Agricultural land management practices impact the physical, chemical, and biological characteristics of soil, including the structure of the community of microorganisms present in the soil. The community of soil microorganisms, in turn, directly influences processes such as nutrient cycling and water infiltration and retention, which ultimately shape the long-term health and fertility of an agricultural landscape. The purpose of this study is to examine the effects of cover cropping and tillage practices on the soil microbial community as well as other various soil physical and biological features that contribute to soil health. We will work with three farmers on the Eastern Shore of Virginia to assess how various practices that they currently utilize (or are considering utilizing) impact the structure and function of the soil microbial community on their farms. More specifically, we will examine how cover crops of varying species diversities (legumes, grasses, brassicas and legume/grass/brassica mixtures) and tillage practices impact on-farm soil microbial diversity and activity, structure and fertility. The results of this study will shed light on how sustainable practices, which are becoming increasingly common in the Southern region, influence soil health and fertility, and the implications for crop productivity. Our results will inform farmers on how to utilize practices that enhance the functioning of the soil microbiome and improve agricultural sustainability.
- To determine how cover crops and tillage practices impact the structure and function of the soil microbial community.
- To determine how changes in the microbial composition of the soil relate to soil physical characteristics and fertility (i.e. soil moisture, soil structure, active and total organic matter, nitrogen mineralization rate, and nitrogen fixation).
- To determine the influence of changes in microbial community structure and correlated soil characteristics on cash crop growth and production.
- To disseminate our findings to a wider audience of farmers, researchers, and others in a practical and meaningful way.
The summer cover crop experiment looked at the effects of two types of cover crops on the soil microbial community, soil health, and crop yield. The experiment involved three treatments – Cowpea, Sorghum Sudan-grass, and no cover crop (control) – replicated three times. The experiment took place at the Eastern Shore Agricultural Research and Extension Center (ESAREC) in Painter, VA. Each plot was 24ft x 65ft. Treatments were arranged in a randomized complete block design. Cover crops were planted on 13 July 2016. The cowpea used was Queen Anne inoculated with Rhizobium at 120lbs/acre and the sorghum Sudan-grass was BMR Sudan Grass F1 OG at 40lbs/acre (Clark, 2008). The field was irrigated using sprinkler irrigation as needed, roughly once a week throughout the summer. The control (no cover crop) plots were maintained by tilling every 3 – 4 weeks. Cover crop stand counts and weed counts were taken 13 days after planting (DAP) using a 1ft x 1ft square (5 per plot). Weeds were also identified at the same time. On 2 Oct. 2016, aboveground biomass samples were taken prior to cover crop incorporation. Weeds and cover crop biomass was determined. A subsample of the dried plant tissue (of both weeds and cover crops) was sent to the laboratory for nutrient analysis.
Cover crops were incorporated on 4 Oct. 2016 by first flail mowing the cover crop to break up the plant residue, rototilling, and then incorporating plant residue with a three-row disc hipper. Following incorporation, soil samples from each plot were taken on October 14th. One sample was frozen to be sent out for microbial community characterization. One sample was dried and sent out for nutrient analysis. One sample was stored in the refrigerator in order to conduct an aerobic incubation study. In addition, soil respiration was measured using the method for field measurement of soil respiration developed by Parkin et al. (1996) using the NRCS Soil Quality Test Kit. The metal chamber was covered with a plastic lid with two sealed rubber stoppers. After 30 minutes, a 100mL of gas was dragged from the chamber through the tube. In addition, the temperature and volumetric water content (% v/v) were also recorded at depths of 2.5cm and 7.5cm roughly 2.5cm away from the soil respiration chamber. Soil moisture and temperature were taken with a waterScout SM 100 soil moisture sensor and an external temperature sensor, respectively, connected to WatchDog 1425 data logger (Spectrum Technologies, Inc. Aurora, IL).
Aerobic Incubation Study
A 5-week incubation study was conducted in October 2016, roughly two weeks after the soil samples were taken from the field. The purpose of the incubation study was to determine the rate of nitrogen mineralization in an aerobic environment over several weeks by analyzing the nitrate and ammonia content of the soil at weekly intervals over a 5-week period. At the time of each measurement, the soil samples were extracted with a 2 M KCl solution and run on a Lachat 8500 to determine their nitrate and ammonia contents.
Soil samples (20g wet weight) of soil was added to a 250mL plastic jar and weights were monitored. The plastic jars were covered tightly with parafilm. Distilled water was added to bring the soil in the jars to 15% moisture. The weight of the jars was taken each week in order to ensure minimal moisture loss. Moisture loss over the 5 week incubation study was less than 0.25% in all of the jars, therefore no water was added to the jars after the first week. Samples were used for nitrate and ammonia determination at 0, 1, 2, 3, and 5 weeks incubation time in Lachat 8500.
Red Sail lettuce was planted in each plot on 27 Sept. in order to determine the effects of the cover crop treatments on lettuce yield. The lettuce was planted into black plastic. The lettuce crop was irrigated as needed via drip irrigation. No fertilizer was used in order to determine the fertility effects of the cover crop treatments. Lettuces were harvested on December 19th, 53 DAP. Four lettuce heads were harvested per plot to determine fresh weight, leaf area (LI-COR LI-3100C Area Meter), biomass, and leaf area/biomass ratio. A subsample of the dried lettuce leaf material was sent out to the lab for nutrient analysis.
