LIMING AND RE-LIMING NEEDS OF ACIDIC SOILS USED FOR CROP-PASTURE ROTATIONS

In a long-term liming experiment in northeastern Victoria, we have re- applied lime and applied gypsum (1992 season) to assess wheat grain yi eld responses with on-going changes in soil pH and extractable alumini um. An acid-sensitive wheat (cv. Oxley) was grown in 2 seasons (1992-9 3), 12 years after initial applications of lime. Where lime (2.5 t/ha) was applied in 1992 to a previously unlimed soil, grain yield was inc reased by 19 and 46% respectively in the 2 seasons. However, the yield from these newly limed plots was well below the yields obtained from plots limed in 1980. Re-liming plots limed in 1980 resulted in further yield increases, with lime re-applied at 2.5 t/ha increasing yields b y 12% in both seasons. Gypsum decreased grain yields on unlimed soil i n the year of application but in the second year gave increases in yie ld. Whilst pH had changed little in the unlimed soil over the 12 years , the concentrations of extractable aluminium in the root zone increas ed substantially such that these concentrations far exceed levels whic h may affect acid-sensitive wheats. Liming at 2.5 t/ha did reduce the aluminium at 0-10 cm depth, but the concentrations at 10-20 cm depth ( 11.7 mg/kg) are likely to restrict grain yield. The data illustrate th e progressive nature of soil acidification and the risk to wheal produ ctivity through delaying treating this soil degradation problem.