Preliminary study of the potential for phosphorus loss with the development of organic soils

Organic Soils (organic matter concentration > 30%) cover 200,000 ha in New Zealand, of which around 67,000 ha have been converted for intensive land use (viz. dairy). However, Organic Soils have a number of properties that can exacerbate P losses including: high porosity and hydraulic conductivities and poor P retention, otherwise known as anion storage capacity (ASC). Studies have indicated that the physio-chemical properties of Organic Soils change quickly after initial development. However, the rate at which properties change and their influence on P loss is not well established. We hypothesized that Organic Soils have a potential to lose more P than other soil orders in the same region, and that the rate of P loss changes with time since development due to changes in soil properties. Ninety three paddocks were sampled containing approximately equal numbers of soils from the Organic, Gley and Podzol Soil orders in a coastal Southland catchment. Water extractable P (WEP), as an indicator for potential P loss in surface runoff, increased in soils with low ASC and/or high Olsen P concentration. Many of these soils were Organic Soils that had only recently been put into production. In contrast to Gleys and Podzols, the WEP concentration in Organic Soils quickly decreased with time since development to reach a near steady state after approximately 25 years. A mass balance confirmed that Organic Soils had lost more P than other soil orders, but also suggested that this decreased with time. The results of this study indicate that P losses from Organic Soils are potentially large but decrease with time since development and that using strategies to mitigate P loss is especially important within the first 25 years of development