FIGURE 12: Hillslope Erosion Risk

FIGURE 12: Hillslope Erosion Risk

Soil carbon

Carbon stocks in cropping and grazing soils in the Cradle Coast Region are broadly higher than elsewhere in Tasmania (Doyle et al 2012), and soils in Tasmania store more carbon than elsewhere in Australia. This is attributed to higher rainfall and lower temperature, which contributes to more plant growth and less oxidation of organic matter. Carbon storage rates of up to 135 tonnes per hectare in the top 30 cm of the soil profile have been recorded for the ferrosols of the Devonport area. Soil carbon storage was found to be strongly linked to rainfall and soil type in the Region, but also that carbon stores could be reduced in the top 10 cm of soil by between 29 and 36 per cent when comparing long term pasture land uses with cropping systems. This reduction indicates a theoretical potential to lift A-Horizon soil carbon storage in coastal cropping ferrosols from average 95 t/ha to 135 t/ha through improved land management practice. McKenzie (2017) reviewed the national findings of the Soil Carbon Research Program (SCaRP) and noted amongst other things that poor application of management practices generally thought to increase soil carbon storage, may in fact result in a loss of soil carbon. Equally, a very good application of a practice that is generally found to decrease soil carbon may result in increased soil carbon stocks if levels of carbon capture and return to the soil are high enough. These findings suggest that practice change needs to focus simply on maintaining relatively high levels of Net Primary Productivity (NPP) in the crop/ grazing cycle. As the greatest potential losses from soils in the Region are in the top 10 cm, the approach to cultivation and stocking rate are pivotal.

Soil acidification

Soil acidification is an underlying problem for intensive agriculture and grazing in the Region. Many of the agricultural soils in the Region are naturally very strongly acidic and intensive use along with poor nutrient management can reduce long-term profitability. Moderate to high levels of annual rainfall across farmland of the region increase the need to effectively manage soil pH, as precipitation itself contributes to soil acidification as basic cations such as calcium and magnesium are leached through the soil profile and replaced with hydrogen. While soil acidification can be readily addressed by applying locally sourced lime or dolomite, monitoring funded by CCA in 2009 and 2014 indicates that underuse of pH amendments may be occurring and that they are not being applied in a targeted manner.

It is possible to use nutrient budgeting and subsequent development of soil monitoring programs to moderate and target the use of chemical fertilisers which will help with the long-term management of soil acidification in the productive grazing and cropping landscapes of the Region.