Farmers are always looking for effective ways to reduce weed competition within their vegetable crops. One method is soil tarping. When tarps are applied in early spring and removed before planting, they can reduce weed pressure for early season crops like onions. Since onions have a very minimal canopy and therefore, do not provide much soil coverage, they have high susceptibility to weed pressure.
SOLARIZATION VS. OCCULTATION
Two tarping methods are solarization and occultation. Solarization uses clear plastic to harness energy from the sun and warm the soil. The goal behind solarization is to exhaust the weed seed bank before planting to reduce the number of weeds coming up during the growing season. In some cases, it can raise temperatures high enough to kill germinated weed seedlings. Occultation uses an opaque material to stop light from hitting weed seeds and therefore, stops germination. It can deprive any existing seedlings or perennial plant parts of light needed for survival. The length of time a tarp is on the ground may also impact weed pressure.
Much of the research and usage of tarping has occurred in more-humid areas in the eastern United States (Kennenbrew et al. 2023). The purpose of this tarping project is to study the performance of tarping to reduce weed pressure and increase yield under sunnier, windier conditions.
Materials and Methods
LOCATION AND EXPERIMENTAL DESIGN
Field research was conducted in Brookings, South Dakota to study the effects of soil tarping on weed pressure and onion yield. Soil type and organic matter can affect the wetting pattern from irrigation as well as nutrient holding ability and microbial composition. The soil type in the area of interest was clay loam (Soil Survey Staff, Natural Resources Conservation Service, United States Department of Agriculture 2024). Soil tests taken six inches deep at the beginning of the season showed soil organic matter ranging between three and four percent.
TARPS EVALUATED
Three types of tarps were evaluated: black silage tarps, white silage tarps, and clear greenhouse plastic. A control (bare ground) treatment plot was also included. The amount of time the ground was tarped was another factor explored. Each type of tarp was placed at six, four, or two weeks before onion planting. The experimental design was a completely randomized block design with onion cultivar as a split plot within the tarping treatment. There were four blocks and ten treatment plots within each block. Blocks were twenty-four feet wide by one hundred feet long, making each treatment plot twenty-four feet wide by ten feet long.
TARP PREPARATION
Opaque silage tarps with one side black and one side white were obtained from Farm Plastic Supply for the occultation treatments. Clear greenhouse plastic (UV-resistant six mil from Farm Plastic Supply) was also obtained for solarization treatments. Tarps were cut into pieces twenty-four feet wide by ten feet long to match plot size. They were laid down on the soil on April 18, May 5, and May 17 of 2023 so each tarp type covered the soil for durations of roughly six, four, and two weeks prior to onion planting. Black-side-up and white-side-up tarps were secured with twenty-five to thirty fifteen-pound sandbags. Clear tarps were secured with sandbags as well as burying the edges.
All tarps were removed May 30, 2023. Data on weed count, height, biomass, and type of weed was collected from each treatment plot. This was done by randomly throwing three 50-by-50 centimeter PVC quadrats into each treatment plot and collecting data on weeds within the quadrat area. Weeds within the quadrat were separated by broadleaves and grasses. For each weed type, three random weeds were selected to be measured for height. All weeds within the quadrats were clipped to soil level, counted separately by broadleaves and grasses, dried, and weighed to measure biomass.
After data collection, clear tarp and control treatment plots were tilled using a BSC tiller to clear out the germinated weeds for planting. These treatments were tilled lightly at a two-and-a-half-inch depth so as to not bring up more weed seeds to the soil surface. Black and white tarped plots did not need to be tilled at removal of tarps, as there were very minimal to no weeds in these treatment plots.
PLANTING ONIONS
Onion (Allium cepa) cultivars Barolo, Patterson, and Candy were seeded in size 128-cell trays in the South Dakota State University campus greenhouse on February 28 through March 2, 2023. Greenhouse temperatures were set 68 to 74 degrees Fahrenheit for daytime and 64 to 70 degrees Fahrenheit for nighttime. Four-hundred-watt, high-pressure sodium lights were active from 6:00 a.m. to 10:00 p.m. each day.
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Categories: South Dakota, Crops