2018 ASHS Annual Conference

Biodegradable plastic mulch is designed to be tilled into the soil after use, eliminating environmental and economic issues associated with plastic mulch removal and disposal. There is a need for a soil sampling protocol that can accurately assess the amount of biodegradable mulch remaining in a field post soil-incorporation. An experiment was carried out at Mount Vernon, WA to develop such a protocol. Three blocks each of five beds measuring 1 m wide and 10 m long, spaced 2 m on center, were covered with a biodegradable mulch (Organix, Organix Solutions, Maple Grove, MN). Sixty-one days later, beds were disced, first in the direction parallel to the beds and then in the direction diagonal to the beds, as is the common grower practice in this region. Blocks were separated from each other by 8 m so the tractor-tiller could turn without entering an adjacent block. In each block, 24 soil samples were collected 1-3 day after discing: 12 were from the center of the three center mulched beds (four samples per bed), and 12 were from the center of the three center alleys (four samples per alley). For each sample, soil was collected from a 1 m² area to a depth of 15 cm using a shovel. The amount of soil sampled was then reduced using the quartering method: the soil was placed on a piece of plywood, mixed thoroughly, divided into quarters by two lines intersecting at right angles at the center of the pile, and two diagonally opposite quarters were discarded. This procedure was carried out a total of three times so that the final sample size (19 L) was 1/8^th of the original sample size. Mulch fragments were then recovered from each soil sample, and the mulch area was measured. There was no difference in the amount of mulch recovered from the beds and the alleys (91-110%), indicating that mulch fragments were distributed evenly throughout the field. The 95% confidence interval for the recovery rate was 100 ± 11%, indicating this method was reliable and repeatable. Thus, soil sampling using the quartering method can be used to assess the amount of mulch remaining in the field after tillage incorporation when the field is tilled parallel to the bed and then diagonally, and the location of the soil sample relative to the bed center will not affect the amount of mulch recovered.