Soil Microbial Response to Seven Different Organic Transition Strategies
The three year transitional period from conventional to organic row cropping can be the most challenging time for an organic farmer. Maintaining or improving weed control and soil health is never more vital than during this time, but it is also the period in which a new organic farmer is likely to have the least experience with organic practices. In this experiment we are examining seven transitional cropping systems to gain information on best management practices for weed control and soil building during the transition into organic row cropping. The overall goal of this research project is to improve the competitiveness of transitional organic grain crop producers by documenting critical information on cropping systems that will help maintain or increase productivity, suppress weeds and build soil health.One parameter we are examining is the soil biological response from each cropping system. This includes microbial activity as determined by beta-glucosidase assay and microbialbiomass as determined by phospholipid fatty acid assay.
In Missouri, many farmers transition into organic production by stopping chemical herbicide and fertilizer use while increasing tillage. Increased tillage has been implicated in soil degradation and erosion and is of major concern in a state with 5.3 tons of topsoil loss per acre per year. In organic row cropping, reduced tillage must be accompanied by increased ground cover to reduce weeds. Organic no-till has been found to be successful in some areas of the U.S. and can improve soil carbon retention, water holding capacity and soil structure. In this experiment we compare organic no-till to conventional tillage and modified tillage/cover crop production systems.
The University of Missouri Bradford Research Center Second Annual Organic Field Day was held August 1, 2014 and was attended by 220. This 3/4 day event featured talks and demonstrations on organic weed and insect control, organic no-till, organic small grain production, permaculture, trap cropping, mycorrhizae, organic certification, soil nutrients, vermicompost, bee keeping and fruit production.
On February 6-8, 2014 Kerry Clark had a booth at the Missouri Organic Association Conference (attendance 400) and provided free soil testing for active carbon levels to participants. Active carbon was explained in an informational poster and related to tillage and crop productivity issues. Free active carbon testing was also provided at all University of Missouri field days in the summer of 2014 (total attendance 550) to get both organic and conventional farmers thinking about soil health and soil water retention.
On February 26-28, Kerry presented a poster on research results at the MOSES Organic Conference in La Cross, WI.
In January-February and July-August, 2014 Kerry gave 16 full-day workshops on soil health and no-till, including organic no-till, throughout the state of Missouri (attendance 480). This was followed up with a state-wide soil health expo in Columbia on August 13-14 (attendance 200), in which the relationships between physical, chemical and biological soil properties and their effects on productivity and pest management were discussed. On July 30, she spoke to 75 agriculture professionals at the MU Crop Injury and Diagnostic Clinic on water retention and runoff in no-till fields with cover crops versus conventional tilled fields. On September 16, she spoke to approximately 300 high school students at the MU FFA field day on organic no-till and water infiltration.
Field research was completed in 2014 and final data analysis is currently being conducted.
Impacts and Contributions/Outcomes
Drought in 2012 and 2013 had major effects on the germination and growth of plots in this experiment. Weed pressure was also high as this was a field recently taken out of a long-term fescue pasture. Perennial weeds became problematic, especially in no-till plots. Overall, yields in this study were very low due to the combined effects of drought and weed pressure. Germination was low in both the conventional and no-till plots and yield was lower in the no-till plots than in the conventional till plots. Corn in the no-till plots in 2013 was visibly yellower, indicating that the cover crop breakdown could be tying up nitrogen. The no-till plots had about 800 more lbs/acre of weed biomass than the conventional tilled corn plots and yielded 27 bu/acre less in 2013. Grain sorghum yields were severely affected in the no-till plots with the rolled cover crop. Germination was reduced and it appeared that the small grain sorghum plants had difficulty emerging through the thick cover crop mat. Because of slow early growth of grain sorghum in the conventional tilled plots, row cultivation was delayed, which allowed early weed growth that adversely impacted grain sorghum yields. The double crop beans planted after the wheat harvest in 2013 had only about 40% germination, probably due to very dry conditions at the time of planting. Growth of the soybean plots was so limited they were not harvested for yield.
In 2014, weather conditions were improved but yields remained low due to weed pressure and poor germination as a result of a buildup of non-degraded biomass in the field from both cover crops and weeds. This biomass was mowed and disked, but still adversely affected seed to soil contact. Although winter cover crops were drilled at a rate of 120 lbs/acre, the three year average of 3830 lbs/acre of biomass is far below the levels generally recommended for good weed control in an organic no-till system. In other studies, we have planted winter cover crops at 180 lbs/acre with rows going perpendicular to each other and still failed to achieve the 8000 lbs/acre biomass recommended by Rodale for organic no-till. Attaining high cover crop biomass is key to successful organic no-till but is difficult to do when in a crop rotation because most summer annual grain and oilseed crops are still growing at the optimum cover crop planting time (mid-September in Central Missouri). Over-seeding the cover crop to attain earlier planting usually results in reduced stands when compared to drilling.
