Template:Chapters 6.7

6.7 - Solarization to kill parasitic weeds, pathogens and nematodes

Elin Hughes, University of Guelph, Canada

Suggested citation for this chapter.

Hughes,E(2022) Solarization to kill parasitic weeds, pathogens and nematodes. In Farmpedia, The Encyclopedia for Small Scale Farmers. Editor, M.N. Raizada, University of Guelph, Canada. http://www.farmpedia.org

Introduction

Weeds, pathogens, and nematodes have serious environmental and socioeconomic implications for smallholder farmers, particularly diminishing yields, as they compete with crops for sunlight, nutrients, water, and/or space (Chauhan, 2020). Controlling weeds is an important task undertaken by farmers, and progressively more methods of weed control are being introduced into modern agricultural practices. Traditionally, the most common approach of weed control by smallholder farmers in Africa involves physical labour – manual hoeing or pulling weeds by hand (Lee & Thierfelder, 2017 p. 6). Such traditional weed control methods are often a burden borne by the women and children, and if there is insufficient access to male labour, livestock, herbicides or mechanization, alternative cost-effective and feasible methods of weed control such as solarization could positively influence the livelihoods of female smallholder farmers (Lee & Thierfelder, 2017 p. 6). In this process, the heat of the sun is used to kill weed seeds as well as soil pests and pathogens while simultaneously increasing nutrient release (Stapleton, 2019).

The Practice of Solarization and its Benefits

The soil solarization process involves covering moist soil with a thin, ideally transparent polyethylene sheet or tarpaulin/silpaulin during the hot summer months prior to crop planting to raise soil temperatures (Patel et al. 2005). To achieve the most fruitful outcome: soil must be heated to between 40-55°C; a thin clear tarpaulin as opposed to a thick one should be used; and between 4-6 weeks of coverage are recommended (Merfield, 2019; Patel et al. 2005). Additionally, it is important to seal the edges of the tarpaulin with the soil to ensure the heat cannot escape. There are two primary forms of coverage, complete and strip coverage. Complete coverage involves covering the complete surface of land, whereas strip coverage involves covering strips or lines of crop fields (Elmore et al. n/d). Solar heating is effective in eliminating many types of perennial/annual parasitic weeds such as bermudagrass and johnsongrass, as long as they are not too deeply rooted in the soil, as the heat has minimal effect if weeds are rooted less than 12 inches (30 cm) below ground level (Stapleton, 2019). As well, solarization has minimal to no effect on some crops because tubers, tough seed shells, and rhizomes are resistant to the heat, making this an ideal method of weed control for crops such as cassava and potatoes (Stapleton, 2019). Additionally, solarization is effective in killing some nematodes – pests that feed on plants and soak nutrients from the soil; however, there are concerns about its effectiveness due to nematode mobility and their ability to submerge deeper into the soil to avoid the heat (Stapleton, 2019). Finally, solarization is highly effective in killing some harmful bacteria and fungi that cause plant disease such as various wilting, rotting, cankers, and scabbing symptoms that decrease yields (Stapleton, 2019). As shown in a video published by the Nigerian Organic Agricultural Network (NOAN)(see link below), solarized fields as compared to fields which are handpicked have noticeable advantages for both the farmers who maintain the land and the yields that grow from the soil.

Across the semi-arid and sub-tropic regions of Africa, parasitic striga weeds, also known as “witchweed” or “the killer weed”, are one of the most common pests endangering smallholder farmer crop yields (El-Dabaa et al. 2022 p. 54). Evidence supports solarization as beneficial in targeting striga weeds in Sub-Saharan Africa, namely in cereal crops such as sorghum, maize, rice, and pearl millet (El-Dabaa et al. 2022 p. 58). Once again, depending on the geographic location, soil type, crop type, and economic position of smallholder farmers, soil solarization could be an overall effective method for tackling the ever-growing concerns of striga infestations.

