Soil is viewed as a three-phase system consisting of solids, liquid and gas. This perspective does not seem to account for all the inherent variability of the soil, in particular, for residual soils and from agricultural perspective. It has been acknowledged that natural soils are heterogeneous consisting of biotic and abiotic factors. These factors are central to terrestrial ecosystem dynamics. In geotechnical engineering, the abiotic factors are accounted for mathematical formulations for soil properties derived from the three-phase conceptual model. However, because the biotic factors are not considered in the three-phase system, its contribution to soil properties are not mathematically represented. Consequently, the associated physics are not adequate to describe all physics needed to adequately model the natural heterogeneity in agricultural soils. This paper idealised soil as four-phase system (i.e. consisting of solid, soil biota, liquid and gas) allowing biotic factors to be adequately accounted for, in particular, for agro-geotechniques applications. It intends to extend the existing mathematical formulations for the basic properties of the soil derived from the three-phase model to include those of the biotic phase, thereby providing framework for modelling agricultural soils. This framework can be applied to develop hydro-soil-biota coupling model to analyse biological, chemical and mechanical stability of soils to address global issues facing agricultural lands.