Compost and Cover Crop Effects in Dryland Organic Wheat

Producers of dryland organic wheat (Triticum aestivum L.) in the western USA struggle to maintain adequate soil fertility due to the high cost of organic fertilizers and concerns over moisture use of cover crops. Low soil fertility results in decreased wheat yield and quality, and increased year-to-year variability in yield and quality based on weather. This study was conducted to measure the effects of, and interactions between, cover crop mixes and a one-time compost application on soil health and winter wheat yield and quality. The study was located on three adjacent certified organic wheat farms near Snowville, UT, each with a calcareous silt loam soil under conventional tillage in a wheat-fallow system. Four composted steer manure treatments (0, 12.5, 25, and 50 Mg ha-1) and three cover crop treatments (oat [Avena sativa]- pea [Pisum sativum] mixture, vetch [Vicia villosa]- pea mixture, and a mechanically tilled fallow control) were evaluated from fall 2014 to 2017. Treatments were arranged in a randomized complete block split-split plot with three replications, with each farm hosting a single replicate. In 2015, the oat-pea cover crop treatment decreased grain yield, while compost amendment caused yields to more than double. However, neither compost nor cover crops affected yield in 2017. The 2015 crop year was much drier than 2017, suggesting that grain yield and quality benefits may be dependent on other factors such as the precipitation patterns of a particular year. The vetch-pea cover crop treatment increased total soil N in 2017 only, and compost amendment increased nitrogen fixation in the vetch-pea cover crop in that year. Compost treatments increased soil N, P, K, and organic C, both years without affecting soil EC or pH long-term. Nitrogen fixing cover crops may be a viable option for producers to increase soil N when sufficient soil moisture is present, however may be detrimental in a dry year. Compost amendment was shown to increase soil nutrition for multiple years, although that may only translate into increased grain yield when prerequisite weather conditions are present