Under a system of conservation/no-till agriculture, mechanical disturbance of soils is kept to a minimum and emphasis is placed on alternative solutions to maintaining healthy farming land (Lee & Thierfelder, 2017 p. 1). However, within no-till agriculture, a major concern arises in terms of weed control, and a well-researched solution is that of solarization. Solarization as a potential alternative for controlling weeds comes with both benefits and consequences for farmers which are situational depending on factors such as geographic location, farm size, access to labour, and economic position. Solarization has been used both in small scale gardening as well as larger scale farms, however there are costs associated with this.

Critical Analysis: Problems and Solutions

For solarization to work effectively, land must be idle for 6-8 weeks in the peak of the summer growing season (Yaron et al. 1991 p. 179). Depending on the crops being grown, this loss of season may be unreasonable for some farmers if specific crops are best suited to the warm weather of the summer months (i.e., okra and amaranth in West Africa). In temperate climates, it is more difficult to pinpoint the ideal solarizing season, however in the subtropics and tropics, it is hot year-round allowing for more leniency in terms of when to solarize the soil. The top 6 inches (15 cm) of soil must reach between 43-52 degrees Celsius for effective results, and since high daily temperatures in the tropics range between 30-35 degrees Celsius, it is important for coverage to be applied for 6-8 weeks as already noted (Stapleton, 2019). Additionally, there is a labour requirement of between 10–20-man days/ha in order to prep, lay, and maintain the solarizing land over the 6–8-week period (Yaron et al. 1991 p. 179). To minimize soil disruption, this practice must be undertaken manually which requires increased labour as compared to alternative solutions such as the use of chemical herbicides or ecological approaches (e.g. cover crops, high density cropping, etc.).

Financially, there are other factors that must be considered by farmers who are interested in solarization. The cost of purchasing polyethylene tarpaulin alone may raise concerns for smallholder farmers. For reference, according to one supplier of polyethylene tarpaulin, one square meter of material will cost roughly 50 U.S. cents – meaning that in order to cover one hectare of land it will cost a farmer roughly $5000 USD for just the tarpaulin alone, not including shipping and labour costs (Alibaba.com, 2022). The tarpaulin is meant to last between 1-5 years due to their UV protection and waterproofing qualities, however it would be an investment requiring replacement every few years. Additionally, in order to extend the life of the material, the tarpaulin will need to be properly removed and stored during the off season in which they are not out on the field. For smallholder farmers in low-income countries, $5000 USD every 1-5 years for one hectare of coverage is not feasible. However, for small home gardens and patches of high value fruits and vegetables, this method of weed control could greatly benefit farmers in terms of benefits to women farmers including helping women grow more high value fruits and vegetables for commercial sales; the profits could then be used to purchase more tarpaulin to expand their gardens over time. With the labour saved on manual weed control, female smallholder farmers would be able to invest into more productive activities, and reduce the burden on children, creating more time for them to be able to go to school during peak weed times.

Solarization is admired as being a more sustainable and environmentally friendly method of nematode and weed control as compared to common alternatives such as chemical herbicides. However, a counter argument can be made based on the requirement to dispose large quantities of plastic material at the end of their life cycle (Russo et al. 2005). Traditional plastic film is often not disposed of in accordance with environmental guidelines, and therefore often contributes to damaging the land and environment in which it was intended to help (Russo et al. 2005 p. 718). As such, a potential solution being tested is the use of biodegradable plastic. The cost of biodegradable film is approximately 3 times higher than traditional film; however, long-term costs may be saved in labor since there would be less maintenance required annually (Russo et al. 2005 p. 718). For more information on Mater-Bi biodegradable film, see the additional links below.

One of the key components of solarization is its ability to kill unwanted weeds and nematode pests. Although solarization has been shown to be successful in doing this, there is another concern. Solar heating is harmful to some important beneficial soil microbes and microbial biodiversity which is vital in maintaining the structure of the soil (Scopa et al. 2009 p. 544-545). Therefore, there is a tradeoff to be made with this method of weed control.

