Most compaction processes in agricultural engineering are always associated with increasing elastic moduli showing definitely non-linear character. At the same time, the non-linear viscoelasticity is still less developed and hardly ever used for describing compaction processes. A new non-linear rheological approach is suggested using varying E-moduli and multiple relaxation times. The E-moduli are expressed as a function of strain and the multiple relaxation times are also dependent on the strain or E-moduli. In order to verify the proposed method experimental measurements on selected agricultural materials (silage, soil, saw dust etc.) were, undertaken. The measurements and calculation have shown the proposed method is suitable to describe compaction processes in a wide range of pressure as a function of loading velocity.