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The accumulation of organic matter in volcanic soils may modify their properties and characteristics. Soil complexity makes difficult to assign the relative importance of organic matter in determining soil behavior. If changes in soil properties can be associated with the soil organic carbon (OC) content, a better understanding of the organic matter role may be achieved. A total of 20 samples of uncultivated volcanic soils, located in Central-Southern Chile, classified as Andisols and Ultisols were studied. Properties such as active/free iron oxide ratio (activity ratio), isoelectric point (IEP), cation ORDER REPRINTS exchange capacity (CEC), and potassium-calcium (K-Ca) cation exchange equilibrium were studied in relation to the soil OC content. The activity ratio increases when OC increases showing the inhibitory effect of organic matter in iron oxide crystallization. The IEP increases when OC decreases, this relationship being useful to follow the soil organic matter destruction in sample preparation. The ÁCEC (CEC pH ¼ 8.2 À CEC soil pH ) is directly related with OC, showing that an important fraction of the negative surface charge is the result of ionization of organic matter active groups when pH increases over field value. Selectivity for K over Ca was observed, but decreases when OC content increases and a Ca selectivity may be expected in soils whose OC content is higher than 12.6%.

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Self-heating in compost piles results in unsuitable doors, smoke production and fires, having strong negative environmental impacts. Field, laboratory and numerical studies were accomplished trying to reproduce and understand the conditions where selfheating and combustion may take place. Inside a compost pile, built from solid obtained after municipal waste water treatment, oxygen, methane, carbon monoxide and carbon dioxide concentrations and temperature change with time and in depth. Electrical conductivity and pH showed only sligth changes. In field piles temperature increased with time until get a maximum about 90°C. While no spontaneous combustion was observed after 6 months in field experiments, in laboratory studies carried out in a closed bottom cylinder, self-ignition was observed and a maximun temperature of about 400 °C was reached.Numerical simulations describe the field piles internal heat generation, concluding that a minimum of 1.8 m height is required to reach a self-ignition condition.

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