Soil degradation - II: Effect of trash and inorganic fertiliser application on soil strength

A long term trial to study the sustainability of a vertisol under continuou s sugarcane monocropping was commenced in 1939 at the Experiment Station of the South African Sugar Association. The design of the trial consists of f our replications of two main plots, each split into four sub-plots. The tre atments comprise various combinations of trashed, burnt, fertilised and non -fertilised cane. The burnt treatment was further subdivided, with tops eit her spread or raked and burnt after harvest. Composite soil samples at dept h 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 penetrom eter resistance, soil water content, soil bulk density, dispersion index, l inear shrinkage index, modulus of rupture and aggregate stability. Soil che mical analysis indicated that the trashed treatments had a slightly acidify ing effect when compared with the burnt treatments, while the fertilised tr eatments had a highly significant acidifying effect on the soil compared wi th the unfertilised treatments. The fertilised treatments had a significant ly larger soil aggregate stability and in situ measured soil strength compa red with the unfertilised treatments. However, the strength of samples remo ulded in the laboratory was larger for the unfertilised compared with that of the fertilised treatments.