Updated May 1, 2021:
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In 2019 the Ag Census reported a 30% increase in mid-Atlantic acres cover cropped between 2012 and 2017. However, timing and method of cover crop planting are critical determinants of nitrogen-capture, biomass and species-dominance in cover crop mixtures. Many cover cropped acres are relatively ineffective. Most cover cropped acres achieve minimal biomass, groundcover and N-capture due to late-planting and/or low soil fertility.
The Weil lab at the University of Maryland has also observed that having cover crop roots clean up soluble N deep in the profile before the onset of winter is critical to capturing N and reducing nitrate leaching all winter. Cover crops planted later than early October in Maryland will be too late to clean up the deep soil profile before winter have little effect on N leaching. Only vigorously-growing, early-planted cover crops can capture the deeper nitrogen before winter drainage water leaches it away.
When we studied soil under 45 mid-Atlantic crop fields and reported that residual end-of-summer soluble soil nitrogen in upper 7 ft averaged 250 lb N / acre. About half of this residual N was found below 2 ft. Despite this large pool of plant available nitrogen in deep soil layers, topsoil may be depleted of nitrogen at cover crop planting time because of leaching, crop-uptake and tie-up by microbes in corn residue. Low N in the topsoil in fall is especially likely on the sandy coastal plain soils such as those common on the mid and lower Eastern Shore. Sandy cropland where low nitrogen in topsoil is most likely covers approximately 400,000 acres of New Jersey, Delaware, and Maryland, alone.
We therefore hypothesized that low N availability in the topsoil may stunt cover crop growth and prevent roots from reaching large pools of residual N deep in the soil profile. Small nitrogen applications to early-planted cover crops in low-nitrate soils may stimulate early grow and deeper rooting. This deeper rooting may allow cover crops to increase N capture by substantially more than the small amount of N applied. For example, an investment of 15 lb of N at the time of seeding a non-legume cover crop (such as rye or radish) might increase the N uptake in fall from a paltry 5 to 15 lb to more than 50 lb N per acre, a net saving of N.
We propose that, under Maryland conditions, small fertilizer applications may stimulate cover crops to provide improved net water-quality, soil-conservation, carbon and soybean yield benefits. Currently, fertilizing cover crops in fall with N is not allowed by the state of Maryland cost-share cover crop program, but if we produce sufficient data that shows the above hypotheses is true, then it is likely that the program rules could be modified to allow small doses of N fertilization under appropriate circumstances (e.g., below a certain fall topsoil nitrate threshold).
Knowledge gaps we propose to address include: 1) How widespread are poorly functioning nitrogen deficient cover crops? 2). Will a small application of nitrogen stimulate deeper root growth so cover crops can enhance the net nitrogen capture by an amount substantially larger than the nitrogen applied? 3). How can we determine where and how much nitrogen application to cover crops would be justified? Several papers documented successful use of a fall nitrate test for fall application of nitrogen to winter wheat showing that when nitrate-N in top foot of soil is less than 9 ppm, fall nitrogen will increase wheat yields. We hypothesize a somewhat similar but earlier nitrate test in late-August/early-September could predict the value of a small nitrogen application shortly after cover crop seeding, especially when interseeding early into high nitrogen uptake and high C/N ratio crops like corn.
Project Objectives
Overall goal is greatly enhanced effectiveness of Northeast cover cropping, especially where manure application is rare and/or soil texture is coarse. We plan to determine extent of nitrogen-deficient cover crops and whether small nitrogen application in fall can increase cover crop benefits in winter and spring. Also develop a practical in-field nitrate-test determining where nitrogen fertilization of cover crops is justified. Results will be developed and shared with farmers, advisors and cover crop policymakers.
Research description.
Major Research Components of the Overall Project.
1. Survey cover crops in 20 to 40 Maryland fields in October of year one to determine growth-stage, biomass and degree of apparent nitrogen-deficiency (stunting and chlorotic older leaves).
2. Conduct a replicated experiment running for three years with a corn/soybean/corn rotation to determine the medium-term effects on N cycling of cover crop fertilization. Split plot randomized design with 4 replications on two soils for 3 years without re-randomization of cover crop treatments; subplots are nitrogen at 0, 15 and 30 lbs/acre and main plots are cover crop types (weeds only, rye, radish, or radish+rye) interseeded into standing corn or soybean late-August/early-September (as proven successful in previous research). Measurements include soil-nitrate (1-foot deep before August20-September10), percent groundcover using the CANOPEA Android app, biomass and nitrogen uptake at end-of-November and end-of-April.
