Risk mapping of soil acidification under Stylosanthes in northern Australian rangelands

The inclusion of Stylosanthes into pastures and cropping systems has proved to be a low cost method of improving product quality in Asia, Africa, South America, and northern Australia. However, there is recent evidence that accelerated soil acidification has occurred under these production systems, questioning their long-term sustainability. In an effort to assist producers and extension officers in identifying soils that are predisposed to accelerated acidification, an acidity risk map of the Dalrymple Shire in Queensland, Australia, was developed using information from a recently completed land resource survey. Validation of a previously derived pedotransfer function that predicts pH buffering capacity was undertaken using an independent set of soil samples collected from the Shire. Excellent agreement between measured and predicted pH buffering capacity was obtained. The pedotransfer function was used to estimate the pH buffering capacity of 44 soil associations in the Shire. These values were used to predict the number of years that it would take for soils to acidify from their current pH to 5.0 assuming a constant net acid addition rate of 2.1 kmol H+/ha. year. Approximately 62% of the total area of the Shire is predisposed to accelerated acidification and would take between 10-20 years to acidify to pH 5.0. In contrast, a relatively minor proportion of the total area of the Shire (17%) had significant internal buffering capacity. However, the degree of uncertainty associated with these estimations on certain soil associations may be too high to be of relevance. In order to overcome this limitation a field test designed to assess the risk of accelerated acidification on a paddock basis is proposed and outlined in the paper.