Using manure fertilizer closes organic yield gap by promoting bottom-up pest control
Organic farmers rely on natural predators to help control their crop pests rather than using chemical sprays. A new study published in the Journal of Applied Ecology found that using organic fertilizer builds important soil fauna that increases local populations of natural predators who provide enough pest control to prevent yield loss to pests. The study compared yield of crops grown with synthetic mineral fertilizer versus organic manure and found that overall, there was no difference in yield between organic and conventionally fertilized plants, but the gap was closed with different underlying mechanisms. Plants grown with organic manure had less herbivory, which counteracted the slower plant growth typical for organic management, while plants under non-organic fertilizer management had more herbivory, but this yield loss was counteracted by faster plant growth. The results indicate that organic fertility management boosts plant growth by also boosting soil biodiversity providing alternative food sources for natural predators. This helps sustain their populations when they deplete their main food resource: crop pests. Recruitment of natural predators like ladybugs and wolf spiders from nearby locations was also important for controlling aphid pests in this study, indicating that a two-pronged approach should be taken to promote natural pest control: 1) support locally emerging natural predators by bolstering soil biodiversity through organic manure application and 2) support the spillover of predators from the surrounding landscape by providing habitat resources throughout the farm. Luckily for organic farmers, these strategies are already common practice.
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