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Improving Soil Fertility through Cover Cropping, Green Manure, and Intercropping
Modern industrial agriculture, as we currently know it, was ushered in by Haber, Bosch, and BASF (Haber Process, 2016) in the 1930’s and John Deere’s moldboard plow a century earlier. A little over 80 years later, time-tested farming practices for maintaining soil fertility were replaced with synthetic fertilizers. This change allowed for unprecedented growth in farm productivity and, along with other factors, the demise of the diversified family farm; farmers were no longer constrained by nutrient requirements and could retool their farms to focus on the most profitable cash crops.
“Between 1930 and 2000 U.S. agricultural productivity (output divided by all inputs) rose by an average of about 2 percent annually causing food prices to decrease. "The percentage of U.S. disposable income spent on food prepared at home decreased, from 22 percent as late as 1950 to 7 percent by the end of the century." (Intensive farming, n.d.)”
These changes have had a profound impact on humanity; We have reduced hunger and starvation, increased nutrition, and improved life expectancy, among other. However, the tenants of modern agriculture have also had a negative impact on our soils and the environment in general. Nationally, soil has been lost to erosion, excess nutrients are damaging our lakes, rivers, and oceans, and the structure and composition of our soil have degraded (Intensive farming, n.d.) and (Dabney, 2001).
Our soils are a finite and perishable resource that are a legacy that we leave to future generations - It up to us to ensure we enhance these resources. The approaches we take must not diminish the productivity gains we have seen or we will be unable to feed an ever-increasing human population. By reintroducing the core practices of cover cropping, green manures, and intercropping we can improve soil fertility while reducing and/or eliminating our dependency on synthetic fertilizers.
Cover crops are crops grown for purposes other than harvest and may include many traditional cash crops like rye, oats, alfalfa, and winter wheat (Unknown, n.d.). They are gown to improve soil fertility and quality, reduce erosion, improve soil water holding capacity, and manage pests, diseases, and weeds (Cover crop, n.d.). The choice of which cover crop to be grown is often based upon the soil needs, time of year, rotational limitations, termination process, and cost - However, they must be carefully chosen. Marianne Sarrantonio (Greg Bowman, 2007), recommends the following process for select a cover crop; 1) Clarify your primary needs, 2) Identify the best time and place for your cover crop, and 3) Test a few options. Creating a targeted goal will help narrow the selection – for example do you want to provide nitrogen, add organic matter, improve soil structure, reduce erosion, etc. Once you have established a goal, you need to determine when to plant your cover crop and where you should grow it; this will help narrow the species and varieties that will meet your goal, where you need them to grow, and when you need them to plant them. A list of common cover crops and their use is detail in table 1 and Chart 1.
The soil quality is often measured by its salinity, pH, organic matter, texture, nutrient levels, and micro flora/fauna among others. For a crop to flourish, the soil quality and fertility should be well aligned with the needs of the crop and if they are not, crop yields will be negatively impacted. Compacted soils, which limit water and air penetration, can be addressed by crops that loosen soils, pans can be remediated by deep rooted plants that will penetrate and breakup pans. Salinity, water holding capacity, pH, and soils that are low in flora and fauna (macro and micro) can be helped by cover crops that add organic matter to the soil (Patrick, 1957). For example, a cover crop composed of oats, daikon radish, and field peas can help improve soil fertility by adding 2,000 to 4,000 Lbs. of dry matter per acre (DM/A) (unknown, n.d.) from the oats, 5,000 DM/A and 90-300 pounds per acre of nitrogen from field peas (Field Peas, 2007), and 7,000 DM/A from the radishes with the added benefit or reduced compaction from root penetration of the radish (Gruver, 2016). Taken together, the dry matter, nutrients scavenged and fixed, loosening of the soil will improve overall soil fertility; when we add in additional side benefits like habitat created for beneficial organisms, weed suppression, etc., the improvements to soil fertility is compounded.
Green manure is a crop grown, terminated prior to maturation, and incorporated into the soil to improve soil conditions and/or fertility for a subsequent cash crop and may be classified as a cover crop. Often the crops chosen are done so because they fix nitrogen, add organic matter, and/or scavenge soil nutrients. When they are terminated, and incorporated into the soil, the nutrients they created or scavenged are release, through decomposition by heterotrophic bacteria (Green manure, n.d.), and additional organic matter is added to the soil. Since natural processes are used to mineralize the nutrients and break down the organic matter, considerations need to be taken to ensure that decomposition has progressed to the point where nutrients will be available and not bound, allelopathic characteristics have degraded, and decomposer will not interfere with the subsequent crop.