3. On-farm strip trials with 4 to 5 replications on each of two farms. These trial will have two treatments (no nitrogen v. 20 lbs N/acre) applied on early-planted farmer-choice cereal, brassica or mixed cover crops (interseeded into early corn hybrid). Measure soil nitrate (30 cm deep before 10 September on two farms), percent groundcover using the CANOPEA Android app, biomass and nitrogen uptake at end-of-November and end-of-April. Farmers will be asked to report the yields of the following crop, usually soybeans, using calibrated yield monitors. Soil dataloggers will be installed to monitor summer soil moisture and temperature to quantify water-conservation benefits from increased biomass of cover crops.
4. Develop in-field nitrate-test to predict where cover crops nitrogen fertilization is justified. Data from the 32 to 40 site years (replication=site) of cover crop nitrogen-response (biomass and nitrogen-capture), along with data from survey sites used to model relationship between late-summer soil-nitrate and biomass/nitrogen-capture responses by cover crops to nitrogen application and rate. We will measure soil nitrate at several depths to build the model.
Results for April2020-March2021:
Our ability to conduct a survey and establish research plots in 2020, especially on the Eastern Shore, was greatly hindered by Covid-19 travel restrictions. Most students were not present on campus (or even in Maryland) and limiting 1 person per vehicle made travel to the Eastern Shore difficult. However, we did establish two replicated field experiments at the Beltsville research farm in which we interseeded two types of cover crops into corn.
At Beltsville, one experiment was conducted on a field with a loamy sand soil and the other on a field approximately 1.5 miles away with silt loam to silty clay loam soils. Cover crop seed was interseeded into a soybean crop in June 2020 using a special drill developed by Penn State University that drill three rows between each row of corn. The main plots were three cover crop treatments: 1) a no-cover control (weeds only), 2) a rye cover at 120 lbs seed per acre, and 3) a three-species mixture (3-way) of 4 bs. /acre of forage radish (rad), 70 lbs rye, and 15 lbs crimson clover (clover). These plots were sub-dived into subplots with three levels of N fertilizer applied after corn harvest: 0, 15 and 30 lb/acre of N as UAN solution.
Data on N contents of the cover crop biomass are not available yet at this writing as samples are still being processed, however, some preliminary observations were made of cover crop response to N fertilizer application. Although only 4 lbs. /acre of radish seed was included the mix, the radish appeared to be the dominant species in the 3-species cover crop vegetation in fall after corn harvest. Overall, across a sandy and a silty site, green groundcover measurements just one week after N application appeared to cause a trend of greater growth, but the differences were not statistically significant (data are not shown). Green groundcover measurements made in early December when the cover crop growth had reached its maximum did show a significant response to the N applied in October when averaged across both fields and both types of cover crops (Figure 1).
The effect of applying N as UAN on 21 October 2020 at 0, 15, or 30 lbs/acre was observed in the more vigorous and greener growth of the N-fertilized 3-species mixed cover crop growing on the silty soil site. On the sandy soil, the 30 lbs. N /acre fertilizer rate caused some leaf burn on radish and clover. Probably because of this injury, the mixed-species cover crop with 15 lbs. N /acre appeared to have more vigorous growth and covered a larger percentage of the ground surface in some of the sandy plots than the cover crop fertilized with 30 bs N/acre.
There were trends towards higher biomass with N application, but the N effect was significant only for the rye cover crop.
We hypothesized that small nitrogen applications to early-planted cover crops in low-nitrate soils may stimulate early growth and deeper rooting. This deeper rooting may allow cover crops to increase N capture by substantially more than the small amount of N applied. Achieving this nitrogen capture bonus will require that N be applied early when there is still enough warm weather to allow the cover crop to grow in response. It will also require the occurrence of soils that are low in available N in the topsoil but hold substantial N in the subsoil.
The greatest N response was seen on the silty soil for the rye cover crop. A similar Fall biomass data response was seen for the 3-species mix on the sandy soils. However, the 445 kg of extra dry matter stimulated by the application of 30 kg N is likely to contain only ~15 extra kg of N taken up (at this point without the actual N content data we assumed 3.5 % N in the tissue). These small responses to applied nitrogen were not what we hypothesized would occur. It is possible that the N content of the cover crop biomass may yet show a greater response to the Fall-applied N than did the biomass (dry matter) sampled in December. We expect to have the data on the nitrogen content of the biomass samples by mid-May 2021.
The lack of a stronger response may be due to the late timing of the N application, after the interseeded cover crops had been growing for several months without any applied N. By the time the N was applied in mid-October there were not enough growing degree days left in the season to allow the cover crop plants to take advantage of this soil fertility boost. Therefore, next year we plan to apply the N to the interseeded cover crop at early corn senescence instead of waiting until after corn harvest, a difference of about 5 weeks or 500 growing degree days. To do this we will apply solid urea instead of UAN solution.