Physical quality of a podzolic soil following 19 years of irrigated minimum-till kikuyu-ryegrass pasture

Dairy production from planted pastures is one of the main farming systems in the southern Cape region of South Africa. Large tracts of natural rangeland were converted to planted pastures for increased fodder production. To sustain these agro-ecosystems, the physical condition of soil needs to be protected. The objective of this study was to assess the impact that conversion of natural rangeland to planted pastures had on the physical condition of a sandy soil after 19 years. An irrigated minimum-till kikuyu (Pennisetum clandestinum)-ryegrass (Lolium spp.) pasture soil was compared with a virgin fynbos soil. Physical indicators of soil quality were measured at 0-100 mm, 100-200 mm and 200-300 mm depth intervals. The planted pasture was highly productive, in comparison to the natural vegetation. Clay content was similar (P ≤ 0.01) between sites within the 0-100 mm and 100-200 mm depths and comparison of indicators between sites and interpretation thereof should be unbiased for those depths. Penetration resistance and to a lesser extent also bulk density of the planted pasture soil showed that conversion of virgin soil to minimum-till pastures adversely affected soil physical resistance by increased compaction. Aggregate stability (water stable aggregate percentage) was higher (P ≤ 0.05) in the 0-100 mm and 100-200 mm layers of virgin soil and the soil microstructure was adversely affected with a lower water stable aggregate percentage in planted pasture soil compared to that of the virgin soil. Infiltration rate did not differ between sites. In comparison to fynbos vegetation, planted pasture increased water holding capacity of the 0-100 mm layer by 5.0% (P ≤ 0.01), but did not affect (P > 0.01) water holding capacity in the 100-200 mm and 200-300 mm layers. Conversion of virgin soil to minimum-till kikuyu-ryegrass pasture degraded the dynamic physical condition of a podzolic soil over 19 years in terms of physical resistance and soil microstructure strength, but enhanced water holding capacity and maintained infiltration rate.