If the soil is infested with many nematodes, soil borne diseases or weeds, these pests can be at least partly removed by using the method of solarization. It is a method that utilizes the heat of the sun to sterilize the soil.
First the soil has to be irrigated to make sure that it is well absorbed with water. Then, the soil is covered with a piece of transparent plastic. The sides of the plastic should be well fixed to the ground by placing some stones or soil on it. Now, depending on the weather, the soil should remain covered for at least 3 weeks, but preferably even 6 to 8 weeks.
The method works best in a period with a lot of sunshine (dry season). The sun rays cause a strong increase of the soil temperature, which will kill large numbers of nematodes, pathogens and seeds of weeds.
Soil solarization is often not practical in big fields. However, it can be a very useful method in small plots that are going to be used for seedbeds or nurseries.
Another method of soil solarization is by packing the soil in plastic bags (mixed with some cow dung and water) and placing these in the full sun for a couple of weeks. This soil can then be used to prepare a nursery seedbed or to fill small bags for growing seedlings.