Conclusion

It is vital to consider the various dimensions which might affect the overall success of solarization on small scale farms. For farmers in hot climates, utilizing the heat to promote healthier soil before growing certain crops could dramatically increase yields, and create long-term benefits for the sustainability of their fields. However, although there are benefits, it is also important to weigh the costs for individual farmers to ensure that this method will encourage long-term benefits to smallholder farmers and their land. It is recommended that the practice first be undertaken on a small parcel of land as a trial.

Practical Links to Get Started

How to Use Solarization to Prepare Areas for Planting

- Prepping the soil, setting the tarps, end of season steps.

https://www.alibaba.com/product-detail/Durable-agricultural-poly-greenhouse-cover-150_1600104741141.html?spm=a2700.galleryofferlist.normal_offer.d_image.2baa120fB0e07z

- Polyethylene Tarp Rolls: 50 cents/m^2 = approx. $5000/hectare

https://materbi.com/en/solutions/agriculture/mulching-film/

- Mater-Bi biodegradable mulching film https://extension.umn.edu/planting-and-growing-guides/solarization-occultation

University of Minnesota article about solarization and important factors for its success

Demonstration of two Weed Control Methods Using Solarization with Plastic Mulch and Manual Weeding. - Nigerian Organic Agriculture Network – video comparing solarization vs. Manual weed picking in garden yields

References

1. Chauhan, B. S. (2020). Grand challenges in weed management. Frontiers in Agronomy 1,3 Retrieved November 3, 2022, from https://www.frontiersin.org/articles/10.3389/fagro.2019.00003/full

2. Elmore, C. L., Stapleton, J. J., Bell, C. E., & Devay, J. E. (n.d.). Soil Solarization - A Non pesticidal method for controlling diseases, nematodes, and weeds. vric.ucdavis.edu. Retrieved November 7, 2022, from http://vric.ucdavis.edu/pdf/soil_solarization.pdf?iframeView=true

3. El-Dabaa, M., Abo-Elwafa, G., & Abd-El-Khair, H. (2021). Safe methods as alternative approaches to chemical herbicides for controlling parasitic weeds associated with nutritional crops: A Review. Egyptian Journal of Chemistry, 65(4), 53-65 .https://doi.org/10.21608/ejchem.2021.98930.4602

4. Lee, N., & Thierfelder, C. (2017). Weed control under conservation agriculture in dryland smallholder farming systems of Southern Africa. A Review. Agronomy for Sustainable Development, 37(5), 1-25. https://doi.org/10.1007/s13593-017-0453-7

5. Merfield, C. N. (2019). Integrated Weed Management in Organic Farming. In Organic Farming (edits Chandran, S., Unni, M.R. and Thomas, S), Chapter 5, p117–180. Woodhead Publishing. https://doi.org/10.1016/b978-0-12-813272-2.00005-7

6. Patel, R. H., Shroff, J., Dutta, S., & Meisheri, T. G. (2005). Weed dynamics as influenced by soil solarization - a review. Agricultural Reviews 26(4), 295–300.

7. Russo, G., Candura, A., & Scarascia-Mugnozza, G. (2005). Soil Solarization with biodegradable plastic film: Two years of experimental tests. Acta Horticulturae, 691, 717–724. https://doi.org/10.17660/actahortic.2005.691.88

8. Scopa, A., Candido, V., Dumontet, S., Pasquale, V., & Miccolis, V. (2009). Repeated solarization and long-term effects on soil microbiological parameters and agronomic traits. Crop Protection, 28(10), 818–824.

9. Stapleton, J. J., Wilen, C. A., & Molinar, R. H. (2018). Soil Solarization for Gardens and Landscapes. University of California Agriculture and Natural Resources. Retrieved December 1, 2022, from http://ipm.ucanr.edu/PMG/PESTNOTES/pn74145.html#:~:text=Solarizing%20Period&text=The%20cooler%20the%20soil%20temperatures,110%C2%BA%20to%20125%C2%B0F