For years, researchers have been looking to polycultures, biodiversity in space, as a way to improve agriculture (Trenbath 1974; Tilman et al. 1997; Cardinale et al. 2011; Finney and Kaye 2016). Behind this research is the idea that nature is the best model for agriculture. Because we find that nature is generally a polyculture, we should mimic this biodiversity on the farm. Natural is now viewed as the best option. Today, however, I want to commend a most unnatural practice, crop rotation.
Crop rotation is the growing of different crops in sequence, such as wheat, then corn, then beans. It can be a cycle, repeating the sequence, or it can by a dynamic sequence with no intentional repetition, wheat-corn-beans-beans-alfalfa-peas… Crop rotation helps control weeds, insect pests and diseases. It can improve soils, increase nitrogen levels, and decrease market risk. It is one of the basics of sustainable agriculture. And it is unnatural, disruptive of natural processes, and depends on crop monocultures.
When a field is planted to a crop, let’s go with wheat, that wheat is by far the most abundant species in the field in terms of mass. As a plant, wheat is a primary producer. Primary producers take energy from the sun and incorporate it into their tissue. This biomass-stored-energy, both above- and below-ground, then drives the food chain in the field, feeding primary consumers such as herbivorous insects, which then feed secondary consumers, such as predatory insects and birds, and so on. The organisms that thrive and have the potential to increase their populations are either those that like wheat, or those that like to eat the organisms that like wheat. The wheat monoculture drives the field’s biology.
Crop rotation takes this wheat field and changes it to a different crop. Now it’s a bean field. Beans produce a unique quality of biomass, both roots and shoots, differing from that of wheat and other species in chemical makeup, nutrient ratios, and display of leaves, stems, and roots, planting and harvest dates, root exudates, response to temperature, growth patterns, etc. As with the wheat, this new biomass of bean plants dictates which consumer organisms will be attracted to the field, which will thrive and which will decrease or leave, if they can. The critters that preferred to eat, or grow alongside wheat, whose population was just starting to increase to take advantage of the abundant and edible wheat crop, is now faced with beans as far as their eyes can see. They must either survive in the bean field, leave if they can, or decline. Organisms that prefer bean fields can start to increase in population if they were already in the field. If they were not, they must first find the field and then begin to increase.
This transformation of the plant biology in a field is what makes crop rotation effective. The change is quick and drastic, unlike anything found in nature. Because it is unnatural, it disrupts the natural life cycles and populations of weeds, insect pests and diseases. In a good crop rotation, they never have time to adjust to the current biological environment and so remain at lower populations.
The quicker the change and the more drastic, the greater the disruption – here, I am speaking of its biological, not economic, benefits. This is where monoculture is crucial. A field that has gone from wheat monoculture to bean monoculture will disrupt pests more than a field that was planted to a mix (polyculture) of wheat and beans for two years. The more mono the monoculture, the more drastic the change when a different crop is planted. Volunteer plants from the previous crop and weeds provide alternative food sources for insects and alternative hosts for disease.
Similarly, the larger the difference between the species in sequence, the greater the disruption in pest cycles. Wheat to corn is good, but they are both grasses. Wheat to beans is better. Grow a spring-planted crop last year? Then disrupt things by growing a summer planted crop this year, and then a fall-planted crop. Change up broadleaf and grass crops, annual and perennial.
A yearly change in crop species is good, but even shorter intervals can work better, because they are even more unnatural. We can make multiple disruptions in a year, as with double cropping of peas-buckwheat, or with a fall mustard green manure crop.
As R. Ford Denison points out in his book Darwinian Agriculture, we have options in agriculture that nature does not have, and some of those options are beneficial. In the productive environments (adequate water supplies and hospitable temperatures) where we most often practice agriculture, nature has no choice but to be biodiverse; diversity in space. We, however, can rotate crop monocultures; diversity in time. Crop rotation is not “natural.” It is as much a human innovation as agriculture itself. It works, not as an example of biodiversity, but because it is, ecologically, a regular extreme disruption of a field’s natural biology.
Cardinale, B. J., Matulich, K. L., Hooper, D. U., Byrnes, J. E., Duffy, E., Gamfeldt, L., Gonzalez, A. (2011). The functional role of producer diversity in ecosystems. American Journal of Botany, 98(3), 572–592. https://doi.org/10.3732/ajb.1000364
Finney, D. M. and Kaye, J. P. (2016), Functional diversity in cover crop polycultures increases multifunctionality of an agricultural system. J Appl Ecol. doi:10.1111/1365-2664.12765
Tilman, D., Lehman, C. L., & Thomson, K. T. (1997). Plant diversity and ecosystem productivity: Theoretical considerations. Proceedings of the National Academy of Sciences, 94(5), 1857–1861.
Trenbath, B. R. (1974). Biomass productivity of mixtures. Advances in Agronomy, 26, 177–210.