Core Ideas Organic amendments are promoted as sustainable alternatives to synthetic fertilizer. Crop yield increased by an average of 43% in the first season after organic soil amendment. Yield benefit from organic amendments was greater in leafy crops than root crops. Poultry manure was commonly used and provided the greatest agronomic benefit. Yield benefit of organic amendment was lower in arid regions with poor soil quality. 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.

Core Ideas Mustard, oat, sudangrass, and buckwheat were the most productive cover crops. Excluding mustard from spring mixtures sometimes increased weed biomass. Excluding sudangrass from summer mixtures sometimes increased weed biomass. Forage radish was less productive, but suppressed weeds in monoculture and mixture. Cover crops can provide many ecosystem services and on‐farm adoption is increasing. Cover crop mixtures are popular, but little is known about the functional contributions of individual species in mixture. On‐farm field studies were conducted across 3 yr (2014–2016) and two locations (organic farms in central and northern Illinois) to assess the productivity and weed suppressive capacity of 12 cover crop species planted in monoculture and in mixture. Species were split into two equal (six species each) and seasonally appropriate groups (cool vs. warm season), planted in all possible monocultures and five‐way mixtures (divided proportionally to monoculture rates), and compared with a weedy control. Mustard [Brassica juncea (L.) Czern.] and oat (Avena sativa L.) were among the most productive (as much as 4.48 and 3.95 Mg ha−1, respectively) and weed suppressive spring cover crops. In 2 of 5 site‐years, excluding mustard from five‐way mixtures resulted in increased weed biomass. Sudangrass [Sorghum bicolor (L.) ssp. Drummondii] and buckwheat (Fagopyrum sagittatum Moench) were typically the most productive summer cover crops (as much as 8.78 and 7.11 Mg ha−1, respectively) and also reduced weed biomass. In 2 of 6 site‐years, excluding sudangrass from the mixture led to increased weed biomass. Forage radish (Raphanus sativus L.) had less aboveground biomass, but reduced weed biomass by 45 to 100%. Results are among the first to document species‐specific contributions to an ecosystem service in cover crop mixtures, and can be used to inform recipes for mixtures designed for greater productivity and weed suppression.

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