The choice of a green manure should be made in a similar fashion to the that of a cover crop, with special consideration paid to termination characteristics. Crops that are difficult to terminate present special challenges when used as a green manure because the time, effort, and processes required for their termination – Green manures that are not fully terminated will not provide the desire fertility boost and may compete with subsequent crops. It should be noted that often times, one crop acting as a green manure may not provide the necessary soil improvements. Just as nature prefers diversity, green manures can be composed of multiple species, which when working together, improve multiple soil characteristics.
Composite green manures are composed of plant varieties that were chosen for how they improve the soil and work with each other. For example, an Oat / peas / vetch green manure will add nitrogen to the soil (peas and vetch), add organic matter and weed control (oats and vetch), and perform soil nutrients scavenging (vetch). The oats in this combination will provide scaffolding for the peas and vetch. The vetch decomposes quickly when terminated and will provide nitrogen and nutrients (potassium and phosphorous), followed by the peas (nitrogen) and lastly the oats. This combination has been shown to fix 90-150 lb./A of nitrogen (Eric Sideman, 2013). Other common composite green manures are radish / oats / pea, where the radish help with soil compaction, or substituting buckwheat for oats for a quick turnover rotation with quick decomposition (Eric Sideman, 2013). Common green manures can be found in Table 1.
Intercropping is the practice of growing more than one crops in close proximity for the purpose of increasing productivity from the land or the primary crop. In this paper, we will focus on the later. Utilizing cover crops and green manures can provide substantial benefits to cash crops, but some of the benefits may be limited in duration. Long duration cash crop (fruits, berries, biannual, perennials, long season annuals, and etc.) nutritional needs will likely extend beyond the time that decomposition of cover crops and green manures provide their maximum benefits. Intercropping helps to address this need by allowing farmers to maximize the productivity and yield of their primary crop by adding supporting crops that provide direct benefits, like nutrients, disease / pest management, support for soil flora / fauna, amount others.
Let’s consider a traditional cash crop like corn with respect to its need for nitrogen. Green manures or cover crops planted in the fall and incorporated into the soil will provide a ready source of nitrogen for the seeds post-germination and into the early life cycle of the corn. However, over time the nitrogen stores will be utilized by the corn or other soil microbes. While the corn has had a great start, to reach its full potential, additional accessible nitrogen stores will be required. The corn will scavenge some nitrogen, but it is unlikely to find enough for optimal growth. Corn following an alfalfa rotation has been shown to fix reserves of ~ 170 kg/Ha-1 of nitrogen, but this still leaves a deficit of up to 34 kg/Ha-1 of nitrogen (Sawyer, 2010). Intercropping corn with kura clover can fix between 74 kg/Ha-1 and 334 kg/Ha-1 of nitrogen; effectively meeting the needs of the corn (Sawyer, 2010). This approach can also reduce overall costs. Kura clover seeded at 4 lbs/Acre and a cost of ~ $4 / lbs (welterseeds, 2015)is well below the average cost of $200 / Acre for fertilizer (Fertilizer Cost move Down, 2013) and can be achieved with the same number of passes over the field.
Selection of the crop to intercrop requires the farmer to consider allelopathy and potential competition along with the other factors we have previously discussed in this paper. Intercropping need not be implemented for only nutritional needs, but can be used to improve overall soil conditions. Intercropping grasses between rows of trees in an orchard can reduce soil erosion, improve soil texture, increase water holding capacity of the soil, and provide habitat for beneficial insects.
Cover crops, green manures, and intercropping have been in used since the earliest days of agriculture and have a proven record of success. Their use can provide modern agriculture with the yields that they are accustom to and at potentially reduced costs, but they will require farms to learn or reacquire lost practices. These practices have the potential to slow, stop, and even reverse the reduction in the fertility of our soils and move us from an era of soil mining to soil fertility building.
Table 1 – Common Cover Crops
Hardy through zone
Seeding rate² (lb/A)
Seeding depth (inches)
N-capture/ fertilizer equivalency (lbs/A)
Grasses (Cool season)
Cereal rye (Secale cereale L.)
