Polyculture

thumb|upright=1.35|[[Asimina triloba|Pawpaw trees growing under mulberry trees, a forest gardening style of polyculture]]

In agriculture, polyculture is the practice of growing more than one crop species together in the same place at the same time, in contrast to monoculture, which had become the dominant approach in developed countries by 1950. Traditional examples include the intercropping of the Three Sisters, namely maize, beans, and squashes, by indigenous peoples of Central and North America, the rice-fish systems of Asia, and the complex mixed cropping systems of Nigeria.

Polyculture offers multiple advantages, including increasing total yield, as multiple crops can be harvested from the same land, along with reduced risk of crop failure. Resources are used more efficiently, requiring less inputs of fertilizers and pesticides, as interplanted crops suppress weeds, and legumes can fix nitrogen. The increased diversity tends to reduce losses from pests and diseases. Polyculture can yield multiple harvests per year, and can improve the physical, chemical and structural properties of soil, for example as taproots create pores for water and air. Improved soil cover reduces soil drying and erosion. Further, increased diversity of crops can provide people with a healthier diet.

Disadvantages include the skill required to manage polycultures; it can be difficult to mechanize when crops have differing needs for sowing depths, spacings, and times, may need different fertilizers and pesticides, and may be hard to harvest and to separate the crops. Finding suitable plant combinations may be challenging. Competition between species may reduce yields.

Annual polycultures include intercropping, where two or more crops are grown alongside each other; in horticulture, this is called companion planting. A variant is strip cropping where multiple rows of a crop form a strip, beside a strip of another crop. A cover crop involves planting a species that is not a crop, such as grasses and legumes, alongside the crop. The cover plants help reduce soil erosion, suppress weeds, retain water, and fix nitrogen. A living mulch, mainly used in horticulture, involves a second crop used to suppress weeds; a popular choice is marigold, as this has cash value and produces chemicals that repel pests. In mixed cropping, all the seeds are sown together, mimicking natural plant diversity; harvesting is simple, with all the crops being put to the same use.

Perennial polycultures can involve perennial varieties of annual crops, as with rice, sorghum, and pigeon pea; they can be grown alongside legumes such as alfalfa. Rice polycultures often involve animal crops such as fish and ducks. In agroforestry, some of the crops are trees; for example, coffee, which is shade-loving, is traditionally grown under shade trees. The rice-fish systems of Asia produce freshwater fish as well as rice, yielding a valuable extra crop; in Indonesia, a combination of rice, fish, ducks, and water fern produces a resilient and productive permaculture system.

Definitions

Polyculture is the growing of multiple crops together in the same place at the same time. It has traditionally been the most prevalent form of agriculture. Regions where polycultures form a substantial part of agriculture include the Himalayas, Eastern Asia, South America, and Africa. Other names for the practice include mixed cropping and intercropping. It may be contrasted with monoculture where one crop is grown in a field at a time. Both polycultures and monocultures may be subject to crop rotations or other changes with time (table).

! rowspan="3" colspan="3" |

! colspan="3" |Diversity in time

! rowspan="2" |Low

! colspan="2" |Higher

!Cyclic

!Dynamic

!rowspan="2" |Diversity in space

!Low

!Monoculture, one species in a field

|Continuous monoculture, monocropping

|Rotation of monocultures

|Sequence of monocultures

!Higher

!Polyculture, two or more species intermingled in a field

|Continuous polyculture

|Rotation of polycultures

|Sequence of polycultures

Historical and modern uses

Americas: the Three Sisters

thumb|A Central American polycultural "milpa" in 2011. Beans are growing among the drying maize; banana trees are in the background.

A well-known traditional example is the intercropping of maize, beans, and squash plants in the group called "the Three Sisters". In this combination, the maize provides a structure for the bean to grow on, the bean provides nitrogen for all of the plants, while the squash suppresses weeds on the ground. This crop mixture can be traced back some 3,000 years to civilizations in Mesoamerica. It illustrates how species in polycultures can sustain each other and minimize the need for human intervention. The majority of Latin American farmers continue to intercrop their maize, beans, and squash.

