Modifications induced in soil porosity and in stability of soil aggregates were studied for 2 years on an Italian sandy loam soil. Aerobic and anaerobic sludges and their composted mixtures with the organic fraction of urban refuse were used and compared with manure. Addition rates were equivalent to 50 and 150 metric tons/ha of manure on the organic carbon basis. A control plot was also present. Porosity and pore size distribution were measured on thin sections prepared from undisturbed soil samples by using electro‐optical image‐analysis equipment. The stability of soil aggregates was determined by a wet‐sieving method.All organic materials increased the total porosity significantly at all sampling times. Differences between the two application rates were generally not significant. The improvement of total porosity caused by sludges and composts was comparable to that of manure. Modifications of pore size distribution were also observed. Stability of soil aggregates increased slightly in treated samples. The best stabilizing effect was shown by the anaerobic sludge.

Porosity, pore size distribution and the orientation pattern of pores were measured on thin sections prepared from undisturbed soil samples by means of electro-optical image-analysis. Total porosity was significantly higher at all sampling times in conventionally tilled plots, but the proportion ofpores ranging from 30 to 500pm, which are considered the most important both in soil-water-plant relationships and in maintaining a good soil structure, was higher in no-tilled plots. Modifications of pore orientation pattern were also observed. Micromorphological observations revealed some differences between the two series of soil samples; the formation of surface soil crusts was strongly reduced in no-tilled plots.

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