112 (2 bu)
Excellent nutrient scavenger (esp. N)
Most cold tolerant of commonly used cover crops; provides living cover in winter and spring, erosion control, weed suppression, nutrient recycling, organic matter improvement, soil tilth improvement; earliest small grain to mature
Regrowth may occur if not completely controlled; explosive growth in spring poses termination challenges; possible following crop suppression due to allelopathy or nutrient tie-up; may attract some insect pests.
Winter wheat (Triticum aestivum L.)
120 (2 bu)
Cold tolerant in most of PA; rapid growth; common varieties not as tall as rye and therefore easier to manage; provides living cover in winter and spring, erosion control, weed suppression, nutrient recycling, organic matter improvement, soil tilth improvement
Accumulates lower amounts of biomass than rye; possible following crop suppression due to nutrient tie-up; may attract some insect pests; matures after triticale
Intermediate between wheat and rye
Intermediate between wheat and rye; matures after barley
Winter barley (Hordeum vulgare L.)
120 (2.5 bu)
Good nutrient scavenger
Cold tolerant in southern parts of PA; common varieties not as tall as rye and therefore easier to manage in spring; provides living cover in winter and spring, erosion control, weed suppression, nutrient recycling, organic matter improvement, soil tilth improvement
Winterkill is possible; accumulates lower amounts of biomass than wheat; possible crop suppression due to nutrient tie-up; matures after cereal rye
Spring oats (Avena sativa L.)
100 (3 bu)
Average nutrient scavenger
Very easy to manage because winterkills; provides erosion control, weed suppression, nutrient recycling, organic matter improvement, soil tilth improvement in fall, rapid growth in cool weather; ideal for quick fall cover or nurse crop with legumes; may produce more biomass in fall than other winter small grains if planted early
Winterkills in most of PA, no living root system in winter and spring; erosion control may be limited in spring; high lodging potential; susceptible to disease and insect pests
Annual ryegrass (Lolium spp.)
Cold tolerant in southern parts of PA; varieties not as tall as rye; provides living cover in winter and spring, erosion control, weed suppression, nutrient recycling, organic matter improvement, soil tilth improvement, high-quality fodder
May winterkill; may be difficult to control; low heat tolerance; may harbor insects; may reseed and become weed
Pearl millet (Pennisetum glaucum)
Heavy tillering, drought-tolerant, and adapted to low-fertility, sandy soils; can grow to 12 feet tall depending on variety; matures in 60–70 days; good forage
Very large biomass production can be challenging to manage
Sudan-grass (Sorghum bicolor)
Quick growth; scavenges nitrogen; competes with weeds; large biomass producer; alleviates compaction; can grow 12 feet tall
Prussic acid production when young, drought stressed, or frosted—do not graze then; large biomass production can be challenging to manage
Japanese millet (Echinochloa frumentacea)
Very fast growth, mature and up to 4 feet tall in 45 days; resembles barnyard grass; suppresses weeds
Can become a weed if let go to seed; grows poorly on sandy soils.
Hairy vetch (Vicia villosa Roth)
Most cold tolerant and high biomass production; above-average drought tolerance; adapted to wide range of soil types; combines well with small grains
Requires early fall establishment; slow to establish; matures in late spring; high P and K requirement for maximum growth; can harbor pests; potential weed problem in winter grain; glyphosate not full-proof for control
Crimson clover (Trifolium incarnatum L.)
Fairly cold tolerant; rapid fall growth; high biomass production; matures midspring; above-average shade tolerance; forage use (no bloat); good nematode resistance
May winterkill; requires early fall establishment; poor heat and drought tolerance; residue has tough stems, difficult to no-till plant into
Red clover (Trifolium pratense L.)
