Promoting Adoption of Cover Crops in Southeastern Farming Systems

Research Team: Soil and plant samples were collected as part of this conservation project by Alan Franzluebbers, USDA Research Ecologist at the Soil Ecology and Management Lab on the campus of North Carolina State University in Raleigh NC.  Technical support for soil and plant analyses in the laboratory was provided by: Erin Silva, USDA Biological Science Technician; Ellen Leonard, NC State Research Technician; and Katie Pritchett, NC State Graduate Student. Several NC State student workers over the lifetime of the project, including Allison Denton, Hannah Frank, Honovi Locklear, Ashton Mizelle, and Ashley Turner.

Demonstration Plots: Working with and through Michelle Lovejoy, NC Foundation for Soil and Water Conservation, contact with local Soil and Water Conservation District (SWCD) staff in several counties was initiated to gain access to field demonstration plots on farmer fields from 2015 to 2019.  Demonstration plots followed guidelines distributed to SWCD staff that were designed to have at least two replicate strips of multi-species cover crops compared with either no cover crop or single-species cover crop on working cropland.  A total of 35 fields were sampled for cover crop biomass production in springtime (April-May) prior to planting a summer cash crop.  A total of 31 fields were sampled for soil conditions in the same time period after maturation of the cover crop.  Repeated demonstrations occurred for several of the participants to gain insight into potential multiple-year benefits of practices on soil health condition. NOTE: Demonstrations of fall 2018 cover corp plantings were supported by SARE funding and are noted by an asterisk (*) .

Data Collection & Analysis Methods: Cover crop biomass production was estimated by harvesting two 2.8 ft² areas from six sampling points in multi-species cover crop portions of field and the same in the comparative no cover crop or single-species cover crop portions of the field.  Biomass was cut about an inch from the ground and placed into cloth bags to be dried, weighed, and analyzed for C and N contents. Dry matter of cover crop mass is reported in Table 1 for each of the demonstration trials from 2015/16 through 2018/19.  In many of those trials that had multi-species cover crops compared with no cover crop, a large statistical difference occurred.  Therefore, one aspect of the demonstration project was fulfilled, i.e. to demonstrate that multi-species cover crops were effective in accumulating biomass. However, demonstrations were also revealing in that not all cover crop plantings led to the same biomass production.  Planting date and weather conditions can play a major role in how well seeds germinate in the fall, how well seedlings develop and persist into the winter, and how well biomass accumulates and which species contribute to biomass production in the spring.  The experiences of participating farmers and SWCD staff will be equally valuable in assessing success, as do these quantitative estimates of plant production.

Biomass Accumulation Results: Although we don’t have firm estimates of how much biomass should be produced to be effective, we can say that some conservationists prefer to limit accumulation to a moderate amount of ~3000 lb/acre and others feel there should be no limit and that more is better.  The data collected in this project yielded a fairly wide range of biomass production, from as little as 400 lb/acre to as much as 9000 lb/acre. On average, these demonstrations showed that 3014 lb/acre of cover crop biomass was achieved.  Most conservationists would consider this successful in covering soil and providing sufficient organic resources to feed soil over the cold, wet winter in North Carolina.

Looking more closely at the data, 33% of the multi-species cover crop demonstrations produced <1154 lb/acre of biomass, 33% of the demonstrations produced 1154 to 3384 lb/acre, and 33% of the demonstrations produced >3384 lb/acre.  The lower one-third of demonstrations could likely be considered deficient in biomass production.  This statement is made, because on average that was about the level of biomass production across all demonstrations without a cover crop, i.e. from growth of overwintering weeds.  One has to ask what might have limited biomass production.  Was it weather conditions?  Could it have been late planting?  Were persistent weed control chemical present?  Did seed lay dormant for a long period and only germinate in the spring?  Was seed viable?  Many conditions could lead to poor performance and growers that want to improve cover crop biomass production will try again and make adjustments.

The middle one-third of demonstrations that had 1154 to 3384 lb/acre of biomass production could reasonably be considered successful and this biomass level could contribute to reasonable soil erosion control, good carbon food-sourcing for soil organisms, and potential to biologically sequester nutrients for effective nutrient cycling to the succeeding summer cash crop. Those demonstrations that had the greatest biomass accumulation often had the highest soil fertility conditions, as will be shown in a later section.  Many times growers had previous years of experience and this can be a valuable asset for good production.  However, not always was cover crop experience a factor in getting good production as evidenced in a few first-time successes by some growers.  Getting good quality seed to plant, putting the seed into the ground for rapid germination at a relatively early planting date in the fall, and having good luck with winter and spring weather conditions are important factors.  Nitrogen availability in soil can be a key limiting factor if cover crops do not have a significant legume component.  However, any cereal cover crop in a multi-species mixture still needs nitrogen in soil for sufficient growth, because legume nitrogen fixation only satisfies the needs of the legume plant and only following decomposition will significant nitrogen be released for other plants to thrive on this biologically fixed nitrogen.

TABLE 1. Biomass Dry Matter of Cover Crop Treatments

None (no cover); SSCC (single-species cover crop); and MSCC (multi-species cover crop) at each location. To indicate significance between means < represents p ≤ 0.05; << represents p ≤ 0.01, and <<< represents p ≤ 0.05; = is not significant.

