Soil Solarization

What is Soil Solarization?

‘Soil Solarization’ means heating the soil with the help of sunlight or solar rays. Soil, being a dynamic medium, not only holds and nourishes plants but is also home to a large number of microbes, insects, and animals. These include both beneficial and harmful organisms with respect to plants. The sole objective of soil solarization is to sterilize the soil from these harmful organisms and make it conducive for the growth and development of plants.

How to achieve?

In general, farmers carry out soil solarization during the summer season when temperatures are high. The success of solarization depends mainly on 3 things, i.e. Soil moisture, Soil temperature and exposure time.

• Soil Moisture- It is a critical variable in soil solarization because it makes organisms more sensitive to heat and also transfers heat to the living organisms in soil thus killing them. Moisture helps in increasing the thermal conductivity of soil. The effect of solarization is more pronounced in depths were the moisture reaches than were it doesn’t.

• Soil Temperature- The higher the temperature in soil, more will be the solarization effect. Higher temperatures kills many harmful microbes and pathogens, which are other wise detrimental to plants. A temperature of above 38⁰C will help in eliminating many harmful organisms which includes microbes, insect pests, nematodes and weed/weed seeds.

• Exposure time: The longer the soil is heated, the better and deeper the control of all soil pests and weeds. A period of 4-6 weeks is effective in killing soil-borne pathogens including weeds and nematodes, whereas a period of 6-8 weeks with temperatures above 38⁰C ensures good pest control.

Technically, one may use transparent PE sheets of 0.038-0.050mm thickness and lay them over the soil for effective solarization. Sheets of this thickness would be resistant to wear and tear due to various environmental factors. Colored plastic sheets can be used but may not be as effective as clear or transparent ones. Prior to laying of sheets, the soil must the irrigated. Irrigation increase the temperature of the soil. Once done, it is not irrigated again during solarization. This sheet then can be removed after 4 to 8 weeks.

Most often farmers achieve soil solarization in open air conditions (as shown in Figure 2), but it may not be as effective as with PE sheets. Here, the farmer ploughs his field soon after the harvest of rabi crops using tractor and leave the field for 4-8 weeks.

Effect of Soil Solarization on:

Pests- Soils serve as a place for insects to thrive. Some insects like fruit flies and pumpkin beetles form pupae in the soil and emerge again during favorable conditions. Some insects also lay their eggs in the soil. Solarization helps in exposing and destroying these stages. Some species of birds make visits to feed on larvae/pupae of insects.

Nematodes are worm like living organisms which are not visible to the naked eye. These are very destructive to crops. Solarization is an excellent control of Root-knot nematode, Root lesion and dagger nematodes. The control is greatest in the upper 12 inches (30cm) of soil. Nematodes that habitat deeper into the soil may survive solarization and damage deep rooted plants.

Diseases- Soil solarization controls populations of many important soil borne fungal and bacterial plant pathogens like certain fusarium spp, phytophthora spp, verticillium dahlia, pythium, agrobacterium tumifaciens and so on. These causes wilting, rotting and gall formation in plants or plant parts. Along with destroying these harmful microbes, soil solarization also kills some beneficial microbes like rhizobium (nitrogen-fixing bacteria), bacillus spp, etc. But adding the inoculum to the soil after soil solarization can revive these microbes again.

Weeds-Solarization kills annual and perennial weeds. Few weeds are sensitive to soil solarization, others are moderately resistant and require optimum conditions for control.

Soil nutrients-Solarization brings changes to physical and chemical features of soil. It fastens up the breakdown of organic material in soil, resulting in the release of nutrients like Nitrogen, Calcium, Magnesium, potassium, Fulvic acid and making them available to plants.

Do note that changes in chemical and physical properties occur when solarization is carried out under PE sheets.

Points to be noted:

• Purchasing of PE sheets can be costlier, and small farmers may find it difficult, henceforth they go for open solarization
• This is not a one step, solution. It is a part of an IPM. Pest/disease causing organisms can start to araise after few months.
• Solarization and mulching both are different. Different types of PE sheets are used in each case. Reflecting type PE sheets are used in mulching, while transparent and transmitting sheets are used in soil solarization.

References:

1. https://vric.ucdavis.edu/
2. https://edis.ifas.ufl.edu/
3. https://extension.okstate.edu/
4. Personal experiences