The objective of the paper was to search for a function which describes relations of the bulk intensity of the soil skeleton to its moisture based on approximation of the Proctor points. Knowledge on this relation enables prediction of the influence of agricultural vehicles wheels on soil. Searching was limited to four polynomials – from the second to the fifth degree. Material for research originated from the subsoil of five soil profiles. The following were determined in material: grain size distribution, density of a solid phase, humus content, soil reaction and limits of elasticity and soil liquidity. Concentration of samples with the Proctor method was carried out according to PN-88/B-04481. Values of the maximum density and optimal moisture were calculated in the place, where the obtained polynomials describing experimental data accepted the maximum value. It was found out that for determination of the value of optimal moisture and the maximum density, polynomial of the fourth degree is the most suitable.