Biomass Nitrogen Results: This question was answered by the results in Table 2.  We analyzed harvested plant material and analyzed nitrogen concentration with a combination of dry combustion and near-infrared spectroscopy.  On average, there was 50 lb/acre of nitrogen stored in multi-species cover crop biomass, which was not significantly different than the 37 lb/acre of nitrogen stored in single-species cover crops, but was greater than the 19 lb/acre of nitrogen stored in overwintering weed biomass without a cover crop planting.  One-third of demonstrations had multi-species cover crop biomass nitrogen accumulation <19 lb/acre, one-third of demonstrations had 19 to 54 lb/acre of nitrogen accumulation, and one-third of demonstrations had >54 lb/ace of nitrogen accumulation.  If nitrogen accumulation in cover crop is desired, then sufficient biomass has to be accumulated and legume species should be a plentiful component of the mixture.  Many of the most effective demonstrations with large accumulation of nitrogen used Austrian winter pea, crimson clover, and hairy vetch in the mixture.  Crimson clover was the single-species cover crop in the Rowan County demonstration in both years and it also accumulated a reasonable amount of biomass nitrogen.

Table 2. Biomass N Content of Cover Crop Treatments

None (no cover); SSCC (single-species cover crop); and MSCC (multi-species cover crop) at each location. To indicate significance between means < represents p ≤ 0.05; << represents p ≤ 0.01, and <<< represents p ≤ 0.05; = is not significant.

The ratio of carbon:nitrogen can be an important characteristic of cover crops to determine how fast it will decompose during the subsequent summer.  Ratio of <30 is often desired to avoid potential immobilization of nitrogen to the succeeding cover crop.  However, this issue is much less important in no-tillage cropping systems that keep residues at the soil surface.  Traditionally, cover crops were incorporated into the soil with tillage and high carbon:nitrogen ratio in this production system can indeed immobilize significant quantities of nitrogen during the early stages of decomposition.  As with all organic amendments, early immobilization is often followed by net nitrogen mineralization later in decomposition.  The question then becomes when this transition occurs – will it be one month, several months, or several years?  Again, the issue of nitrogen immobilization is less critical in surface-placed residues in long-term, no-tillage management systems.

Table 3 shows the carbon:nitrogen ratio that occurred in these demonstrations.  One-third of demonstrations had carbon:nitogen ratio of <23, one-third of demonstrations had carbon:nitrogen ratio of 23 to 30, and one-third of demonstrations had carbon:nitrogen ratio of >30.  Carbon:nitrogen ratio of multi-species cover crops were on average very comparable to that of overwintering weeds, but with starkly different amount of total biomass and total nitrogen accumulation.  The positive effect of legume components of multi-species cover crops was evident in the lower carbon:nitogen ratio compared with single-species cover crops.

Table 3. Biomass Carbon:Nitrogen (C:N) Ratio of Cover Crop Treatments

None (no cover); SSCC (single-species cover crop); and MSCC (multi-species cover crop) at each location. To indicate significance between means < represents p ≤ 0.05; << represents p ≤ 0.01, and <<< represents p ≤ 0.05; = is not significant.

Measurable Impacts to Soil Properties:

A diversity of soil physical, chemical, and biological properties was measured in each multi-species cover crop demonstration.  The full list of properties is in Table 4.  This report focused on five properties of key interest in soil health evaluation – total organic carbon, soil-test biological activity, net nitrogen mineralization, soil-test phosphorus, and soil-test potassium.  We also only report here the values for 0 to 2 inch soil depth, although we also measured values at 2 to 6 inch depth.  A peer-reviewed manuscript is being prepared that will contain more of the soil properties and soil depths.

Total organic carbon represents soil organic matter.  Soil organic matter is dominantly comprised of carbon, i.e. 58%.  Multiplying the total organic carbon values by 1.72 would yield soil organic matter.  For example, a value of 2.0% total organic carbon would be equivalent to 3.4% organic matter.  Soil-test biological activity is considered the most active portion of organic matter and is determined from the flush of carbon dioxide (CO₂) during the first three days following rewetting of dried soil.  Net nitrogen mineralization is the quantity of nitrogen mineralized during a 24-day incubation of soil at standard temperature (77 °F) and water (50% water-filled pore space).  Soil-test phosphorus and potassium are from Mehlich-III extraction of soil as routine evaluation of fertility.  Therefore, these five properties represent the important fertility elements of carbon, nitrogen, phosphorus, and potassium, as well as a direct estimation of soil biological activity that drives nutrient cycling.

Table 4. Soil Properties: Measured from samples collected from six areas within each test strip of each cover crop demonstration field.  Samples were collected at 0 to 2 inch depth and 2 to 6 inch depth.  Soil cores were 1 9/16 inch in diameter and a total of eight cores spaced three feet apart were composited to make a sample of an area.  A typical field demonstration had a total of 48 samples analyzed each year.  Soil physical and biological properties (and inorganic nitrogen species) were analyzed by the Soil Ecology and Management Lab and soil chemical properties (and sieved density) were analyzed by the Soil Testing Laboratory of the NC Department of Agriculture and Consumer Services.

Conclusions: Biomass production with multi-species cover crops spanned the range from relatively poor production in one-third of sites (<1154 lb/acre) to good production in one-third of sites (1154 to 3384 lb/acre) to excellent production in one-third of sites (>3384 lb/acre).  Nitrogen accumulation in the cover crop biomass was equally well distributed and reached excellent levels in one-third of sites at >54 lb/acre.  Since demonstrations were relatively short-term in nature, we didn’t anticipate seeing many soil differences, but there were some of notable significance.  At six sites, consistent cover cropping effects across years at the same location were not observed at all.  However, there were four sites that consistently had greater soil-test biological activity under multi-species cover cropping than with no cover crop.  There were also two sites that had consistently lower soil-test K under multi-species cover cropping than without cover crops, as well as one site with consistently lower soil-test P under multi-species cover cropping than with single-species cover crop.  These short-term multi-species cover cropping effects suggest that soil biological activity is enhanced and nutrient dynamics can be altered.  Soil-test P and K are simply indices of availability, not mass balances.  Further experimentation is warranted to better understand the unique soil biological changes that may be occurring with these cover crop mixtures.