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Replacing Summer Fallow with Cover Crops

By Dunling Wang, PAg, PhD, Provincial Specialist, Alternative Cropping Systems, Regina

July 2020

Before 1980, summer fallow was a normal part of a dryland farming system in Saskatchewan. Summer fallow leaves the soil idle for a growing season aiming to help soil moisture retention, weed control, pest and disease management. Summer fallow aerates the soil, stimulating microbial activity, resulting in an accelerated soil organic matter breakdown and nutrient release. As such, summer fallow was quickly recognized to cause a loss of soil organic matter and nutrients, in addition to making the soil more susceptible to wind and water erosion.
Summer Fallow Field
Summer Fallow Field

In Prairie soils, over the past 50 to 100 years, summer fallow as part of a crop rotation has been a key contributor to the decline of soil organic matter by 50 per cent along with a reduction in soil organic nitrogen by 60 per cent. Using summer fallow to increase soil nutrient supplies for the subsequent crop comes entirely at the expense of a reduction soil organic matter. The loss of soil organic matter causes soil degradation, including increased soil erosion, destruction of soil structure and shrinks soil biodiversity.

Research has found that summer fallow did not significantly increase moisture and nutrient reserves nor control weeds any better than growing a competitive crop. For example, wheat yielded higher after sweet clover comparable to summer fallow [1], which was clearly related to the nitrogen fixed by the legume cover crop. Experiments also showed that black lentil (Indian Head variety) as a green-manure crop appeared to be a suitable alternative to the summer fallow. The black lentil terminated at full bloom produced dry matter of 3,170 kg/hectare, which became part of the soil organic matter and significantly increased the soil nitrogen supply for the following crops. Both peas and lentils were high-value crops, when grown in place of summer fallow, the economic return was comparable with more traditional cereal crops. In addition, wheat following the pea or lentil yielded similar to that wheat following summer fallow without additional nutrients supplied [2].

Polyculture Cover Crop
Polyculture Cover Crop

Polyculture cover crops, where multiple species crops are grown together, can be incorporated into crop rotations to create a more biodiverse system. In polyculture, crop species include nutrient scavengers, pollinator attractors, nitrogen fixers and carbon providers. Together, they capture solar energy, convert atmospheric CO2 into organic matter to build soil health and restore soil microbiology. Increased soil organic matter and improved soil structure also enable more water to be absorbed into the soil, which has been critical for crop production in drought years.

Producers who still believe practicing summer fallow helps to increase soil fertility and crop yield should evaluate the cost in the long run. The short-term benefit of summer fallow can be quickly eroded by the long-term costs. Before you decide to continue summer fallow next year, make sure you consider the following:

  1. The loss of soil organic matter as a result of fallowing has significantly reduced soil water holding capacity. According to United State Department of Agriculture, increasing soil organic matter by one per cent is equivalent to adding 25,000 gallons of available water to the crop per acre.

  2. Tillage can decrease organic matter by 0.1 per cent each year [3] and reduced tillage can gain back that organic matter. Growing a good cover crop can potentially add another 0.1 to 0.2 per cent soil organic matter per year. Replacing the summer fallow with a cover crop can potentially add one per cent of organic matter to the soil in five years. A one per cent of soil organic matter gain in the top six inches of soil contains 500 pounds of organic nitrogen per acre.

  3. Cover crops can suppress annual and perennial weeds as effectively as summer fallow. Selecting quick growing species that produce large amounts of biomass, can compete for resources that would otherwise go towards weeds. Seeding a blend of cover crops is often more effective as some species grow quickly others take longer to establish but live longer. Adding legumes in the mix can improve the soil nitrogen supply.

  4. Cover crops can suppress pests and diseases by: a) extending the length of a crop rotation; b) providing a physical barrier to the host-specific pests and diseases; and d) reducing the pathogen density because of increased biodiversity. Using cover crops to attract beneficial insects and repelling pest can be an integral part of a sustainable crop production system.

In conclusion, it is worth it to replace summer fallow by growing cover crops. This will add organic matter back to soils, improve soil health and increase biodiversity. Building and maintaining healthy soils is the foundation for a farming system to be productive, profitable and sustainable.

For more information, please read:

[1] Spratt et al., 1975. Summer fallow substitutes for western Manitoba. Can. J. Plant Sci. 55: 471-484.

[2] Brandt, 1996. Alternatives to summer fallow and subsequent wheat and barley yield on a Dark Brown soil. Can. J. Plant Sci. 76(2):223-228.

[3] Brown, 2014. Healthier soil can reduce nitrogen costs, offer drought tolerance. Conservation Technology Information Center, Jan 29, 2014.

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