Africa: cowpeas and complex mixed cropping

In Africa, polyculture has been practised for many centuries. This often involves legumes, especially the cowpea, alongside other crop plants. In Nigeria, complex mixed cropping can involve as many as 13 crops, with rice grown in between mounds holding cassava, cowpea, maize, peanut, pumpkin, Lagenaria, pigeon pea, melon, and a selection of yam species. The prevalence of polycultures declined greatly in popularity at that time in more economically developed countries where it was deemed to yield less while requiring more labor. Polyculture farming has not disappeared entirely, and traditional polyculture systems continue to be an essential part of the food production system, especially in developing countries. Its principal advantages, according to Adamczewska-Sowińska and Sowiński 2020, are: However, the yield of each crop inside the polyculture is lower, not least because only part of the land area of the field is available to it. and artificial fertilizers into the environment.

Tillage, which removes essential microbes and nutrients from the soil, can be avoided in some forms of polyculture, especially permaculture. Land is used more productively.

Weed management

Both the density and the diversity of crops affect weed growth in polycultures. Having a greater density of plants reduces the available water, sunlight, and nutrient concentrations in the environment. Such a reduction is heightened with greater crop diversity as more potential resources are fully utilized. This level of competition makes polycultures particularly inhospitable to weeds. Because different plants are susceptible to different diseases, if a disease negatively impacts one crop, it will not necessarily spread to another and so the overall impact on yield is contained.

Annual

Intercropping

thumb|[[Companion planting of carrots and onions. The onion smell puts off carrot root fly, while the smell of carrots puts off onion fly.]]

When two or more crops are grown in complete spatial and temporal overlap with each other, the approach is described in agriculture as intercropping, and in horticulture as companion planting. Intercropping is particularly useful in plots with limited land availability.

Cover cropping

When a crop is grown alongside another plant that is not a crop, the combination is a form of cover cropping. If the non-crop plant is a weed, the combination is called a weedy culture.

Living mulches

thumb|A [[living mulch planted to reduce weed growth between rows of maize plants.]]

A living mulch is a polyculture involving a second crop, used mainly in horticulture. A main crop is grown to harvest; a second crop is sown beneath it to cover the soil, reducing erosion, and to form a green manure. Living mulches have been popular under orchard trees, and beneath perennial vegetables such as asparagus and rhubarb. It is considered suitable also for annual crops which grow for a long period before harvest and where the harvest is late in the year, such as aubergine, cabbage, celery, leek, maize, peppers, and tomato. Marigolds have a special place among weed-suppressing living mulches as they produce thiophenes which repel pests such as nematodes, and provide a second cash crop. A different polyculture system is used for coffee in Mexico, where the Coffea bushes are grown under leguminous trees in the genus Inga.

Rice, fish, and duck systems

thumb|Ducks with free access to [[rice paddies in Bali, Indonesia provide additional income and manure the fields, reducing the need for fertilizer.

Rice-duck farming is practised across tropical and subtropical Asia. A variant in Indonesia combines rice, fish, ducks and water fern for a resilient and productive permaculture system; the ducks eat the weeds that would otherwise limit rice growth, reducing labour and herbicides; the water fern fixes nitrogen; and the duck manure and fish manure reduce the need for fertilizer.

Integrated aquaculture

Integrated aquaculture is a form of aquaculture in which cultures of fish or shrimp are grown together with seaweed, shellfish, or micro-algae. Mono-species aquaculture poses problems for farmers and the environment. The harvesting of seaweed crops in mono-species aquaculture releases nitrates into the water and can lead to eutrophication. In seafood mono-species aquaculture, the greatest problem is the high cost of feed, more than half of which goes to waste, causing nitrogen release and eutrophication or algal blooms.