Significant decreases in weed biomass occurred in years when a summer cover crop was grown. In the cover-crop-only plots, weed biomass was reduced from 5182 lbs/acre in year one to 12 lbs/acre in year three. Both sorghum-sudangrass and sunn hemp have proven to be excellent at reducing summer annual weeds. Some nitrogen deficiencies in subsequent crops have been observed in plots following sorghum-sudangrass. For this reason, grass cover crops should usually be grown in conjunction with a legume to provide an improved C:N ratio and less N immobilization.
In addition to examining how different transition strategies affect weed dynamics, this study is also looking at how the various transitional rotation strategies affect soil quality. When all soil quality data is analyzed, results will be used to determine the soil quality index for each treatment, using the Soil Management Assessment Framework (SMAF). SMAF is a quantitative evaluation tool that assesses the impact of soil practices on soil function. Our hypothesis on this project is that the CCO plots will give the highest SMAF scores. Some of the indicators used in SMAF are active carbon, available P, pH, total organic C and N (TOC, TON), PLFA, Beta-glucosidase enzyme activity, aggregate stability and nitrate and ammonium-N. These are all being assessed in this study but none of the data is ready for index development in SMAF. The data that we have gathered and analyzed is presented here in a relational format. Active carbon is a subset of soil organic matter and can rapidly change according to management practices. In each treatment, we saw an increase in active carbon from 2012-2013. This could be due to improved soil moisture conditions and to additions of compost. We do not currently see treatment differences in active carbon levels.
Many of the changes in soil fertility can be attributed to the addition of compost to the plots. The CCO plots had compost added only in year 1 and show the lowest change in P. The MC plots had little change in K, a loss in Mg and the lowest increase in Ca. It is unknown why this occurred. The pH of all treatments increased over the three years of the study due to the addition of compost with a pH of 8.65. Organic matter decreased in every plot, including the no-till plots. This likely occurred because all plots had at least some tillage. In the no-till plots most of the OM decrease occurred in year 3, when plots were disked for weed control and double crop soybean was planted after wheat. This field started out with relatively high organic matter levels because it had been in pasture for many years and was disked for the first time only the year before this experiment started. Soil fertility, like all other soil quality results from this experiment will be utilized in developing a SMAF index for each treatment.
Total organic carbon and nitrogen levels from 2012 and 2013 showed no significant differences between treatments but there was a significant rise in levels of both after the 2013 growing season. Again, this is likely due to addition of compost.
Normally, active carbon and total organic carbon levels will decline when soil is tilled due to increased microbial activity and rapid oxidation of the carbon stored in the soil. In this study, we have not seen a tillage effect on carbon levels. This may be because additions of compost help mediate this loss by providing an ongoing food source for soil microbes. Also, because the no-till plots have greatly reduced crop growth and yield, they may have reduced additions of carbon to the soil when compared with the higher biomass producing tilled plots.
Soil microbial biomass can be assessed using phospholipid fatty acid analysis (PLFA). Each microbial class has fatty acid chains present in the cell wall that are unique and identifiable through an extraction process and gas chromatography. PLFA results are presented on the following page. In 2012-2013, some slight treatment differences have been observed but their significance has not yet been evaluated. Across all treatments, gram positive bacteria levels significantly increased from 2012 to 2013. This is very likely due to either improved soil moisture conditions or compost additions.
Drought was probably the most significant factor in years one and two of this research. Dry weather impeded crop growth and reduced yields. Poor germination may be an effect of the cover crop. The rolled cover crop impeded milo germination significantly in 2013 but corn germination was equal in rolled plots and plots where the cover crop was mowed and disked. Sorghum-sudangrass has proven to be an excellent biomass producer and very effective weed control. After very bad crop growth in year one, we were afraid that the entire plot area would need to be abandoned. However, in plots where sorghum-sudangrass and sunn hemp were grown, weed control improved and after the three year transition may be well suited for improved crop productivity.
After three years of using organic no-till in multiple experiments we have concluded that researchers need to design methods to deal with mid and late season weed emergence through the cover crop mat. In a new NIFA study we are using between-row mowing in no-till plots. This achieves about the same level of weed control as cultivation in tilled plots.
University of Missouri Dept of Soil, Environmental and Atmospheric Science
Columbia, MO 65211
Office Phone: 5738826607