Factors affecting anaerobiosis in soil have been studied by means of computer simulation applied to a model soil composed of spherical aggregates in a hexagonal packing. This schematized geometry implies that two different types of pores are distinguished: intra- and interaggregate pores. The moisture characteristics for the interaggregate water and the non-water-covered area of the aggregates, the so-called air-exposed area, were calculated in general terms. The results give a tool to derive curves for a certain aggregate radius. The interaggregate water content is used to calculate the diffusion properties of the model soil down the profile. The diffusion from interaggregate pores into the aggregates is taken proportional to the air-exposed area of the aggregates.
Results of the model are given for an aggregate radius of 0.5 centimeters. They indicate anaerobiosis to occur below a depth of 10 centimeters and in the profile as a whole, before and during rainfall, respectively. The anaerobic soil volume rapidly responds to a shower of rain. However, it is not affected as seriously by the water regime as it is by the magnitude and distribution of the respiratory activity or the diffusion coefficient in the aggregates. The model soil was also used to derive, for a particular case, a soil anaerobiosis characteristic relating soil moisture suction to the anaerobic soil fraction. Some indications are given for further experimental research.