Cover Crops as Partial Replacement of Summer Fallow

Cover crops are one tool to improve soil health and long-term agricultural sustainability, especially when grown in place of fallow. This MontGuide summarizes the information in the MSU Extension bulletins Cover Crops: Soil Health, Cover Crops: Management for Organic Matter and Nitrogen, and Cover Crops: Soil Water and Small Grain Yield and Protein. See those bulletins for more details and all references. Our focus is on covers planted as partial fallow replacement in dryland systems of the northern Great Plains.

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Cover Crops

Cover crops, also called covers or green manure, are crops grown between cash crops, generally for soil conservation purposes. They increase plant diversity, keep the soil covered, and increase the time that living roots are in the soil, especially compared to fallow. Covers are part of a regenerative approach to improve soil health. Benefits achieved depend on many factors including soil type, soil moisture, species planted, and when and how the cover is terminated. Cover success depends on the goal, which determines cover species selection and management. For example, decreasing nitrogen (N) fertilizer needs might require different cover selection and management than increasing infiltration or soil organic matter (SOM). It is unreasonable to expect a cover to provide all the potential benefits.

Soil Organic Matter

The key to healthy soil is having adequate SOM. Soil organic matter is composed of humus (stable organic matter) plus plant, animal, and microbial tissue in various stages of decomposition. Humus is the end product of decomposed microbes and plant residue. It makes up around half of the SOM. Healthy soil structure (aggregation) and water infiltration rates and water holding capacity increase with more SOM. Humus in particular has high nutrient supplying capacity and helps resist pH change. Plant residue is the fuel that feeds the soil microbes which drive SOM production.

Building SOM is slow; after 10 years of an alfalfa-grass or no-till recrop in Montana, SOM only climbed from an initial level of 1.40% to a final level of 1.47% in the top foot. In an extensive Saskatchewan study, SOM increased only 0.11% over 22 years, despite a wide-scale conversion to no-till. However, a small change in SOM can lead to large improvement in soil health.

RESIDUE AMOUNT

In Montana dryland cropping, each ton of aboveground residue formed about 0.4 ton SOM over a decade (Figure 1). The remaining 0.6 ton was used as energy by soil microbes and lost as carbon dioxide to the air. Soil organic matter can only build when residue input is greater than soil microbial appetite for fresh residue and existing SOM.

In the 10-year Montana study (Figure 1) and a 10-year Saskatchewan study both in 16-inch annual rainfall zones, SOM was maintained with 1.8 ton/acre/year dry, aboveground biomass input. The 1.8 ton/acre/year threshold is an annual average over several years. For example, 36 bu/acre continuous wheat, 72 bu/acre winter wheat-fallow, or one ton cover crop biomass plus 56 bu/acre wheat the following year are all rotations that could lead to 1.8 ton/acre/year residue. These production levels can be hard to meet, especially the crop-fallow yield, which is why it is nearly impossible to build SOM with fallow in rotation. The break-even amount is likely less than 1.8 ton/acre/year in locations and years with less rainfall because decomposition is slower in drier conditions.

Figure 1: Average annual soil organic matter change in top foot over 10 years relative to annual above ground residue input (Engel et al. 2017, MT).

Both the cash and cover crops on a given field need to be included in residue calculations. The longer covers grow, the more water is taken from the soil, causing lower grain yields the following year. Lower cash crop growth leaves behind less stubble (see yield section). Cover crops only increase SOM if their residue is greater than the loss of cash crop straw caused by the cover’s use of soil water.

In wet years or locations with around 6-inch growing season precipitation (April to July), a 2-year cycle of early terminated covers and wheat can return more residue than fallow-wheat. However, in dry years or locations, there is the risk that cover- wheat will not produce substantially more total residue over a 2-year cycle than fallow-wheat.

In regions with unreliable precipitation, the key is to select single or mixed species with reliable and acceptable biomass production. Well-suited, single specie covers often produce more biomass than a multi-species mix. However, mixes provide a better chance that something will grow well and that biomass will be more consistent across growing seasons. Timing the seeding of mixes can be a challenge if they include both warm and cool season plants and some species may produce a residue that challenges cash crop seeding with the equipment at hand.

