WEEDS and soil-borne pathogens pose a major threat to crops. Soil solarisation offers a cost-effective and non-chemical method of checking their growth. It is achieved by covering (mulching, tarping) the soil with transparent polyethylene during the hot season, thereby heating it and killing the pests. Soil solarisation is a recently developed approach for soil disinfestation, while the two other main approaches, soil steaming and fumigation, were deve- loped at the end of the 19th century (Indian Farming, Vol 46, No 2).
Soil solarisation evolved after some farmers in Jordan valley noticed the intensive heating of the polyethylene- mulched soil. Under this method, soil is mulched during the hottest months (rather than the coldest, as in conventional plasticulture which is aimed at protecting the crop). The resultant temperature increase proves lethal to soil- inhabiting pathogens, nematodes (creatures like roundworms, pinworms and hookworms) and weed seeds.
To derive maximum benefit out of solar heating, transparent polyethylene should be used since it transmits most of the solar radiation that heats the soil. Soil should be kept wet during mulching to increase the thermal sensitivity of resting structures and improve heat conduction. For eliminating pathogens living in the deeper levels of soil, the mulching period has to be extended sufficiently.
The solar heating method for disease control is essentially similar to that of artificial soil heating by steam or other means normally carried out at 60- 1OO'c. But there are some biological and technological differences. Soil solarisation does not require transportation of heat from its source to the field. It can be carried out directly in the open field. Unlike artificial heating, soil solarisation is carried out at relatively low temperatures so that its effects on living and non-living soil components are likely to be less drastic. In certain cases, soil steaming leads to phytotoxicity due to the release of manganese or other toxic products and a rapid soil reinfestation due to the creation of a biological vacuum. So far, these side effects have not been reported with solar heating, though such possibilities cannot be ruled out.
Soil solarisation also leads to effective weed control lasting in some cases for a whole year or even longer. There ar ,e diverse weed populations, including species that may differ in heat sensitivity. In general, most of the annual and pearennial weeds including Amaranthus, Anagallis, Chenopodium, Cynodon, Digitaria, Eleusine, Fumaria, Phalaris, and Xanthium, can be effectively controlled by solar heating.
Better results can be achieved when herbicides are combined with solar heating. Weed control due to solarisation is influenced by soil type, temperature, moisture content, size and depth of seeds or vegetative propagules (any structure, like a leaf bud, that is capable of giving rise to a new plant) in soil during treatment, besides film type and thickness.
However, solarisation has its limitations. The weather and locality of the field plays a crucial role during the treatment period of the soil. Besides, solarisation requires high soil moisture content and the effect is relatively low for deep-rooted crops and for pathogens inhabiting deeper soil layers.