Distantly related crops are not better rotation partners for tomato.
Although crop rotation has been used for centuries to enhance agricultural production, there are surprisingly little data justifying the use of one rotation over another. Growers typically avoid growing plants in succession that belong to the same genus or family, but it is not clear whether closely related crops are indeed poor rotation partners and whether evolutionary history, overall, predicts legacy effects in agricultural soils. Here, we use a plant-soil feedback framework to test the relationship between species relatedness and crop growth. Using tomato Solanum lycopersicum as our focal crop, we determined how 36 common crop and weed species that vary along a spectrum of phylogenetic relatedness influence tomato growth in subsequent plantings. We also tested whether soil conditioning affects the performance of an above-ground insect herbivore, the tobacco hornworm Manduca sexta. Phylogenetic relatedness did not predict plant-soil feedback effects on tomato biomass or hornworm performance; rather, impacts of soil conditioning were highly species- or family-specific. For example, tomatoes growing in soil previously containing plants in the Asteraceae family were notably resistant to caterpillar feeding. There was also a disconnect between which plant species caused negative feedbacks on tomatoes vs. hornworms (e.g. thistle Cirsium discolor soil had strong negative effects on herbivory but no impact on plant growth). Hence, negative feedbacks on hornworms are likely due to enhanced defence instead of simply reducing leaf availability. Synthesis and applications. These data demonstrate that, despite being widely recommended by agronomists in most cropping systems, phylogenetic relatedness is a poor predictor for the success of crop pairings in rotation, especially in tomato. Better understanding of species-specific effects of soil conditioning will lend insight into how polycultures can be better designed to optimize crop growth while reducing susceptibility to insect pests, which is particularly useful on diversified farms that cultivate a variety of crop species.