UC Davis

Organic soil amendments are increasingly promoted as a sustainable alternative to synthetic fertilizers and as a tool for building soil quality through improved chemical, physical, and biological properties. However, short-term yield response to organic amendments is highly variable. A meta-analysis of 53 studies was conducted to (i) develop a global estimate of first-season crop yield response to organic amendments, and (ii) determine the effect of crop type, amendment characteristics, soil properties, cultural practices, and climate on the magnitude of this yield response. Yield response ratios were calculated (organic amendment yield compared to a non-fertilized control) and differences among groups were determined using 95% bootstrap confidence intervals (CI). Across all studies, crop yield increased 43±7% (95% CI) in the first-season after an organic amendment. Yield response was greatest for leafy crops (71±26% increase) and lowest for root/tuber/bulb crops (29±10% increase). Poultry manure/compost was the most commonly used amendment and provided a yield increase of 76±21%. In contrast, plant-based amendments increased yield by only 27±9%. Amendment application rate alone was not an effective predictor of yield response, and there were not enough studies available to explore the possible interaction between amendment type and rate. Yield benefits of organic amendments were muted in soils with high organic matter and in arid climates. These results help identify options for maximizing the agronomic value of organic amendments, and suggest research is needed to improve agronomic efficiency of amendments in arid regions with poor soil quality.