Fluorine (F) is a key environmental contaminant associated with the long-term high application rates of superphosphate fertiliser particularly on soils used for dairying and horticulture in New Zealand. These elevated soil F concentrations may have the potential to negatively impact soil microorganisms which underpin New Zealand’s agricultural economy. Ongoing soil F monitoring requires an accurate and simple test to establish comprehensive soil F management practices, and quantify both the total and bioavailable concentration of F in soil. In order to assess both total and bioavailable F concentrations in New Zealand agricultural soils, we developed standard methodologies at Massey University. We found that the bioavailable F concentrations in New Zealand pastoral soils are ranging from 1.70 to 6.45 mg/kg (F^- concentration in soil solution < 0.58 mg/L); two orders of magnitude lower than that of total F.

Rhizobium leguminosarum, is a Nitrogen fixing soil bacteria which is a fundamental component of New Zealand’s legume-based pastoral farming. Currently, there is no data available to determine whether New Zealand’s bioavailable F concentrations in soil are harmful to Rhizobium leguminosarum. We found that the normal rod-shaped bacterium cell was morphologically altered when Rhizobium was exposed to fluoride concentration above 100 mg/L through a laboratory incubation experiment. This concentration is orders of magnitude higher than those recorded for New Zealand agriculture soils under ‘normal conditions’. Therefore, there is no indication of an imminent risk by soil F to Rhizobium leguminosarum.

Allophanic soils in NZ where increasing F concentration is of great concern, due to significant fertilisation history and F retention. Our glasshouse trial with two white clover varieties growing in allophanic soil at varying levels of soil F (80-400 mg/kg), indicated no significant influence of soil F on plants or soil microbial activities. The study suggests that F concentrations equivalent up to 50 years of F accumulation, through the application of phosphate fertiliser to potentially high-risk New Zealand soils, doesn’t have immediate effect on soil or environmental health.

This information seeks to provide better understanding of soil F and its influence on legume based pasture crops in New Zealand.