Including legumes in the cover is important when the cash crop receives little or no N fertilizer, as they provide N for subsequent crops.

Small grain yield

In semi-arid regions of Montana, the soil water used to grow covers is usually greater than soil water saved through reduced evaporation or increased snow capture. Therefore, subsequent wheat yields are often lower following covers than fallow (Figure 2). An exception is in shallow or sandy soils with limited water holding capacity where soil water, thus grain yield, is more likely to be similar following cover, recrop or fallow. If the goal of a cover is to increase SOM, then some cash crop loss can be expected. Over time the increased SOM should increase soil water to increase cash crop yields.

In water-limited conditions, early termination of covers by small grain boot, or legume by early to mid-flower stage, preserves soil water for the next crop. In several Montana sites, wheat yields were an average 8.6 bu/acre less after cover terminated by first bloom than after fallow. Small grains following late-terminated covers (pod to almost seed set) yielded an average 15.3 bu/acre less than after fallow. In higher rainfall sites (greater than 6.5 inches April to July), grain yields were not significantly less after covers than fallow, regardless of cover termination timing.

Figure 2: The differences in wheat grain yield and spring soil water (to 3-ft depth) after a cover (CC) compared to fallow. The zero on either axis means there was no difference between cover and fallow. For example, the value (spring soil water, grain yield) (-1,-7) means the cover had 1 inch less soil water the following spring and produced 7 bu/acre less wheat than fallow (in Cover Crops: Soil Water and Small Grain Yield and Protein).

Wheat yields can vary depending on the cover composition. Wheat yields tend to be higher after legume cover than non-legume cover, and lower after tap-rooted or fibrous-rooted covers than legume and brassica covers.

Nitrogen

Nitrogen is released as cover residue decomposes. This can be used by future crops, lost to leaching if no crop is in place to take it up, or slowly accumulate in SOM. Covers can add available N when legumes are included and maintain available soil N by trapping residual N (catch and release N). Although legumes can fix N, they will use available soil N if levels are high, before fixing much N, so they do not always increase spring plant available N (PAN) compared to fallow.

Plant available N is high (after substantial cover decomposition) when the cover biomass is 75 to 100% legume and low if legumes make up less than 25% (Figure 3). A minimum of 40% legume biomass in a mixture has been suggested to supply, rather than tie up N, in soils following covers. Nitrogen fixation varies by legume species and relies on healthy root nodulation. The bacteria responsible for nodulation (rhizobia) thrive and increase N fixation in soil conditions that support healthy plant growth.

Covers can reduce soil N compared to fallow; therefore, soil testing in the spring after covers is important to avoid over- or under-fertilization. The amount of fertilizer N a producer can back-off of a fertilizer N recommendation based on a spring soil sample is called ‘N credit’. This is the additional amount of N that will be provided by legume residue during the following growing season and is difficult to measure. As a rule of thumb, the N credit from an annual legume cover grown once is 20-30 lb N/acre. After a third time in rotation with small grain, the N credit from an annual legume cover may be 30-50 lb N/acre.

Figure 3: Plant available N (PAN) by cover legume content and plant stage at termination (Sullivan and Andrews 2012, OR)

FOR SMALL GRAIN YIELD

In cool semi-arid regions, the N release can be too slow to meet small grain’s early growth N requirements. Yet, if fall conditions are moist and relatively warm, legume cover may decompose quickly and lead to vigorous early small grain growth. If the growing season turns dry, that growth cannot be sustained resulting in low grain yields.

Nitrogen benefits to subsequent crop yields may be realized only after multiple years of legumes in rotation. Plant available N from pulse residue takes time to build but tends to be available for an extended time; N from legume residue may still supply N for grain planted at least three years later.‌

Fertilizer N rates can be reduced from standard guidelines when legume covers are used due to increased fertilizer N recovery and N supplied in the long term. Reducing fertilizer N rates in turn will minimize leaching loss and slow soil acidification from ammonia-based N fertilizer which is a growing soil health issue in Montana croplands (MSU Extension: Cropland Soil Acidification).

