The starchy root of Manihot esculenta, cassava, is a workhorse of the developing world. It forms the basic diet of about half a billion people and can grow with reasonable yields in poor quality soils where many other crops fail. The secret to this ability lies not so much in its own genetics, but the relationship it has struck up with a particular family of soil inhabitants: arbuscular mycorrhizal fungi. The thin strands of these fungi colonise plant roots and transport phosphorous and other micronutrients to the crop from beyond the root structure’s natural reach. These fungi aren’t picky, nor are they new. They’ve been engaging in symbiotic relationships with numerous plants for up to 460 million years, and may well have been critical in helping the plant kingdom creep out of the primeval ooze onto land.
Such symbiosis is completely unremarkable. It is nature’s default setting; a mesh of interdependent species of all scales that, like traders in a busy market, willingly give up some resources, exchange others, and at some point might put each other out of business. Examples of these interactions can be seen in insects pollinating plants for the promise of sugary nectar and protein-packed pollen. In agriculture, even the most fundamental practices are underpinned by symbiosis: humans protect and feed ruminants who turn grasses inedible to humans (and perhaps grown on lands unsuitable for other crops) into digestible foods: meat and milk. Microorganisms in the animals’ guts help them break down vegetation and, once the animal has extracted all the goodness it can, its faeces spread seeds, nutrients and a bunch of microorganisms that can help scores of tasty plants prosper in new soils.
“In agriculture, even the most fundamental practises are underpinned by symbiosis.”
Monoculture-focused agriculture has disrupted the balances on which such relations depend and foiled nature’s ability to buffer attacks by pests: a lack of genetic variation helped the late blight of the 1840s wreak devastating havoc in the Irish Potato Famine; in 1970 the southern corn leaf blight destroyed about 15% of the US Corn Belt’s crop production because of a prevalence of one maize cultivar; in the late 1980s witches’- broom disease swept through Brazil’s cacao plantations, eradicating around 70% of the country’s crop and inflicting economic and social catastrophe. Brazil went from being one of the world’s largest exporters of cacao beans to a net importer. And there are many more examples.
“In Mesoamerica, the ‘three sisters’, have long been grown as companions: maize stalks serve as stakes for beans to climb, while squash plants discourage weeds from growing beneath their leafy shade.”
Polyculture seeks to ameliorate some of the worst effects of monoculture by returning symbiosis and balance back to the centre-stage. A family of farming methods — which includes techniques like companion planting, multi-cropping, intercropping and alley cropping — the polyculture principle goes that by planting different types of crop with complementary characteristics and functions, it is possible to boost the natural immunity of land to pests and diseases, reduce inputs, improve soil quality, and help increase biodiversity and dietary diversity, albeit with slightly higher labour costs than current practices require.
In Mesoamerica, maize, beans, and squash, or the ‘three sisters’, have long been grown as companions: the maize stalks serve as stakes for the beans to climb, while squash plants discourage weeds from growing beneath their leafy shade. In addition to its relationship with soil fungi, cassava can reap the benefits from being planted alongside legumes that fix nitrogen from the atmosphere and provide their soils with a source of ammonia (it’s actually the bacteria on legume roots that do this; yet another symbiotic relationship). In pilots in Thailand, Vietnam, and India, intercropping with cowpeas, mung beans or groundnuts has increased cassava yields and economic revenues per hectare farmed. Such successes are not unique to cassava. Intercropping systems with other cereals, including wheat and rice, have been extensively investigated with similar results.
The ‘sloping agricultural land technology’ approach — a form of alley cropping developed in the Philippines — involves planting hedgerows of nitrogen-fixing shrubs and trees, the prunings from which serve as mulch and organic fertiliser, feeding alleys of permanent crops including coffee, cacao, citrus and banana, and rotating crops such as maize, sorghum, melon and pineapple. The system maintains soil fertility and provides farmers with multiple harvests throughout the year. Critically, polyculture’s use of large quantities of bulky organic matter as fertiliser tends to make its soils more resistant to drought. The Rodale Institute, in tests stretching back to 1981, has produced yields 31% higher in times of drought stress with these high organic matter soils compared to those treated with conventional fertilisers.
