NOVELTY - Biodegrading sludge containing hydrocarbons converts the sludge into a dispersion through chemical reaction powder to be mixed with a structure material as a layer in a compost heap.    USE - The method is for decontamination of soil which has been fouled with oil and other hydrocarbons.    ADVANTAGE - The technique biodegrades the contaminated soil in a short time and in a trouble-free operation.    DETAILED DESCRIPTION - To biodegrade hydrocarbons in sludge, quicklime and water are added together with heat to give a finely dispersed and homogenous powder containing hydrocarbons and calcium hydroxide as a dispersion through chemical reaction (DCR) powder. The powder, together with a structure material with a given water content, is prepared into a mixture which is permeable to gas, and applied to a compost heap (2) in a layer (4) over a layer (3) of biodegradable and rotting compost. A gas containing carbonic acid is passed through the layers of the mixture, and the calcium hydroxide is converted to calcium carbonate with the pH value brought down to a physiological range. The mixture is treated with microorganisms to give a biological action to the hydrocarbons to degrade them under required limits.    The gas with carbonic acid is taken from an incinerator or internal combustion motor or composting matter, and delivered to the spread layers. The layers of the mixture are with other layers of composting matter, for the gas with a carbonic acid content to rise through the stack through the layers of the mixture. The structure material is compost, or other suitable organic or inorganic material, and especially waste and rubbish materials. The mixture, cleaned of hydrocarbons, can be used as the structure material. The structure material has a water content of 10-35%. The gas with a carbonic acid content reduces the pH value to 8.0-9.5.    DESCRIPTION OF DRAWING(S) - The drawing shows a compost heap for neutralizing and biodegrading.    compost heap (2)    compost layer (3)    layer of DCR powder and structure material (4)