Improve Soil Health by Replacing Summer Fallow with Cover Crops

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

June 2024

Before 1980, summer fallow was part of a dryland farming system in many parts of the world including Western Canada. Summer fallow is defined as a practice where no crop is seeded in the growing season, and all weed growth is controlled by only tillage or herbicides. Summer fallow, along with the associated tillage, stimulates microbial activities resulting in an accelerated soil organic matter breakdown and nutrient release.

However, the use of summer fallow to increase soil nutrient supplies comes entirely at the expense of reduced soil organic matter. The loss of soil organic matter directly causes soil degradation, including increased soil erosion, destruction to soil structure and reduced soil biodiversity.

After much research, this farming practice was identified as causing loss of soil organic matter and nutrients, in addition to making soil more susceptible to wind and water erosion. Soils in the Prairies had lost organic matter by 50 per cent and organic nitrogen (N) by 60 per cent after several decades of crop production utilizing summer fallow practices.

Research also found that summer fallow did not increase fall moisture and nutrient reserves significantly, nor control weeds any better than growing a competitive crop. For example, wheat yielded higher after sweet clover compared to after summer fallow [1], which was clearly related to the N fixation by the legume cover crop. Experiments also show that black lentil as a green-manure crop was a suitable alternative to summer fallow. The termination of black lentil at full flowering stage produced dry matter of 3,170 kg/hectare, which became part of the soil organic matter and significantly increased the soil N supply for the following crops.

Both peas and lentils can fix a large quantity of N. When grown in place of summer fallow, the economic returns were comparable to growing a traditional cereal crop. Wheat following the pea and lentil crop yielded similar to that following summer fallow, without additional nutrients supplied [2].

Polyculture cover crops, where multiple species crops are grown together, can be incorporated into crop rotation to create a more biodiverse system. In polyculture cover crops, some species are nutrient scavengers. Others are pollinator attractors, N fixers, organic carbon generators or providers. Together, they capture solar energy and 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 stored in the soil, which becomes critical for crop production in drought years.

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

The loss of soil organic matter resulting from summer fallow significantly reduces soil water holding capacity. According to the United State Department of Agriculture, an increase in soil organic matter by one per cent is equivalent to an additional 25,000 gallons of water available to crops in one acre.

Tillage and summer fallow can decrease soil organic matter by 0.1 per cent each year [3] and reduced tillage will avoid that organic matter loss. Growing a good cover crop can potentially add up to 0.1 per cent organic matter per year. Every one per cent gain in organic matter in the top six inches of soil converts to an additional 500 pounds of organic N in an acre.

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 to growing weeds. Seeding a blend of cover crops is often more effective as some species grow quickly, while others take longer to establish but live longer. Adding legumes in the mix can improve soil N supply for other species in the area to grow better.

Cover crops can also suppress insect 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 c) reducing pathogen density because of increased soil biodiversity. Using cover crops to attract beneficial insects and repelling pests can be an integral part of a sustainable crop production system.

Growing cover crops provides increased returns and improves soil health by adding organic matter back into the soil and increasing biodiversity. Building and maintaining healthy soil is the foundation for a farming system to be productive and sustainable.

For more information, refer to:

• [1] Spratt et al., 1975. Summer fallow substitutes for western Manitoba. Can. J. Plant Sci. 55: 47l-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; and,
• [3] Brown, 2014. Healthier soil can reduce nitrogen costs, offer drought tolerance. Conservation Technology Information Center.