Sustainability of New Zealand high‐country pastures under contrasting development inputs. 2. Soil inorganic components

Abstract

Analyses of the <2 mm soil fractions for S, P, pH, Al, and exchangeable cations were made to 40 cm depth 10 years after commencement of two grazed multiple pasture species mixture trials on a Tekapo/Pukaki high country soil. One trial was 30 combinations of 5 superphosphate rates (0-500 kg ha -1 yr -1 ) × 3 stocking rates × 2 stocking methods, and the other used 27 combinations of P and S fertiliser (0-100 kg ha -1 yr -1 ). Olsen and inorganic P of topsoil increased with increasing P fertiliser rate, and was independent of S fertiliser rate. High levels of P (800-1200 ppm) in subsoil following sample ignition were interpreted as being release from humic complexes rather than organic P. There was an increase in extractable Al. Elemental S was the main S fertiliser used. Sulphate concentrations increased in all soil layers with increasing S fertiliser, and increased slightly with P fertiliser rates. There were high rates of residual elemental S (0-800 ppm). There were large increases in extractable Al and a decrease in soil pH below 5 cm with increasing S fertiliser rates. The pH decrease was attributed to oxidation of elemental S and legume N fixation. Grazing management treatments had small effects on soil inorganic components. The pasture development options were sustainable considering soil nutrient concentrations, except perhaps for increasing extractable Al and decreasing subsoil Ca and pH.

Sustainability of New Zealand high‐country pastures under contrasting development inputs. 6. Fertiliser efficiency

“…In all instances the "Outlook" model estimated decreasing total P pools and Olsen P soil test values whether based on an unfertilised soil, maintenance of fertiliser treatments for a further decade, or changing the fertiliser regime to one of the other treatment levels after a decade. The observed results were that Olsen P soil test values were increasing (Scott 2000a , fig. 1).…”

“…The predictions of the "Outlook" model failed to correspond with the empirical data on observed soil Olsen P, sulphate S, and extractable organic S tests (Scott 2000a), or with observed changes in stock carrying capacities in both trials (Fig. 1).…”

“…Elemental-S fertiliser did decrease the 1 Oth-year soil pH values in this study and increase extractable aluminium (Scott 2000a, fig. 11-12).…”

“…Previous papers have described the trial designs and fertiliser levels used (Scott 1999), the changes in soil inorganic components after 10 years (Scott 2000a), sheep carrying capacities over 15 years (Scott 2000b), and organic components including pasture response (Scott 2000c). These could be considered as referring to a maintenance phase after an approximately five-year initial pasture development phase.…”

“…These interests have been particularly true for South Island high-country rangeland pastoral farming. The present series of papers attempts to quantify some of these production and sustainability aspects based on two long-running grazing pasture trials under contrasting management inputs of fertiliser and grazing management (Scott 1999(Scott , 2000a(Scott , 2000b(Scott , 2000c(Scott , 2000d.…”