With the increase in consumer awareness and subsequent demand for cultivated mushrooms, there now exists a need for alternative, cost-effective strategies of mushroom cultivation. The global mushroom industry is forecast to grow from a value of about $35 billion USD in 2015 to nearly $60 billion USD in 2021, thus clarifying the need for a diverse range of production styles in order to develop the industry in a sustainable manner (Research and Markets 2018). Alternatives to more traditional high-volume production techniques include the inoculation of logs, cultivation in forest understories, and use of managed forests, as well as field-grown mushrooms.
Field-grown mushrooms offer an effective use of space, allowing for the production of mushrooms in agricultural fields, alongside crops, or between cropping cycles. Offering additional income to farmers, this practice has become popular with rural development programs (Zhang et al. 2014a, b; Brum and Brum 2017). Additionally, field cultivation allows for improved soil systems in agricultural fields due the increased rates of nutrient cycling and provision of organic matter into the soils (Phan and Sabaratnam 2012; Zhang et al. 2012).
In this section, we outline some of the basic principles and considerations for field cultivation while listing some of the key species that can be successfully cultivated in a field environment. We will limit our discussion to the cultivation of mushrooms in agricultural fields and soils, not the broader practices of outdoor cultivation using bags or inoculated logs.
There are a range of mushrooms that can be cultivated outdoors, and appropriate species can be selected according to local growing conditions. For example, Coprinus comatus or Stropharia rugosoannulata can be grown during the autumn and winter months when temperatures are milder, and Volvariella volvacea should be grown during the warmer summer months. Consideration for soil pH, texture and health is also critical. The pH and texture will partially determine which species of mushroom will be grown, and also the degree of irrigation required to maintain desired soil water levels. Soil health is crucial to the final product, as many species of fungi can hyperaccumulate heavy metals (Cocchi et al. 2006; Turkekul et al. 2004).
After the selection of an appropriate growing area, a few basic steps are required for the preparation of the field and subsequent cultivation. After light tilling of the field, compost is applied, followed by the application of the fungal material (spawn or colonized bags), this is then covered in compost and a final layer of soil over that (a mulch of straw or shade cloth could further protect against desiccation and drying out).
There are a wide range of applications for the field cultivation of mushrooms, but most relate to the intensification of land use systems, generation of additional sources of income, and appeal to low income groups that cannot afford the costs of establishing grow houses. This method of production has become popular in many parts of Asia where climates are suitable and smallholder farmers need alternative sources of income. For example, in Cambodia, farmers have adopted the practice of spreading spawn produced from Lentinus or Volvariella species over rice straw waste in the paddies (with supplemented watering). This provides an additional harvest of mushrooms from the paddies during the off season. Another program run in Honghe County, Yunnan Province, China, assists local farmers with the cultivation of Stropharia rugosoannulata in their fields; this is a seasonal mushroom crop produced mostly in autumn and spring when the temperatures aresuitable.
The nature of growing mushrooms outdoors means that the mushroom crop is subject to ambient environmental conditions, with no climate controls in place, for example, temperature and humidity fluctuate with no real mechanisms in place to control these parameters. In addition to climatic variability, outdoor cultivation exposes the mushrooms to non-sterile conditions, allowing easy access for secondary infection by slime molds, predatory fungi, and insects.
Field cultivation of mushrooms is applicable to a wide range of habitats and conditions, depending on the desired species to be grown. By offering novel and exciting alternatives to plant-based agriculture, and providing additional income to farmers, field cultivation of mushrooms is becoming popular in programs aimed at improving the livelihoods of smallholder farmers and for incorporation into projects looking to diversify the agricultural output of a unit of land. However, growing mushrooms outdoors is not without challenges and requires careful species selection and training of practitioners before any such programs can commence.