Winter cover crop suitability for improved soil health in northwestern Washington

Toby Una

doi:10.7273/000002444

Cover cropping is a promising soil conservation practice, but finding suitable species can be difficult given environmental and logistical constraints. Challenges like short shoulder seasons, saturated winter soils, disease and pest pressures, and intensive tillage in northwestern Washington’s cropping system limit potential candidates. Additionally, soil quality must be maintained to sustain the production of high-value crop such as fresh-market potatoes (Solanum tuberosum) and vegetable seed crops. To identify winter cover crops suitable for northwestern Washington, two trials including a no-cover control, eight single species (including brassicas, grasses, and legumes), and a grass-legume mixture were conducted in Mount Vernon, WA from 2018-2020. Following the cover crop incorporation, potatoes and a spinach seed (Spinacia olercea) crop were planted in years one and two of the trials, respectively. In both years of the trials, the examination of spring cover crop and weed biomass production determined that cover crops suppress weeds and annual ryegrass and the mixture produced the most biomass. During the potato portion of the trials, short-term soil health and productivity impacts were examined by measuring permanganate oxidizable carbon (POXC) and inorganic nitrogen (N) before cover crop incorporation and midseason at 0-30 cm. Both POXC and inorganic N were higher at midseason, but did not vary among cover crop treatments. These findings were expanded upon during the spinach portion of the trials by examining POXC, mineralizable carbon (Min-C), and inorganic N at 0-30 cm, 0-15 cm, and 15-30 cm monthly from before cover crop incorporation (April) to spinach seed harvest (August). Comprehensive spatial and temporal measurements can better inform appropriate sampling protocols to best track soil health changes over time. All three measurements varied by depth and temporally, and Min-C and inorganic N varied by cover crop treatment. Depth and time should be considered when soil sampling for soil these soil health metrics and sampling in April with routine fertility sampling will give values representative of the season for these metrics. Potato tuber yield varied by year, but not cover crop treatment, while spinach seed yield was higher in the control, daikon radish, and mixture treatments than winter pea and crimson clover.

Winter cover crop suitability for improved soil health in northwestern Washington

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