SLP (2–3 yrs)
Survives winter; deep taproot; soils; tolerates wet soil conditions and shade; forage use, especially if mixed with grass
Needs to be established before midsummer because initial growth slow; high P and K requirements for maximum growth; hard seed can persist creating volunteer problems; pure stand forage causes bloat; vulnerable to some pathogens, insects
Field peas (Pisum spp.) (e.g. Austrian winter pea)
Rapid growth in cool weather; versatile legume; interseed with cereal and Brassica spp.; used as food or feed
May winterkill; shallow root system; sensitive to heat and humidity; susceptible to diseases, insect pests
Cowpea (Vigna uncuiculata)
Also known as black-eyed peas; adapted to wide range of soil conditions; deep taproot can extract moisture from deep in profile
Performance has been erratic in PA trials
Sunnhemp (Crotelaria juncea)
Fast growing, up to 9 feet tall in 60 days; competitive with weeds; drought tolerant; can reduce root knot nematode populations
No seed production in contiguous U.S.; stems fibrous with age
Buckwheat (Fagopyrum esculentum Moench)
Spring or late summer
Fair to good nutrient scavenger (esp. P, Ca)
Grows on wide variety of soils (infertile, poorly tilled, low pH); rapid growth; quick smother crop and good soil conditioner
Limited growing season; not winter hardy; limited biomass accumulation; frost sensitive; poor growth on heavy limestone soils; occasional pests
Brassicas (Cruciferae family) (e.g. rape, radish)
Spring or fall
Good nutrient scavenger (esp. N, P, Ca)
Quick establishment in cool weather; prevent erosion in fall (radish) and spring (canola, rape); radish easy to manage because winterkills; deep, thick root systems; compaction alleviation; nutrient cycling; weed suppression
Radish winterkills in all of PA, while canola/rapeseed may winterkill in northern parts of PA; radish leaves soil bare in spring, therefore mix with a small grain
Source (Unknown, Characteristics of Common Cover Crops, n.d.)
Chart 1 – Performance and Roles of Cover Crops
(Greg Bowman, 2007)
Cover crop. (n.d.). Retrieved 11 27, 2016, from Wikipedia: The Free Encyclopedia: http://en.wikipedia.org/wiki/Cover_crop
Dabney, S. M. (2001). Using winter cover crops to improve soil quality and water quality. Communications in Soil Sciences and Plant Analysis, 1221-1250.
Eric Sideman, P. (2013, July). Maine Organic Farmers and Gardeners Association. Retrieved from Maine Ogranic Farmers and Gardeners Association: http://www.mofga.org/Publications/MaineOrganicFarmerGardener/Summer2013/GreenManures/tabid/2621/Default.aspx
Fertilizer Cost move Down. (2013, August 8). Retrieved from AgWeb: http://www.agweb.com/article/fertilizer_costs_move_down/
Field Peas. (2007). Retrieved from SARE: http://www.sare.org/Learning-Center/Books/Managing-Cover-Crops-Profitably-3rd-Edition/Text-Version/Legume-Cover-Crops/Field-Peas
Green manure. (n.d.). Retrieved 11 27, 2016, from Wikipedia: The Free Encyclopedia: http://en.wikipedia.org/wiki/Green_manure
Greg Bowman, C. C. (2007). Managing Cover Crops Profitably. Beltsville, MD: Sustainable Agriculture Network.
Gruver, D. J. (2016, February 26). Radishes – A New Cover Crop for Organic Farming Systems. Retrieved from Extension: http://articles.extension.org/pages/64400/radishes-a-new-cover-crop-for-organic-farming-systems
Haber Process. (2016, Oct 30). Retrieved from Wikipedia: https://en.wikipedia.org/wiki/Haber_process
Intensive farming. (n.d.). Retrieved 11 21, 2016, from Wikipedia: The Free Encyclopedia: http://en.wikipedia.org/wiki/Intensive_farming
Patrick, W. (1957). The effect of longtime use of winter cover crops on certain physical properties of commerce loam. Soil Science Society of America, 366-368.
Sawyer, J. E. (2010). Intercropping Corn and Kura Clover. Agronomy Journal, 568 - 574.
Unknown. (n.d.). Characteristics of Common Cover Crops. Retrieved from PennState Extension: http://extension.psu.edu/agronomy-guide/cm/tables/table-1-10-6
Unknown. (n.d.). Cover Crop Plants. Retrieved from USDA: http://plants.usda.gov/java/coverCrops
unknown. (n.d.). Oats. Retrieved from SARE : http://www.sare.org/Learning-Center/Books/Managing-Cover-Crops-Profitably-3rd-Edition/Text-Version/Nonlegume-Cover-Crops/Oats
welterseeds. (2015, june). Retrieved from welterseeds.com: http://welterseed.com/wp-content/uploads/2015/06/Retail-Price-List-2015.pdf
Wunderlich 15ica spp.; used as food or feed
Fast growing, up to 9 feet tall in 60 days; competitive with