SOlL DEGRADATION - II: EFFECT OF TRASH AND INORGANIC FERTILISER APPLICATION ON SOlL STRENGTH

reported resistance values of 2,8 to 3,2 MPa for tillage pans in A long term trial to study the sustainability of a vertisol under continuous spgarcane monocropping was commenced in 1939 at the Experiment Station of the South African Sugar Association. The design of the trial consists of four replications of two main plots, each split into four sub-plots. The treatments comprise various combinations of trashed, burnt, fertilised and non-fertilised cane. The burnt treatment was further subdivided, with tops either spread or raked and burnt after harvest. Composite soil samples at depth intervals of 0-50, 50-100 and 100-200 mm were collected in 1997 from each plot and chemically analysed for pH, exchangeable P, Al, Na, K, Ca and Mg, soil organic matter and CEC. Soil physical analyses included cone penetrometer resistance, soil water content, soil bulk density, dispersion index, linear shrinkage index, modulus of rupture and aggregate stability. Soil chemical analysis indicated that the trashed treatments had a slightly acidifying effect when compared with the burnt treatments, while the fertilised treatments had a highly significant acidifying effect on the soil compared with the unfertilised treatments. The fertilised treatments had a significantly larger soil aggregate stability and in situ measured soil strength compared with the unfertilised treatments. However, the strength of samples remoulded in the laboratory was larger for the unfertilised compared with that of the fertilised treatments.