Of all developing world crops, rice is undoubtedly the most important — it’s the main component in the diets of three billion people globally. But rice paddies are the leading agricultural source of greenhouse gases in the developing world, in part because of the excess use of fertilisers.
In a rice paddy in Zhejiang Province, China, a carp knocks carelessly into a rice stem, knocking morning dew from the plant. A few feet away another rice stem shakes and a planthopper falls into the water, providing breakfast for the carp. These seemingly innocuous occurrences were captured on video and quantified in a bid to understand the potential benefits of the 1,200-year old technique of rearing fish in rice paddies. The rice provides shelter and cool water for the fish and attracts insects for them to consume. In return, the fish uproot and eat weeds and eat the plants’ insect pests.
In a five-year study that compared hundreds of fish-rice and monoculture paddies in 31 villages in Zhejiang, the presence of the carp decreased the number of pests on the plants by 26% and reduced the risk of certain diseases proliferating in their leaves. The result? Chemical inputs needed by farmers were dramatically reduced — 68% less pesticides and 24% less fertiliser for almost identical yields. In other studies, rice yields under such methods have been some eight to 15% higher. Although they use more water, fish-rice paddies have been shown consistently to increase farmers’ income, provide a vital source of dietary protein and fatty acids, and serve as natural mosquito control.
Even the best technologies are of scant value if they aren’t fully adopted. Uptake of fish-rice systems has been low in Bangladesh for example, with farmers put off by implementation costs and the lack of knowledge surrounding the practise and its benefits. To encourage smaller-scale farms in developing countries to adopt new technologies they need access to financing and educational support. For the former, better IT systems can make background checks, loan approval and performance monitoring much quicker and more effective. In terms of funding, public-sector investment that is free from short-term commercial pressures and biased thinking is also critical.
Food is of equally scant value if no-one eats it. Once produce has been grown it is vital for farmers to have access to well-developed wholesale markets and storage, efficient transport connections, and marketing expertise, particularly if new co-products are involved and especially in developing countries. Developments in infrastructure must be symbiotic with agricultural improvements.
Supporting the efficacy of polyculture schemes requires a localised approach: what challenges are faced by farmers working in particular climates and what affect their particular cultural or historical pressures? Similarly, the research and development that underpins such schemes should be location-focused and foster collaboration between farmers, scientists, policymakers, social movements, and technologists to scale up research discoveries and maximise outputs with the proviso of carefully minimising damaging inputs and their consequences.
“The biggest revolution in polyculture may happen when we start to literally dig deeper into the rhizosphere.”
Of course, to make polyculture as successful as possible we will need to make use of many of the tools that conventional agriculture has given us. Currently, monoculture seems more viable simply because the last 50 years of agricultural research has sought to maximise its development, working with species of plants and animals that flourish in its characteristic environment. Most modern crop varieties bred for this purpose are unlikely to exhibit traits that are optimal in polyculture farming. Using the tools of conventional agriculture to breed specific high-yielding polyculture varieties is essential, and may well include the development of high- performance GM varieties.
The benefits of adjusting what is grown and where will also be enhanced by in-field technology advances, not least how we feed and water our crops. While rainfed crop production comprises as much as 60% of total agricultural outputs, irrigated agriculture produces yields up to three times higher per unit area of land. Drip and micro-irrigation systems that dose specific amounts of water to where plants need it the most are already available for those with the money. Making them cheaper to produce and implement is the next step. Additionally, similar systems can be used to judiciously deliver chemical inputs: it has been shown that only around 27% of mineral fertiliser applied to rice paddies in China is uptaken, and whereas in Europe and North America soil monitoring is routine, the lack of it in developing countries results in its overuse. This is both expensive and environmentally damaging.
These steps are critical if we are to give polyculture approaches the best possible chance of success, but the biggest revolution in polyculture may happen when we start to literally dig deeper into our fields. The rhizosphere, the area around a plant’s root, is teeming with a bustling ecosystem of bacteria, fungi, protozoa, and nematodes that provide access to nutrients, ward off pathogens and might even make some produce tastier than others. Our understanding of such microbes is limited and little consideration has been given to their role in agriculture. If the last century saw humanity mastering the chemistry of our soils and the individual needs of organisms, then the next looks set to see us better harnessing their biology and complex interactions.