Porosity and pore-size distribution in cultivated ustolls and usterts

Soil management systems affect soil porosity and pore sizes, changing soil hydraulic properties by loosening or by compacting different soil layers. Changes in porosity and pore-size distribution following cultivation were studied in six Ustolls and two Usterts of the prairie in the Upper Missouri River Basin. Soil pores were morphologically described. Water infiltration was measured at 0.03- and 0.06-m tensions. Soil bulk density and moisture retention at 1-m tension were determined in undisturbed and in remolded soil cores. In Ustolls, cultivation decreased soil porosity and pore sizes. Steady-state water infiltration rates were higher in grasslands than in cultivated soils. In no-till and till systems, both very fine macroporosity and microporosity were reduced when compared with grasslands. No-tillage relative to tillage increased soil porosity between the 0.05- and 0.30-m depth. More very fine tubular pores were present in no-till than in tilled Ustolls, indicating increased biological activity in pore formation. In Usterts, total pore space, quantity, shape, and size of macropores, water infiltration under tension, and moisture retention at 1-m tension did not show significant changes related to different management systems.