Effect of fertiliser type on cadmium and fluorine concentrations in clover herbage

Abstract

Summary. This study investigated whether changing phosphatic fertiliser type affects the accumulation of cadmium (Cd) and fluorine (F) in pasture herbage. North Carolina phosphate rock, and partially acidulated fertilisers derived from this rock, generally have higher Cd and F concentrations compared with single superphosphate currently manufactured in Australia.

Clover herbage from sites of the National Reactive Phosphate Rock trial was collected and analysed for concentrations of Cd (11 sites) and F (4 sites). A comparison was made between pastures fertilised with 4 rates of single superphosphate, North Carolina phosphate rock, and partially acidulated phosphate rock having Cd concentrations of 283, 481 and 420 mg/kg P respectively, and F concentrations of 170, 271 and 274 g/kg P respectively. One site used Hamrawein (Egypt) phosphate rock (HRP) having a Cd and F concentration of 78 mg Cd/kg P and 256 g F/kg P respectively. To help identify differences in herbage Cd concentrations between sites, unfertilised soils from each site were analysed for total and extractable Cd contents. At one site Cd concentrations in bulk herbage (clover, grasses and weeds) were related to infestation of the pasture by capeweed (Arctotheca calendula L. Levyns).

There were no significant differences between F in herbage from plots fertilised with either single superphosphate, partially acidulated phosphate rock or North Carolina phosphate rock, or between sites. Concentrations of F in herbage were low, generally less than 10 mg/kg. However, there were large differences in Cd concentrations in herbage between sites, while differences between fertiliser treatments were small in comparison. The site differences were only weakly related to total or extractable (0.01 mol CaCl₂/L) Cd concentrations in soil.

Significant differences in Cd concentrations in clover due to fertiliser type were found at 5 sites. North Carolina phosphate rock treatments had significantly higher Cd concentrations in clover compared with single superphosphate at 2 sites. Partially acidulated phosphate rock treatments had significantly higher Cd concentrations in clover compared with single superphosphate at 4 sites. At the site where Hamrawein was tested, this treatment had significantly lower Cd concentrations in clover compared with both single superphosphate and North Carolina phosphate rock treatments.