Although compost is widely used as an organic soil amendment or conditioner, little is known of how it affects the characteristics or interactions among soil constituents. To address this, mixture theory was used to describe the mass–volume–density–porosity attributes and interactions among bulk soil, the mineral constituent, and the organic matter constituent of a sandy loam soil in a no-till corn field that had received one-time additions of yard waste compost at rates of 0 (control), 64, 154, and 380 dry t ha^-1. Bulk density (BD, 0–10 cm depth) decreased consistently and near-linearly with increasing soil organic matter (SOM) mass fraction (F_OM) for all six growing seasons (2012–2017) after compost addition. Fitting mixture theory expressions to BD vs. F_OM data and to soil particle density vs. F_OM data for 2013–2017 yielded constant mineral and SOM self-packing densities of D_M = 1.673 Mg m^-3 and D_O = 0.335 Mg m^-3, respectively, and constant mineral and SOM particle densities of ρ_M = 2.760 Mg m^-3 and ρ_O = 1.409 Mg m^-3, respectively. On a self-packing basis, soil mineral and SOM domain porosities were constants at n_M = 0.39 and n_O = 0.76, respectively. On a bulk soil volume basis, soil mineral and SOM porosities and volume ratios were linear functions of F_OM. The porosity and volume characteristics of the SOM domain differed substantially from those of bulk soil and the mineral domain, and may therefore control the agri-environmental performance of soil, given that organic matter influences soil functioning more than mineral matter.