In low rainfall areas of the northern Great Plains cover cropping is appropriately aimed at greening the summer fallow period, which is an economically risky practice. Over time, soils may improve sufficiently so that the increased crop return outweighs the cost of cover crops. This study was begun in 2012 (SARE Project SW11-099) at two on-farm sites in Montana to compare different plant functional groups’ effects on soil properties. Eleven treatments are included in this replicated plot-scale study, including paired plant species representing four plant functional groups (brassicas, fibrous rooted, nitrogen fixing, and tap rooted crops) in various combinations and sole pea and chemical fallow as controls. The experimental design provides a unique opportunity to test each functional group, whereby 1) each group appears separately as four functional group treatments (‘presence’), 2) they appear together in a complete mixture, and 3) the complete mixture minus each functional group (‘absence’). Due to the semi-arid climate, more time is required to see soil differences. We will be completing the final soil sampling in spring 2019, after four cycles of cover crops, in association with my M.Sc. thesis. This should have allowed enough time to elapse for a battery of biological, chemical, and physical soil assessments to be differentiated by functional group and what that does to the subsequent wheat quality. This research will fundamentally discover the role of functional groups within cover crop mixtures which will allow for more strategic design of cover crop seed mixes for targeted soil improvement. By making this research widely available, producers will be provided valuable information regarding methods of sustainable soil management. If soil benefits from long-term cover cropping can be proven to offset the short-term economic loss, then this more sustainable management technique can be more easily adopted into our agricultural systems.
Soil and Agronomic Objectives
1. To investigate how biomass production compares among the ten cover crop treatments included in this study.
2. To measure how plant functional groups affect biological, chemical, and physical soil properties, and subsequent wheat yields and protein, differently.
3. To increase local producer knowledge of the value, or lack there-of, for cover crop mixtures compared with less diverse cover crops for biomass production, soil water and nitrogen use, and change in biological, chemical, and physical soil properties.
4. To provide producers with information regarding the potential benefits of alternative cover crops through scientific publications, extension publications, popular press articles, social media, public presentations, and research summaries and factsheets.
• The number of viewers at each presentation will be tracked to assess the impact of presentations on producers and others present. The number of people who “like”, “follow” or “view” the information provided on social media will also be tracked.
The results of this research will provide fundamental knowledge on the ability of different plant functional groups to change soil properties distinctly. This base knowledge will be helpful in optimizing strategies for deploying cover crop mixtures over time and to serve specific functions
to the soil.
The study takes place on two private farm fields located near Amsterdam and Conrad, Montana. Both sites had extensive no-till farming histories prior to this study and were managed as commercial farms. The Amsterdam field site receives an average of 14 inches of precipitation annually and has a silt loam soil type which is classified as frigid, Aridic Calciustoll. The Conrad site receives an average of 10-12 inches of precipitation annually and has a clay loam soil type that is classified as frigid, Aridic Argiustolls.
The study consists of four randomized complete blocks with 11 treatment plots. Four plant functional groups are being examined: Brassicas, Fibrous root crops, Nitrogen fixers, and Tap root crops. Each were grown in individual plots (25 X 40 ft) with two species in each group along with a full mix plot containing all four functional groups (8 cover crop species). There were also 4 additional treatments that contain the full mix minus each functional group (containing 6 cover crop species). The last two treatments were a sole species cover crop (Pea) and a chemical fallow plot, which were used as control plots.
Each of the 11 treatments were randomly assigned to an 25 x 40-ft plot in each of the four replicated blocks. The cover crop plots were planted in early May and all species were planted at a common depth of 0.25 – 0.75 inches. All treatment plots were seeded at the same seeding rate of 11 plants per ft2 to eliminate biasing biomass in favor of plant species with higher grain seeding rates. The subsequent year, following cover crop treatments, wheat was grown to assess a cash crop response to the different cover crop treatments. Wheat was seeded in rows perpendicular to the cover crop rows. Nitrogen fertilizer was applied to the wheat crop at three different rates, 0 lbs-N-ac-1, 54 lbs-N-ac-1, and 108 lbs-N-ac-1. The study began in 2012 with the first round of cover crop treatments and has followed this rotation since.
Cover crop plots were terminated with glyphosate in early July after approximately 60 days of growth. 24 hours after the cover crops were terminated the above ground biomass was sampled. Four transects were taken in each treatment plot, one transect from the high N, one from the low N and two from the medium N sections. Plants were cut at the soil surface (the root bulb of turnip was cut at the soil surface), separated by functional groups, and dried for 72 hr at 122°F. The dried plants were then weighed, ground to pass a 2-mm sieve, and will be analyzed with a LECO combustion analyzer for carbon and nitrogen.
Soil samples will be taken in early April 2019 at the time of wheat planting (about 10 months after cover crops were terminated). The soil measurements will be taken at this time to measure what will be available or present in the soil for the wheat (or cash crop). Biological, chemical, and physical soil measurements will be taken. The soil water measurement were also taken at the time of cover crop termination. All measurements will be taken at the middle nitrogen fertilization rate (54 lbs-N-ac-1), unless otherwise stated. The fallow, sole pea, and full mix, treatment groups will be measured at all three fertilization rates.
