Written by Julia Pridmore

Introduction

Increasing plant diversity has been an easy and effective way to improve the environment and crop health in both organic and conventional agriculture. In many natural thriving ecosystems, plant diversity is present, and is essential to plants and the other organisms. Learning from dynamic ecosystems is essential to improving organic agricultural practices as the resulting benefits promote processes that reduce the need for fertilizers and pesticides. The health and productivity of the primary crop depends not only on the cultivation practices but on its surroundings, the soil, the pollinators, and the pests that threaten it. This chapter outlines types of plant diversity practices and their benefits, overall benefits of plant diversity on the environment and the farmer, and discusses plant diversity in the organics field.

Types of Plant Diversity Practices

Intercropping

Intercropping is growing two or more species in close proximity, each having their own clearly defined rows or sections in the same field (Li et al. 2014). Species’ interactions may vary depending on the field’s intercropping design, and there may be limited or no species interaction (Connor et al. 2011) or there could be positive or negative interactions, expanded on below, between the species (Li et al. 2014). Types of intercropping design are row, mixed, strip, and relay. Row design involves planting crops in regular rows, strip design is when crops are grown in wider strips but still alternate throughout the field, mixed uses no row arrangement, and relay is when the second crop is planted after the first crop has matured (Mousavi and Eskandari 2011). Intercropping is a straightforward method of achieving the goal of increasing plant diversity in the organic agricultural system because the different crops are split into separate rows or sections, depending on the field design, which allows for easier seeding and harvesting.

Companion Cropping

Companion cropping is the practice of growing an additional crop alongside the main crop to provide it with benefits (Pickett et al. 2010). Also known as polyculture, where multiple types of crops are grown simultaneously, this type of cropping can protect against erosion, increase water retention, provide biological pest control, and improve soil conditions (Adamczewska-Sowińska and Sowiński 2020). The soil can profit in many ways when using companion cropping, such as by improving nutrient balance, providing soil temperature regulation, and increasing allelopathic benefits provided by plant-plant interactions (Adamczewska-Sowińska and Sowiński 2020; Pickett et al. 2010).

Cover Crops

Cover crops are unharvested crops planted in rotation with a cash crop in the off-season, most often in the fall or early spring, and are expanded upon in Chapter 3c on cover crops (Finney and Kaye 2017). One or more cover crops can be grown simultaneously and increase plant diversity in the organic agricultural system and provide benefits to the system that the crop plants may not. Many of these benefits pertain to soil and environmental maintenance and amelioration, and can reduce inputs required and the costs associated with them.

Complementarity

Considering complementarity is important in polyculture to ensure benefits are achieved without negative side effects like competition. These interactions occur both above ground between the canopies of the crops and below ground in the root system. Picasso et al. (2008) describes complementarity as a series of “positive interactions among species” in diverse ecosystems that augment productivity. Complementarity is the practice of creating an ecosystem diverse enough that the production of plants in the polyculture systems have a greater yield than when the same crops are grown in monoculture (Picasso et al. 2008). Complementation can be achieved in three ways: making use of the differences in spaces (root types), differences in growing times, and differences in sensitivities (e.g., drought, temperatures, disease, moisture, etc.) (Connor et al. 2011). Nutrient requirements are another factor to consider when choosing crops that will complement each other. In polycultures, choosing crops that will complement each other requires planning by the farmer.

Additive versus Substitutive design

In addition to planning, experimentation by the farmer can also be useful for achieving complementarity and in finding the optimal cropping system for a specific organic agricultural system. Additive and substitutive designs are often used when experimenting with different types of cropping systems. In a meta-analysis on the pros and cons of polyculture, Iverson et al. (2014) looked at both additive and substitutive design. In additive designs, the primary crop’s density is held constant and secondary crop species are used to increase crop density (Iverson et al. 2014). In substitutive designs, the number of plants in the polyculture is the same as in each mono-culture, where the change in density can confound the effects of mixed species (Iverson et al. 2014). These crop designs can overlap with the practices listed above, the difference being that farmers adapt to the ecosystem by trying different designs. Considering experimental design is not only for researchers but for individuals who want to improve their knowledge and immediate surroundings.