FOR SMALL GRAIN PROTEIN

Small grain protein is usually greater after legume covers than fallow (Figure 4). Legume cover benefits to small grain protein occur after fewer rotations than necessary to see N benefits to small grain yields. Wheat grain following pea cover may reach or exceed the protein cut-off level required to avoid protein discounts, despite receiving less fertilizer N than wheat after fallow.

Figure 4: The number of times small grain protein was lower or higher following a legume cover (CC) than fallow over 47 site- years in Montana and Saskatchewan. Positive protein means protein was higher following cover than fallow (in Cover Crops: Soil Water and Small Grain Yield and Protein).

Other Benefits

Soil aggregation increases the soil’s ability to absorb and hold water and resist wind and water erosion. Covers provide living roots, plant residue, and microbial activity which all contribute to improved aggregation and water infiltration.

Microbial activity appears to be more dependent on residue amount than the diversity of plants making up the residue. However, as N concentration of the residue increases, so does microbial activity. Terminating covers before plant maturity and including legumes in the cover are ways to increase residue N.

Arbuscular mycorrhizal fungi (AMF) help plants take up water and nutrients. They are largely responsible for forming soil aggregates. Oat cover or mixes containing oats may increase AMF root colonization and growth of the following crop. The AMF benefit directly from soil N provided by legumes, and indirectly if ammonium-based N fertilizer is reduced because of legume supplied N. Fungi biomass decreased when more than 45 lb N/ acre was added as ammonium nitrate mid growing-season because of localized soil acidification by the N fertilizer.

Soil temperatures at a 2-inch depth was lower by 5 to 15°F with covers than fallow from late June through late August (the last temperature measurement) even well after cover termination (Figure 5). Lowered summer afternoon temperatures should decrease evaporation and likely benefit biological activity.

Legume cover residue may release more P and provide it earlier in the next growing season than mature legume or non-legume cash crop residues. In an Alberta study, clover and pea covers terminated at 50% bloom released 10 to 11 lb P2O5/acre for the next crop, while canola, wheat and pea grain residue released less than 2 lb P2O5/acre. Grass covers are well suited to catch P (and N) runoff or deep seepage in acidic or sandy soils, which do not bind P.

Figure 5: Summer soil temperature at 2-inch depth at 4pm in fallow, pea, and mixed species cover crop (Miller et al. 2016, MT).

Summary

Covers are a tool to provide residue for soil health, build soil organic matter, and provide plant available N. However, in water limited systems, growing covers uses soil water and often reduces subsequent cash crop yields.

• In water limited systems, cover termination by first flower minimizes next crop yield loss due to cover water use.
• Residue gained by growing past flower does not offset the water use and subsequent residue loss due to lower cash crop yields.
• Cover species should be selected that produce reliable biomass, keeping in mind seed costs and equipment availability.
• Legumes need to comprise more than 40% of a cover crop biomass to contribute soil available N.
• Soil testing should occur in the late fall/winter or the following spring to adjust N fertilization rates following covers.
• It usually takes several legume cover rotations to provide an N benefit for subsequent cash crop yields. In a wet fall/early spring, N release may be early and go to yield. Generally N release is later and cash crop protein is often higher after legume covers rather than fallow.

A good way to start regenerating soil health is to reduce fallow, increase the frequency of live roots and the amount of plant residue on a given field, and include legumes in rotation to supply N.

Acknowledgements

We thank the Western Sustainable Agriculture Research and Education program and the Montana Fertilizer Advisory Committee for funding MSU studies, and Eric Miller and Colleen Buck (MSU Extension Agents), Lance Lindbloom (Certified Crop Adviser), Korey Fauque (farmer) and MSU Communications for their time and expertise in reviewing and producing this bulletin.

References

See the Montana State University Extension bulletins Cover Crops: Soil Health, Cover Crops: Management for Organic Matter and Nitrogen, and Cover Crops: Soil Water and Small Grain Yield and Protein for all references, including sources of data for figures. Montana State University Extension publications are available online https://store.msuextension.org/ or call 406-994-3273.

Engel et al. 2017. Soil Sci. Soc. Am. J. 81:404-413 http://dx.doi.org/10.2136/sssaj2016.06.0194

Miller et al. 2016. https://projects.sare.org/sare_project/sw11-099/ Sullivan and Andrews. 2012. https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/pnw636.pdf