Long-Term Compost Carryover in Organic Dryland Wheat Production.

A major challenge of organic farming is soil fertility, which is a function of nutrient sources and crop management. Compost improves soil fertility and plays a major role in soil fertility planning in organic systems but major drawbacks include transportation costs, bulkiness and most importantly, the challenge of accurately predicting how much compost to apply and how long beneficial effects will persist. This research investigates compost carryover on yield and soil quality of certified organic dryland wheat. The compost was applied 20 years ago (1994) as a one-time application of dairy manure compost at the rate (0, 50 Mg DM ha^-1) and 10 varieties of wheat. The experimental trial was a random complete block design (RCBD), with split plot and three replicates. Compost was the whole-plot factor and variety the split-plot factor. In 2014, yield was higher in compost amended plots and root and shoot dry weight was higher. Compost amended soil was higher in available phosphorus (P) at all soil depths measured (0-5cm, 5-10cm, 10-30cm). Available phosphorus was higher in 2012 than 2014 at all soil depths. Microbial activity as measured by dehydrogenase activity was greater at all depths in compost amended plots. The findings show a strong persistence of a compost effect 20 years after an initial one time application. Further work needs to be done to determine the mechanisms of long-term compost carryover in dryland soils and to determine its reproducibility.