Soil Solarization – a novel technique of weed management
The impact of pesticide use on the environment is now well documented, and a more wide spread adoption of integrated weed management strategies and tactics is recommended in sustainable agriculture systems. Soil solarization is a novel technique of controlling soil borne pests including weeds. This hydrothermal process occurs in moist soil which is covered by a transparent plastic film for 4-6 weeks and exposed to sunlight during the warm summer months. The practice was first reported from Germany in 1888 and was first used commercially by USA in 1897. Soil propagules, as weed seeds, offer a wide range of tolerance to high temperatures, conditioning the long term success of weed control by solarization. It seems probably that not only harmful organisms, but also beneficial organisms are killed by using soil fumigants for soil disinfestations.
Principles of solarization
Solar heating involves the use of heat as lethal agent for tarps for capturing solar energy by means of transparent polythene soil mulches. The thin polythene mulch allows the maximum transmission of solar radiation through moist soil and reduces moisture loss from soil through evaporation. Increase in soil temperature was achieved through improved heat conduction within the soil owing to its higher moisture levels. The principle behind the soil solarization is enhancing the diurnal heating and cooling cycle of the soil.
Solar heating method
The soil solarization method for weed control is similar in principle to that of soil steaming.
Soil preparation: The area to be solarized should be leveled and free from large clods of soil or plant debris.
Soil moisture: Soil moisture constitutes a critical element in the success of soil solarization. Appropriate soil moisture (exceeding 60-70 per cent of field capacity) increases sensitivity of target weeds and organisms to heat and improves soil heat conductivity.
The plastic sheet: Efficacy of plastic cover tarping is affected by plastic type, transmittancy, width and color. Low density transparent plastic mulch is considered as ideal because it transmits maximum solar radiation and reduces the escape of heat from the soil.
Soil covering: The entire field is covered with plastic film and the edges are buried in the soil to the depth of about 20 cm.
Elements influencing effectiveness of solarization
Effectiveness of solarization is influenced by certain characteristics of three major elements, namely, the solar energy, the plastic cover and soil characteristics.
Increased effectiveness of soil solarization
The following steps should be taken to increased effectiveness of soil solarization:
- Solarization in closed green houses.
- Use of double layer plastic cover- The air layer between the two sheets provides insulation against escape of heat, moisture, and volatile gases from the soil. This treatment leads to an increase of 3-100 C over the single layer cover.
- Addition of soil amendment with organic matter prior to plastic tarping.
Benefits and limitations of soil solarization
The primary advantage of this method is non-chemical management of weeds and soil borne pathogens. In addition to the lethal effects of radiant heat energy on weed seeds and other pests in the soil, solarization may increase crop yields through biochemical and biophysical changes which occur in the rhizospheres, especially in relation to nutrient availability. The method is simple, safe, effective, and not too costly, leaves no toxic residues, and does not harm the environment.
Limitations of the methods are:
- Certain unfavorable environmental conditions.
- Non cropping for a relatively long period, especially in areas of summer cropping.
- Shortage of supplementary irrigation water during solarization.
- Survival of weed seeds deeper in the soil.
- Pollution of plastic residues resulting from the treatment.
- Lack of adequate machinery for large scale soil mulching
- Dinesh Sah and Debashish Sen