Amelioration and prevention of agriculturally generated subsurface acidity in sandy soils in Western Australia

[Truncated abstract] A long-term strategy for managing agriculturally generated soil acidification below the depth of normal cultivation will be vital to the sustainability of agriculture on sandy soils of the Western Australian wheatbelt. The need to prevent subsurface acidification is demonstrated by severe subsurface acidity which occurs naturally in restricted areas of this region. In these soils, aluminium toxicity to roots of many economically important crops can decrease yield by limiting access to water and nutrients at depth. This thesis describes investigations of materials and methods for managing subsurface acidification. These include three phases of field experimentation, and comparative studies of lime dissolution in solution, in leaching columns, and in undisturbed soil using a novel technique of measuring carbon dioxide production. Field experiments initially focussed on both immediate amelioration and medium-term prevention of subsurface soil acidity. Proven techniques of rapid amelioration with surface applied gypsum, or segmental liming of the subsurface to create non-toxic pathways for roots, resulted in one case only of an increase in yield of 8% with Al-sensitive wheat, but no consistent effect on yields over three seasons. With 41 ha'1 limesand mixed at 0-10 cm, a small increase in pH of 0.1-0.3 units at 10-20 cm after 3 years contrasted with the rapid downward transfer of sulfate or bromide tracer. A survey of 21 field trials 2-15 years after lime was mixed at normal cultivation depths established that pH increases to about 30 cm had occurred within 4-7 years of applying lime at gross rates of up to 51 ha*1. Three lime trials established 7-9 years beforehand were re-limed on a split-plot design to partition any residual benefit into surface and subsurface effects. Canola yield increased by 29% at the most acidic site, with 13% attributed to increased soil pH to a depth of 10- 15 cm.