This article gives a target temp and time to thoroughly cook.
Too high a temp destroys soil structure
Low pressure steam sterilization
This has much more application for smaller growers, professional and amateur gardeners.
Here shredded loam was evenly spread in a 25-30cm (10-12in) layer on perforated steel sheeting over a shallow tray of water kept boiling by a coal fire, the steam passing slowly through the soil, being trapped by a tarpaulin or sack covering (now PVC). It took several hours for the soil at the top of the layer to reach the necessary 82-88°C (180-190°F) and careful checking was necessary to ensure that the whole batch of soil was evenly sterilized. Various models were available, most of which are no longer manufactured; low pressure sterilizers are now usually improvised.
Another form of steam sterilization is to drench shallow layers of soil on a clean surface with boiling water, covering with clean sacks or PVC sheets to retain the heat. On a still smaller scale suspending small sacks of soil in the steam given off by a water boiler can be remarkably successful. The efficiency of all these methods depends much upon the care which is exercised in ensuring even distribution of temperature
