Amelioration of acidity with time by limestone under contrasting tillage

Farmers in south-eastern Australia have been advised, on one hand, to sow by direct drilling so as to conserve soil structure. On the other hand, farmers have been advised to thoroughly incorporate limestone into the soil so as to ameliorate acidity and obtain an economic return on the expense of liming. To help to resolve this conflict we investigated lime movement on a long term cropping trial, which included treatments that had been under 12 years of direct drilling. It was hypothesised that there would be a relatively rapid movement of surface applied limestone into the soil under the direct drilled treatments and hence a minimal time delay in amelioration compared with incorporation by various intensities of tillage. However, it took between 2 and 4 years after liming (1.5 t ha(-1)) before there was no statistically significant difference in pH at 5-10 cm depth between surface applied limestone and limestone incorporated with offset discs (and retained stubble). Thus, the acidity constraint at 5-10 cm depth was amended slowly by surface application compared with incorporation of the limestone by discs. Where incorporation of limestone was by scarifier (and stubble burnt), the lag period for surface applied limestone to match the soil amelioration was unclear in that the pH and exchangeable Al data gave different apparent findings, but the increase in pH at 5-10 cm was small compared with incorporation by discing. In a wheat-clover rotation where scarifying took place every second year (before wheat) there was no significant pH difference between cultivated and surface applied limestone below 0-5 cm depth during the 8 years. Surface application of limestone allowed an acidic, high Al-ex layer to remain in the subsurface soil. We propose two compromise courses of action to enable the effective use of limestone in conjunction with direct drilling: (a) apply limestone at a higher rate than calculated or measured for where limestone is incorporated, so as to drive the leaching of bicarbonate; (b) apply the limestone earlier than under conventional tillage, so that the lime movement might precede the appearance of exchangeable Al at depth. (C) 2003 Elsevier Science B.V. All rights reserved.