be at field capacity, meaning 60% of the pore space in the soil is filled with water. Field capacity is generally reached in about a day after a soaking rain. Soil pH should be between 6 and 8, and the soil temperature should be warm, above 60° F. Finally, aeration is needed to provide oxygen to microbes and ensure aerobic decomposition.

Renowned growers Anne and Eric Nordell incorporate green manure into the top two inches of the soil instead of burying it deeply. They have found that shallow tilling of cover crops and compost “promotes aerobic decomposition, simplifies weed management, and improves tilth and moisture retention” (1). They have also seen their cash crops get off to a faster start, because the nutrients released through microbial decomposition are placed near the plant roots where plants can better make use of them.

Good incorporation of a green manure into the soil at the proper time should provide for optimum nitrogen release. For a legume or a grass cover crop (or mix), a rule of thumb is to turn the cover crop under when it is at about mid-bloom. At this stage, the bacteria population decomposing the organic matter will explode, and can double within a week. For monoculture legumes, expect a large release of nitrogen, some of which may be lost through volatilization or denitrification. High-carbon organic matter, such as mature grasses, straw, etc., will tie up nitrogen, as the bacteria use it to break down the fibrous material. The delay of nutrient availability to crops can last several weeks. But, when a legume is combined with a small grain, the added carbon of the grass will slow down nitrogen release from the legume and make the system more manageable, better timing the availability of nitrogen being released with the demand of the newly planted cash crop.

The optimal carbon to nitrogen ratio in the soil for biological activity is about 24:1. When the ratio gets above 24:1, the bacteria will begin consuming extra nitrogen in the soil to digest the extra carbon, effectively tying up nitrogen for several weeks. When the appropriate cover crop mix is planted, soil moisture and temperature are adequate, and the cover crop is tilled into only the first few inches of soil. The stage is set for providing fertility on-site. Then, it’s just a matter of timing for incorporation. Wait at least a week to plant into a plowed-in legume green manure, one to two weeks for a legume-grass mix, and up to three weeks before planting into a high-carbon green manure. These recommendations are variable, as soil moisture and temperature play a role in microbial decomposition. If a cover crop is incorporated when nighttime temperatures in the spring are in the 40s, for instance, decomposition will slow down. In this situation, the interval between termination and cash crop planting should be increased.

Screenshot 2024-02-20 102457.png Undercutting barley cover crop after mowing. Photo: Eric Nordell

In addition to incorporation by mowing, followed by rototilling or discing, cover crops can also be undercut with a cultivator fitted with sweeps to cut the roots just below the soil surface and then covered over with a furrower. This is a method the Nordells have used for incorporation, followed by a spring-tooth harrow. For market garden beds, a flail mower followed by a rototiller is very effective. Walk-behind tractors can be efficient for small farms, and rototiller and flail mower attachments are available for them.

The resources below will help growers at most scale think through the best way to terminate and use green manure this year. Whether covers are mulched on the surface or tilled in depends on the goals of the grower, the time of incorporation, the subsequent crop, soil temperature, and many more factors.

I invite growers to respond to this post with your thoughts and reflections, how you terminate cover crops, and how green manures fit into your summer cropping system.


(1) Nordell, Anne and Eric Nordell. 2018. Organic Surface Tillage. Cover Crops Incorporated. Rural Heritage, Cedar Rapids, IA. https://covercropsincorporated.wordpress.com/2018/05/16/organic-surface-tillage/


Pressman, Andy and Omar Rodriguez. 2023. Equipment and Tools for Small-Scale Intensive Crop Production. NCAT ATTRA Sustainable Agriculture.
Equipment and hand tools for soil preparation, planting, weed management, and harvesting. Lists of further resources and tool and equipment suppliers are included.

Terminating Winter Cover Crops in Organic Crops. By Mary Barbercheck and Kristy Borrelli. Penn State Extension.
This article focuses on ways that organic growers can terminate their winter cover crops before planting cash crops in the spring. In organic systems, the most common methods used to terminate winter cover crops is by winter-kill, mowing, tillage, a combination of mowing and tilling, roller-crimping, and grazing.

Warman, P. 1980. The Basics of Green Manuring. Ecological Agriculture Projects.
Excellent general overview of benefits and limitations of using green manure.

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