Residual Effects of Compost on Soil Quality and Dryland Wheat Yield Sixteen Years after Compost Application

Significant hectarage of certified organic dryland wheat (Triticum aestivum L.) is planted in the western United States. Yields are severely constrained by lack of precipitation, so many dryland organic wheat farmers do not grow cover crops or apply fertilizer due to the inability to recoup application costs in the short term. Composts have a strong carryover effect, however, and the long-term benefits of infrequent applications to soil quality, yield, and economics on dryland wheat-fallow systems have not been evaluated. Soils were sampled in 2008 and 2010 from the site of a previous compost response trial located on a commercial dryland wheat farm in Snowville, UT. Dairy manure compost was applied in 1994 at the rate of 50 Mg dry matter ha(-1) in a randomized complete block design with three replicates. Sixteen years later, soils from compost-amended plots contained 1.6-fold higher total organic C (1.43% vs. 0.89%, p < 0.002), higher microbial biomass, dehydrogenase, acid and alkaline phosphatase activity and higher plant available P, K, and Zn relative to amended controls in the top 5 cm. The 5- to 10-cm layer contained 1.6-fold higher total organic C, higher plant available P and K and higher dehydrogenase, acid and alkaline phosphatase activity in 2010. The 2-yr average yield increased from 0.5 Mg ha(-1) in the control plots to 1.0 Mg ha(-1) in the amended plots (p = 0.004). When compared with the yield gain of 2.3 Mg ha(-1) measured in 1995 to 1997, the estimated half-life of the compost effect was 6 yr. Despite the long-term benefi ts measured in this study, the viability of using compost in dryland wheat production will depend on the availability and proximity of growers to sources of compost.