Data was analyzed by ANOVA using SAS 9.4 (SAS Institute Inc., North Carolina).
Figure 1. Summer cover crops, ESAREC 2016
Sam measuring soil respiration. ESAREC 2016
Lettuce crop after incorporation of summer cover crops. ESAREC 2016
Cover Crop Biomass
The average cover crop biomass incorporate into the field by each treatment were significantly different between the cowpea, sorghum sudangrass, and no cover crop (control) treatments. Sorghum sudangrass had the highest average biomass (597 g), followed by cowpea (219 g), which had the second highest biomass. As expected, the no cover crop treatment provided the least biomass (38 g) since those plots were tilled every 3 – 4 weeks to minimize weed growth (Figure 1).
The results of the laboratory soil analysis indicated that, while differences in organic matter (OM) and total organic carbon (TOC) between cover crops and control, there were no differences in total Nitrogen and C/N ratio. Similarly, there were no differences between the two cover crops and both cover crop treatments were significantly higher in OM and TOC than the no cover crop control. In contrast, extractable nitrate (NO3-N) was significantly different between all three treatments. Cowpea was highest in nitrate (9.3 ppm), followed by the no cover crop control (4.7 ppm). Sorghum sudangrass was significantly lower in nitrate than both of the other treatments (1.0 ppm). In addition, both cover crop treatments were significantly higher than the no cover crop control in both Potassium (K) and Magnesium (Mg), however the differences between the Potassium and Magnesium contents of the two cover crop treatments was not significant. There was no significant difference in Phosphorous (P) and Calcium (Ca) contents between the three treatments.
Soil microbiome analysis is in progress.
The average soil respiration rate of the cowpea treatment (314 kg CO2ha-1∙d-1) was significantly higher than that of the no cover crop control (100 kg CO2ha-1∙d-1). The average soil respiration rate of the sorghum sudangrass treatment was higher than the no cover cop treatment but lower than the Cowpea treatment and did not differ significantly from either of the other two treatments.
Fresh weight and leaf area were both significantly higher for lettuces that followed the cowpea cover crop and no cover crop than those that followed the sorghum sudangrass cover crop. There were no differences between cowpea and no cover crop. These results may be explained partially by the soil extractable nitrate-nitrogen.
Educational & Outreach Activities
On-farm studies where soil samples were taken:
- Blenheim Organic Gardens, Westmoreland County, VA.
- Lois’s Produce in Westmoreland Co., VA.
- Quail Cove farm in NorthHampton Co.
- Copper Cricket Farm in NorthHampton Co.
- Eastern Shore AREC tour, 7th National Small Farm Conference. Sept. 21, 2016. Painter, VA. Participants: 30
- Eastern Shore AREC tour for young farmers from Bedford County, VA. Aug.13, 2016. Painter, VA. Participants: 21
This project will generate and disseminate information about the microbiome structure in soils managed with different cover crops and the crop performance associated with them. Cover crops are expected to improve soil characteristics based on the type of microbiome structure and nutrient status. An economic advantage can be expected from N fixation with the legume cover crops and mineralization by the soil microbiome, but also by recycling of soil nutrients and avoiding losses by deep percolation beyond the reach of the root system. Therefore, an increase in economic sustainability is expected by adopting cover crops rotations in vegetable crops production.
Similarly, the environmental sustainability is expected to improve mainly by the following:
- Reduction in Nitrogen fertilizer used.
- Reduction in nutrient leaching due to the microbiome nutrient uptake.
- Reduction in soil erosion due to cover crops.
Food supply and security are key problems of regional, national and international importance. Strengthening the production capacity of specialty crop farmers is expected to increase supply of locally grown fresh and healthy food, and reduce the dependency on distant production areas, therefore, increasing food security in local markets. Therefore, this project is expected to enhance the quality of life of farmers, farm workers and society as a whole.
I have always been a proponent of sustainable agriculture but had never really conducted any controlled, scientific research on the methods and practices employed in sustainable agriculture up until the start of this project. My knowledge and skills base of the intricacies and challenges of collecting valid, useful data, particularly in such a multifaceted study as ours, was increased dramatically through the course of this project. My advisor, on the other hand, has conducted research in the sustainable agriculture field for many years and is a strong advocate for increasing the efficacy and impact of using sustainable methods in agricultural production.
From the get-go our project was designed to be as grower-focused as possible, which is why we incorporated a measurement of cash crop growth and production into the study. Through the course of talking to farmers and other agricultural research specialists, it became apparent that one of the key goals in sustainable agricultural research must be to demonstrate to farmers, through the use of concrete and robust data, that sustainable agricultural methods and technologies will benefit them from both an environmental and economic standpoint. Our project demonstrates the productive and environmental benefits derived from using cover crops as well as other sustainable agriculture production technologies to enhance soil health and crop yield in the production of vegetable crops. Our project also highlighted the need for further study into the relationships between cover crops, soil microbiology and long-term soil health, and vegetable crop production in the field of sustainable agriculture.
Cover crops have been shown to enhance soil health and productivity and this study is expected to provide additional evidence of microbiome function and nutrient recycling. Therefore, incorporating cover crops into vegetable crop rotations is recommended to improve sustainability.