Potentially mineralizable nitrogen (PMN) will be analyzed by calculating the difference between plant available nitrogen at the time of soil collection and after an incubation period, measured on a Lachat auto analyzer. Six composite samples will be taken in each treatment plot and put through a 2 mm sieve. Six flasks will receive 5 g of soil, three of which will be analyzed immediately using a 2 M KCl extraction and analyzed using a Lachat auto analyzer. The additional three flasks will be pumped with N2 gas to create anoxic conditions and kept in a dark incubator at 86°F for 14 days. These samples will then be analyzed using the same KCl extraction method and Lachat. Enzyme analyses will be performed on all treatments at the 54 lbs-N-ac-1 fertilization rate and at all three nitrogen fertilization rates for the fallow, pea, and full mix treatments. Mycorrhizal infectivity potential (MIP) will be measured only on fallow, sole pea, and full mix and the brassica/ minus brassica treatments (because brassicas does not have a mycorrhizal association), using the gridline intersection method on a compound microscope.
Soil water was measured to a 36 in depth using the gravimetric weight loss method at the time of cover crop termination in July and will be measured again at time of wheat seeding in early April. Four depth increments were measured: 0-4 in, 4-12 in, 12-24 in, and 24-36 in and two composite samples were collected for each treatment. Soil bulk density was calculated for all four depths as well. Prior to spring wheat planting, water infiltration will be measured for the big three treatments (fallow, sole pea, and full mix) at the middle N rate only, using double-ring infiltrometers. Soil penetration resistance measurements will be taken within (or in close proximity to) the infiltrometer ring 24-48 hours after infiltration measurements were taken, using a hand-held static cone penetrometer. Soil penetration was measured in this way because consistent soil moisture is important to accurately compare soil compaction when using a hand-held static cone penetrometer.
The chemical parameters will be measured at all three nitrogen fertilization rates for the sole pea, fallow, and full mix treatments. The chemicals parameters will only be measured at the 54 lbs-N-ac-1 for the remaining 8 treatments. Soil nitrate will be measured in four increments for the top 36 inches in early spring at the time of wheat planting. Depth increments will include: 0-4 in, 4-12 in, 12-24 in, and 24-36 in and two composite samples will be collected for each treatment. Nitrate was measured using the cadmium reduction method and was analyzed using a latchet auto analyzer.
Wheat from each treatment plot will be measured after fully maturing. Wheat biomass samples will be collected for a 3 ft-row for each subplot. This allowed net primary production and harvest index to be measured. The rest of the subplot will be harvested to measure grain yield, grain protein, and test weight. Grain moisture, protein, and test weight will be analyzed with an Infrared 1241 Grain Analyzer at a 120 g kg–1 water content standard.
The R statistical package will be used for all statistical analyses for the objectives in this study. The ANOVA function will be used to compare cover crop treatments and Fisher’s least significant difference with a 90% confidence interval, will be used for individual comparisons between treatments.
Although a large portion of the results will be completed following the final round of soil testing in April 2019, cover crop biomass data and cover crop soil water use have both been analyzed. In figure 1 you can see that the legume treatments yielded best at both the Amsterdam and Conrad locations. This indicates that nitrogen was limiting plant growth. Two orthogonal contrasts were run on the biomass in tons per acre between nitrogen fixers and minus nitrogen fixers for the medium N rate legacy (one for Conrad, one for Amsterdam). At the Amsterdam site, there was 2.5 times more biomass for the nitrogen fixer treatment than for the minus nitrogen fixer treatment (p-value < 0.0001). At the Conrad site, there was 2 times more biomass for the nitrogen fixer treatment than for the minus nitrogen fixer treatment (p-value = 0.01). This supports the previous claim that the system was nitrogen limited.
An additional contrast was run between the 2 species treatments (functional groups) and the 6 species treatments (minus functional groups) for each site. The Amsterdam site revealed no statistical difference between the 2 and 6 species treatments (p-value = 0.33). At the Conrad site, the 6 species treatments out-yielded the 2 species treatments by about 20% (p-value = 0.01), however there was greater variation due to the higher volume of weeds at this site.
Figure 2 shows the available soil water to a 3-foot depth after the cover crops have been terminated. By looking at figures 1 and 2, water use is proportional to the amount of biomass each treatment grew, which is to be expected.
Figure 3 shows soil nitrate-N levels after cover crop termination for the fallow, pea, and full treatments. There were no statistical differences between pea and the full mix at either site. The observed soil nitrate values at Conrad were very noisy due to drought experienced in the previous wheat stage in 2017.
Educational & Outreach Activities
MSU Graduate Symposium – In April 2018, I presented my preliminary research at a seminar sponsored by Montana State University – Dept. of Land Resources and Environmental Sciences (MSU-LRES). Informing my fellow graduate students as well as professors and extension staff about my research to raise awareness at MSU of the need for this research and education. I will present my research again at the April 2019 MSU Graduate Symposium.
Soil Science Society of America and American Society of Agronomy – In November 2019, I plan to present a poster or talk outlining my research and findings at these combined national annual meetings which typically attract more than 3,000 attendees from around the world. This will allow me to share my work directly with other professionals, fellow students and researchers.