Benefits of Plant Diversity

Species biodiversity aids in natural ecosystem regulation and nutrient cycling. These results can also be useful in agricultural settings to gain benefits in terms of assisting with nutrient fixing, increasing yields, and improving root and canopy diversification as expanded upon in the next section.

Nutrient Fixing

Polyculture can be planned to include species that chemically mobilize commonly unavailable nutrients such as phosphorus (P), nitrogen (N), iron (Fe), zinc (Zn) and manganese (Mn) (Li et al. 2014; Kaye and Quemada 2017). Since these nutrients can be present but unavailable to crop plants, having other species that can take up nutrients into their biomass will make them more available to future crops as the biomass decomposes. This has the secondary benefit of decreasing fertilizer costs for farmers. For example, intercropping P-mobilizing species, like Fava beans (Vicia faba), and non-P-mobilizing species, like maize (Zea mays),  on soils with high concentrations of unavailable P can improve overall soil health and crop health while also financially benefiting the farmer (Li et al. 2014). Additionally, planting a legume crop followed by a non-legume crop is a common technique to improve nitrogen fixation (Adamczewska-Sowińska and Sowiński 2020). Improving nutrient fixation by increasing plant diversity is an easy way to improve crop production,  the environment, and a farmer’s financial stability.

Root and Canopy diversification can allow the plant community found on the field to use the surrounding resources more completely (Picasso et al. 2008). In choosing various root depths for instance,  species X is a shallow-rooted species, species Y is a deep-rooted species and species Z has both shallow and deep roots.  When X and Y are planted in polyculture, the entire soil profile can be better utilized, whereas species Z can take advantage of the soil profile in a monoculture (Conner et al. 2011).

Likewise, in canopy coverage, a low growing species (a) and a tall growing species (b), when mixed in (c) improve competition with weeds since both are competing for limited sunlight among plants (Conner et al. 2011) that can help reduce competition and encourage resource partitioning (Bybee-Finley and Ryan 2018). Complementation is a use of space by, selecting different root depths and canopy coverages to maximize the available space without overcrowding (Conner et al. 2011). Maximizing space, both above and below  ground, is a smart way to take advantage of all the naturally available resources for the crops to utilize such as existing nutrients in the soil, soil water, and allelopathic properties of different crops. Root diversification is when a deep taproot species is planted and uses hydraulic lift to supply water to the other plants located in the topsoil (Bybee-Finley and Ryan 2018).

Polyculture can also be planned to maximize canopy coverage as a weed management practice. Diverse crop canopies that have been planned to utilize a variety of nutrients also increase competition for weeds and decreasing available weed niches (Colbach et al. 2021). Overall, root and canopy diversification are very important to nutrient balance, water access, and weed management within polyculture crops. However, it is important to note that although the benefits of this type of diversification are plentiful it does require more knowledge and planning to ensure all goes efficiently and does not result in negative effects on the crops.

Yield Increases

Ecological studies in non-cultivated systems have shown that plant diversity has been directly related to increased biomass production (Picasso et al. 2008). These yields were also found to be more stable over longer periods when compared to monocultures. Increased yield, or overyielding, is also often a result of the interactions resulting from complementarity discussed above. A crop over yields, “when its biomass production is greater than that of the average monoculture of the species contained in the mixture” (Schmid et al. 2008 ). One mechanism of the complementarity that contributes to overyielding is that one crop can increase the availability of a limiting nutrient to itself as well as the crop it is intercropped with (Li et al. 2014). For example, in a 4 year long field intercropping experiment, fava beans, which are able to mobilize phosphorous and provide it to neighbouring plants, were intercropped with maize (Li et al. 2014). As a result, maize over yielded by 46% and fava beans by 26% compared to monoculture (Li et al. 2014). Iverson et al. (2014) also found overyielding when using a substitutive design. The primary crop yield increased by 40% in polyculture versus monoculture (Iverson et al. 2014).

Conclusion

In summary, plant diversity is a useful and essential cultivation technique that can be used in both organic and conventional agriculture. It is essential in organic agriculture since the resulting benefits and stability promote natural processes that reduce the incentive to use synthetic fertilizers and pesticides. Polyculture requires increased planning to prevent competition, but provides farmers the opportunity to adapt to their individual environment to create a more robust and resilient agroecosystem. A combination of the practices outlined above can be used to optimize interactions between plants to reap benefits of optimization of resource-use and pest management.

References

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