Green Manure Cover Crops Organic Sustainable Soil Fertilizers

Cover crops have been used for years to organically enhance soils and add nutrients. The practice is said to be one of the more sustainable ways to produce crops worldwide.

A cover crops or green manures is when you grow plants specifically for the purpose of reducing soil erosion, increasing soil fertility and water retention while assisting in the management of weeds, pests and diseases. [1] [3] Depending on the species used as a cover crop they may provide a variety of the benefits we have just spoken about.

In order to get the maximum benefit from the practice cover crops are often tilled into the soil or mowed just before or after the crop has flowered. Common crops used are covered in Eliot Coleman book Four Season Harvest. In this book Mr. Coleman provides great information on common cover crops and how to use them.

There has been a wealth of research done investigating cover crops. A great paper was published in the journal of Weed Management where researchers from Penn State University discussed and summarized the research that has been completed on cover crops. [2]

So now lets go through the results of the scientific investigation of the benefits of cover crops. I am going to investigate them in the following order:
1. Erosion
2. Weed management
3. Organic Material Addition
4. Beneficial Organisms
5. Nitrogen Fixation
6. Disease Management
7. ***Impact on Soil Organisms (allopathic and beneficial)
8. Cost effectiveness and overall environmental impact


Plants like cover crops have been shown in studies from all around the globe to reduce erosion. In order to use cover crops to reduce erosion generally they are planted where traditionally there would be open soil.

Their roots stabilize soils while the plant above ground helps dissipate the energy of rain water when falling and flowing over the surface. Slowing the water down helps the soil absorb more water while the stability prevents the water from carrying away the soil particles and nutrients.

Weed management

An added benefit of using cover crops to help prevent erosion is they grow where traditionally weed species may try to establish. The presence of these established cover crop plants helps prevent weeds by simply out competing them for the space to grow.

When cover crop living mulches were compared to herbicide practices a similar weed suppression performance was achieved. These results are significant as the weeds were less likely to adapt to a cover crop weed management strategy as they do to commercial herbicides.

Cover crops have been shown to reduce erosion and help keep weeds down. So what benefits do the plants themselves add?
Organic Material Addition

Low levels of organic material in soils have been shown to be correlated with lower crop production due to increased compaction and poorer water and nutrient retention. The use of cover crops can help add organic material to the soil by leaving the dead plant and root material in place. After the effective time, generally right before or after flowering, the plant can be left to die in the winter or simply chopped and dropped in place.

The organic material will return nutrients to the soil that may have otherwise leached while improving the soils structure as it decays.
Influence on soil organisms.

When cover crops are chopped and dropped and left in the mulch layer they have been show significantly increase the numbers of beneficial organisms. For instance earthworm biomass was increased in a study where cover crops were used with a corn production. The researchers thought the 7 time increase in earth worm biomass was likely to do with the use of cover crops paired with a no-till method of cultivation.

Earthworms eat the decaying plant material leave behind plant available nutrients, beneficial bacteria and plant growth hormone. The worms recycle surplus nitrogen that the cover crops took from the soil that may otherwise be lost through denitrification or leaching. [4]

Now that we have taken a look at the benefits the cover crop plants can have on the surface let’s look below the soil.

Nitrogen Fixing

Legume family cover crops like peas and beans form a beneficial relationship with soil born bacteria. The plant trades sugars it produces in photosynthesis for organic nitrogen. The bacteria have a special ability to use atmospheric nitrogen that is not available to plants and covert it to an available form.
The nitrogen fixing bacteria that form the symbiotic relationship with legume family plants live freely in small quantities in the soil. These bacteria greatly increase in numbers when they are in the presence of legume family crops and go back to baseline levels after.

Often when purchasing cover crops distributers will have an “inoculant” that goes with them. There is generally no need for this inoculant. There are plenty of well adapted bacterial in your soil already that will if given the opportunity form this relationship and produce organic nitrogen. The bacteria in the products often are killed through shipping and storage and even if they make it alive they are usually only a few species and will often get out competed by the native species.

Cover crops not only can feed your crop plants directly they can also protect them.

Disease Management

Disease management and pest management has also been shown to improve when green manures are present. Their presence reduced predation on our crops by improving conditions for the pest’s predators while helping to mask or hide the crop of interest.

There are also reports that some common cover crops have allopathic properties helping to reduce or eliminate nematode pests in the soil. That said I was unable to find supporting research while putting this episode together.

The use of cover crops for diseases control is a very specific relationship and care must be taken to select the correct species of cover crop for the crop or pest of interest.

So far cover crops help manage diseases prevent soil erosion, reduces weeds and nutrient leaching while adding organic material and fixing nitrogen in the soil. This all sounds great but is it cost effective?

Cost Effectiveness and Environmental Impact

Unlike the other free and local resources we have been speaking about you will have to purchase the seeds to use as cover crops. It can be done with common varieties that can also produce food however cover crop specific varieties are generally are more effective.

If you use herbicides the weed control of cover crops can replace the need and costs of herbicides. If you grow organically like I do using cover crops for weed management can help save the time of weeding. You’re also able to limit or prevent erosion while adding more organic material to the soil.

Next let’s compare the amount of nitrogen that is fixed vs equivalent synthetic products. When studied in a number of combinations optimal harvests were achieved when cover crops were used with a 1/3 reduction in synthetic fertilizer application. [2] The 1/3 reduction of synthetic fertilizers has been shown to more then cover the costs of the cover crop seeding in commercial farming.

So yes you are saving money. You are also reducing the risk of environmental damage cause by the leaching of nutrients and herbicides.

Today we spoke about cover crops in general and they did not disappoint. They reduce soil erosion, increasing soil fertility and water retention while assisting in the management of weeds, pests and diseases. One downside that cover crops do have is they will compete for available water and nutrients. In a home garden these two drawbacks are likely not going to cause an issue as we do not always have to rely on rain for moisture and generally speaking most home garden soils have a surplus of nutrients through the growing season especially if you used the free and local resources like I do.

Through this investigation process we have we have been able to support some of the practices and resources that are available to organic gardeners while eliminating ineffective or damaging practices. By adding cover crops we now have a tool that can add high amounts of nitrogen to our soils.

This practice has similar benefits to the use of comfrey and used coffee grounds. However because the plants are grown in place the phosphorus and potassium are recycled in place while adding new fixed nitrogen directly to the soil. The use of cover crops may be an effective tool for balancing soil nutrients.

All four of the organic practices that have proven to be supported by our work add organic material that improves soil structure, increases the beneficial organism populations that improve the nutrient cycle ultimately allowing me to avoid the need to buy any organic products or synthetic fertilizers.
If you missed any of these episodes I will provide a link to the playlist. The videos in the playlist have both the supported practices as well as the ones that are not.

[1] General information about Cover Crops.
[2] Cover Crops and Living Mulches
[3] General information about Green Manure
[4] Denitrification

About Stephen

The Alberta Urban Garden Channel hopes to promote organic gardening that is simple, sustainable and does not have to cost a lot. We do this by investigating the Science behind gardening, methods, practices and products to make sure that you will have the best chance of successfully growing your own food at home.