Sulfur Application for Weed Specific Suppression
Berry Brook Organic Lowbush Blueberry Farm has major concerns with weed control and little is known about how to combat many perennial weeds that compete with the carpet of blueberries. The organic recommendation is to apply elemental Sulfur to lower the pH to a level at which the weeds can no longer out compete the blueberries. The University of Maine recommended rate is 100 lb/ac for every 0.1 pH drop desired, to a pH level as low as 4.0.
The goals of my SARE grant project are to see how different specific weeds are affected by a change in pH. By applying varying rates of 85% Sulfur to multiple replicate plots, we can measure the initial and any change in weed species present as a result of the Sulfur application. Another goal is to monitor the speed of pH adjustment, as Sulfur slowly acidifies the soil over time. Taking initial and annual soil samples, and testing pH, allows us to monitor the changes in soil acidity.
This past spring, I set out 40 plots in 8 blocks with 5 treatments per block. I took soil samples in each plot, 20 subsamples per plot, and submitted them to the Soil Testing lab for pH evaluation. The five experimental treatments replicated in each block were: a control plot, 600 lbs S/ac, 800 lbs S/ac, 1000 lbs S/ac, and 1200 lbs S/ac. I designed a plot plan for the study as a randomized complete block design and treatments were assigned to each plot. The first 4 blocks were established in an area in which bracken and sweet ferns are the biggest weed problems, while the second set of 4 blocks were located where goldenrod is a predominant weed. I ordered Sulfur in bulk, and had it delivered to the farm and unloaded it by hand. Farm assistants helped apply the Sulfur to each plot using a hand-pushed drop fertilizer spreader. Jennifer D’appallonio assisted in weed identification and data analysis of weed cover.
In June and August, I assessed for blueberry and weed cover in 4 subsamples per plot. I used the Daubenmire rating scale, which uses percentages of plant cover in a given area, to assign a weed rating to each subsample. Weed species within each subsample were categorized and identified as: ferns, woody weeds, grasses, broadleaf weeds, and goldenrod. In June, we composed a weeding team, which consisted of 2 to 5 people. This team of weeders was timed at weeding each plot in a specific block. The first 2 blocks were weeded by 2 people, and we quickly realized it was far too much work for 2 people in the heat of the season. The second 2 blocks weeded in June were done by a group of 5 people, thereby limiting the effects of different weeders to ‘block effects.’ The weeding time was multiplied by the number of weeders, resulting in an amount of time it would take an average weeder to weed a plot (1200 square feet). This weeding will be redone during the next prune cycle to determine any decreases in weeding time based on weed control efforts. In August, the second set of blocks were weeded manually and talls weeds were cut mechanically. Times were recorded. Blocks hand-weeded in June were assessed separately from those mechanically weeded in August. Goldenrod was not assessed in the June weed assessment because goldenrod has not yet produced flowers in June, so identification compared with other asters is very difficult.
In addition to weed assessment and soil sampling, leaf tissue samples were collected from a few of the plots with no (0 lbs S/acre), medium (600 lbs S/acre), and high (1200 lbs S/acre) Sulfur inputs before the study, to serve as a representation of initial plant health. Leaf tissue analysis was done by the Analytical Lab at UMO. This information will serve as a baseline for plant health and will be sampled again after the next prune cycle when the Sulfur has begun to affect the pH.
Impacts and Contributions/Outcomes
All the collected data has been entered and preliminary assessments have been made. Because it could take 3 to 4 years to see any pH change due to S application, I didn’t expect to see any effects on plot pH or differences in weeds as a result of the Sulfur treatment applications this year. Therefore, only baseline data has been collected and will be presented in this interim report.
The results of this project, to date, show no significant differences in weed cover as a result of the Sulfur treatment applications. I proposed assessing for ferns and goldenrod in response to changes in pH, so I did assess for both ferns and goldenrod. However, I also included other weed assessment categories: woody weeds, grasses, and broadleaf weeds, and assessed blueberry plant cover. The pH level in each plot was determined from early spring soil samples, and I correlated the types and percent of weed cover with the initial soil pH levels. The lowest pH level in any of these plots was 4.6! Most of the plots ranged in pH from 4.8 to 5.15. There were no significant trends in June weed cover based on soil pH level, however the pH of 4.9 to 5.05 had more ferns than the lower or higher pH values represented (see Figure 1). Although there are some differences, the pH ranges of the plots is minimal, so it may not be representative of a larger sample size. August weed assessments showed a trend in that goldenrod percent cover was significantly lower when pH was 4.85 or less as compared to most pH above 4.9. See attached graphs for details of all the weed assessments.
This is only preliminary data, but I hope other farmers could assess their weed types and soil pH so that we could compile a larger sample size of weed pH preference in lowbush blueberry fields. Over time, as soil pH levels change in these Sulfur plots at Berry Brook, we hope to identify changes in the types of weeds to determine their pH preference and at what soil pH level weed types can no longer survive. This information can begin to help growers understand how pH affects weed type in their field. I will assess weeds again in June and August of the following prune year. Extensive and multiple weed assessments cannot be accurately performed during a crop year, as plant damage can occur from walking through developing plants.
There were no significant differences in weeding time by pH level or between the treatment types. Average hand weeding times ranged from 56 minutes to 78 minutes for one person to weed one 1200 square foot plot. The average time for cutting weeds in plots with power tools ranged from 10.5 to 12 minutes per 1200 square foot plot. See attached graph for a visual representation of weeding times.
One problem that has arisen is that multiple University researchers sampled soil and leaf tissue throughout my first 14 plots, due to a miscommunication.
I don’t yet have any economic data on the project, but should have some implications next year during harvest. Certainly, I was able to employ some people for the weeding aspects of the project, and that has allowed me to extend the employment period beyond harvest. I think this will help me to gain a larger base of persons interested in seasonal work.
In July, SARE and UMO co-sponsored an outreach event with MOFGA, Maine Organic Farmers and Gardeners Association, of which I was the farm host. This was one component of my outreach activities. Quite a few organic blueberry growers attended, and I was able to meet more fellow growers. I explained my proposal and how the grant process works, and everyone was very receptive to the idea of evaluating the specific weeds’ response to the change in pH. They also offered weeding suggestions such as leaving the weeds in the field once they are pulled; our weeders loaded a wheelbarrow and dumped the weeds in the woods on the edge of the field. Another facet that I discovered needs to be addressed is the issue of elemental Sulfur purity and the speed of incorporation of Sulfur based on the percent of purity. For example, does the pH drop faster when the S is 85% pure with more clay component than when the S is 99% and a rock hard pellet? I would think that the 85% would break down more rapidly, uncovering more surface area for breakdown, but this has not been examined.
Furthermore, I question whether I should wait another 2 years before pruning again, as was my original plan. I am anxious to see how fast the pH adjusts, and soil sampling needs to occur in the spring of a prune year to be comparable to the initial soil pH. Therefore, this is a decision that must be made by next year’s interim report. Harvesting will occur this year, 2011, and then the project will continue with either another year of harvest or pruning. I think it would be beneficial to continue the study for a total of 6 years, so that weeds and harvested mass can be assessed 3 times to understand any longterm pH changes. Once the study is complete, in 2013, data will be compiled, analyzed, and graphed. I will present these results to MOFGA via a written document, and present my results at the annual Maine Blueberry Growers’ meeting in July.
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