The many benefits of growing a mixture of crop varieties together have now been demonstrated for many crops under many conditions. Latest entry is in a kind of specialised niche — organic tomatoes for processing — and the results are a little underwhelming. Three scientists at the University of California, Davis, grew one, three or five tomato varieties in soil that was either fallow or had a mustard cover crop the preceding winter. ((Barrios-Masias, F., Cantwell, M., & Jackson, L. (2010). Cultivar mixtures of processing tomato in an organic agroecosystem Organic Agriculture, 1 (1), 17-30 DOI: 10.1007/s13165-010-0002-z)) Although there were differences between the mixtures and the monocrop, they were not very pronounced: more shoots, and more fruits that were somewhat redder.
The 3-cv mixture thus had some minor advantages compared with the monoculture, but overall, there was little evidence of higher ecosystem functions from mixtures vs. monoculture.
Leaving aside the question of whether the growing conditions on the farm, as described in the paper, accord with organic ideals, given that they do abide by the rules, it is hard to know what to make of the poor showing of mixtures. The authors concede that there may simply be no benefit to be had because the conditions on California organic processing tomato farms don’t stress the crop to the point where a mixture might be a good thing. It is also possible that the varieties — AB-2, CXD-19, H-2601, H-8892 and Red Spring — do not actually encompass enough trait diversity to offer any benefits. But then, they were chosen as simply the best-performing processing varieties. If mixtures were assembled deliberately to deliver potential benefits, the results might well be different.
Still, good to know.