By Stephan Kamrad
A while ago Joanna reported on a chemical free, organic pest control method that has a lot of advantages to conventionally used pesticides. Studies have shown that organic and comparable agriculture is more sustainable, as measured by indicators like species richness, soil fertility and nitrogen uptake. But even by most experts it is usually dismissed as a fantastical ideal that conflicts fundamentally with our need to feed the growing human population. This month, a new meta-study, published in the Proceedings of the Royal Society B, by scientists from the University of California reveals that the productivity gap between organic and conventional farming might be much smaller than widely believed.
The researchers analysed 115 studies covering over 30 countries and 50 crop species. Organic farming, defined by having no synthetic inputs, was found to be on average 19% (±4%) less productive than conventional farming. But interestingly, this obviously quite a drastic gap shrinks down to 9% (±4%) when the organic farmers use a polycrop system compared to a conventional monoculture. In polycrops, multiple species are grown together, e.g. in alternating rows, resulting in a greater biodiversity than conventional monocultures. This makes them less susceptible to disease and pests and certain combination of crops can act as biological pest repellants and natural fertilisers. In Joanna’s example in Kenya, maize was planted together with Desmodium (which repels the vicious Stemborer moth and also fixes atmospheric nitrogen). Another popular example found in British gardens is intercropping of tomatoes, onions and marigold.
The yield gap was also much smaller (8±5%) when organic farmers used crop rotations, i.e. planted a different crop in each growing season, a system which was once (in the Middle Ages) quite popular in Europe.
But where is the catch? If these techniques are so effective, why are they not used everywhere? More diverse systems are much more difficult to manage. Massive machinery cannot easily be used with companion crops and it is often advantageous for farmers to sell only one or a few crops in bulk. For small farmers in developing countries these techniques are easier to adapt but farmers often are not aware of the possibilities.
All this of course might be slightly too optimistic. After all, non-organic agriculture can also make use of intercropping (rare) or crop rotations (more common). In studies where conventional farming (i.e. the use of pesticides, weed-killers and synthetic fertilisers) was combined with polycropping or crop rotation, the yield gap returned to its original value or was even higher.
Interestingly, the yield gap also depends on what type of crop is under consideration. The yield ratio of organic to conventional farming is lowest for cereal crops, where a lot of effort has gone into the development of high intensity, large scale monocultures but often comes close to 1 for fruits and nuts, were less effort has been made in developing high output systems.
In our world, it is very hard to convince a farmer that he should tolerate a 9% or even 20% yield decrease for the prospect of a healthier agro-ecosystem, that is diverse, unpolluted and resilient to stress and disease. Diversification (be it over time as in crop rotations or over space as in polycrops) can raise organic farming yields and make it more competitive to conventional farming. With more investment it may be possible for the yield gap to be reduced even further.
Ponisio LC, M’Gonigle LK, Mace KC, Palomino J, de Valpine P, Kremen C. (2015) Diversification practices reduce organic to conventional yield gap. Proc. R. Soc. B, 282:20141396. DIO: dx.doi.org/10.1098/rspb.2014.1396