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The effects on soils and vegetation of 13 years of fertilising and grazing on previously unfertilised South Island high country are described. Vegetation cover, and the values of organic carbon, nitrogen and pH in topsoils at 38 sites sampled on a high country farm in 1978 and 1979, before any topdressing or oversowing had been begun, were compared to values at the same sites in 1992, after soils had been progressively fertilised with a total of 1100 kglha of 28% sulphur-superphosphate and oversown with legumes. In 1992 the cover of native species such as silver tussock (Poa cita), narrow-leaved snow tussock (Chionochloa rigida) and scabweed (Raoulia australis) and the area of bare ground had declined relative to cover of these species in 1978179, whereas cover of introduced pasture species tolerant of grazing had increased. Mouse-ear hawkweed (Hieraciurnpilosella) was recorded at only 4% of sites in 1978179 but was present at half the sites in 1992. Organic carbon in topsoils rose from a mean value of 3.3% in 1978179 to 5.5% in 1992. There was insignificant change of total nitrogen values. The pH of topsoils (0-7.5 cm) declined at 36 out of the 38 sites. The average pH decline was 0.41 units (from 5.81 in 1978179 to 5.40 in 1992), and the largest pH decline was 0.7 pH units. Results from an additional seven sites outside the main survey area suggest that the pH decline occurs on upper midslopes and near-ridge sites (about 10% of the land area) rather than on lower midslopes and toeslopes. If topdressing and the associated pH decline continue, then by the year 2005, pH will be close to 5.0 in topsoils, and lime will be needed to maintain legume-based pasture production on the affected soils. Approximately 1 tonne of lime per hectare every four years is estimated to be required to offset the pH decline. The present (1993) cost of applying lime is about $5.50 per stock unit per year and on present returns from livestock and wool, liming would be uneconomic. |
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