EFFECT OF PHOSPHATE FERTILIZER TYPE ON THE ACCUMULATION AND PLANT AVAILABILITY OF CADMIUM IN GRASSLAND SOILS

Cadmium (Cd), a potentially toxic heavy metal for humans and animals, accumulates in the liver and kidneys of older animals grazing New Zeal and and Australian pastoral soils. Phosphorus (P) fertiliser is the ma jor input of Cd into these farming systems. A study was conducted to e valuate the effects, over 10 years, of annual application (30 kg P ha( -1) yr(-1)) of four forms of P fertilisers having different solubiliti es and Cd contents [41, 32, 10 and 5 mu g Cd g(-1) for North Carolina phosphate rock (NCPR), single superphosphate (SSP), diammonium phospha te (DAP) made from low Cd phosphate rocks and Jordan phosphate rock (J PR) respectively] on soil and herbage Cd concentrations. Ten years of fertiliser application caused a marked increase in surface soil Cd con centrations. Total soil Cd was significantly higher in SSP and NCPR tr eatments compared to control (no P fertiliser), JPR and DAP treatments in the 0-30 and 30-75 mm soil depths. Plant-available Cd (0.01 M CaCl 2 extractable Cd) was higher in SSP treatments than in control and oth er fertiliser treatments. Chemical analysis of herbage samples showed that there was no significant difference in Cd concentration in pastur e grasses between treatments in the second year of the trial but in th e eighth and tenth year, plots fertilised with SSP and NCPR had signif icantly higher Cd in pasture grasses in most of the seasonal cuts comp ared to control, JPR and DAP. Cadmium recovery by both grasses and clo ver was less than 5% of Cd applied in fertiliser. Clover Cid concentra tion and yield were much lower than those for grass and therefore its contribution to pasture Cd uptake was very low (< 7%). A strong season al effect on grass Cd concentration, which is inversely related to pas ture growth rate, was observed in all three sampling years - Cd concen tration was highest during autumn and lowest in spring. Total Cd conte nts of the fertilisers and their rate of dissolution rather than soil pH [pH (H2O) at 30-75 mm depth of 5.39, 5.20, 5.11 and 5.36 for NCPR, SSP, DAP and JPR treatments respectively] influenced soil and herbage Cd. These results showed that the use of P fertilisers with low Cd con tent will reduce herbage Cd levels and has the potential of reducing C d levels in grazing animals and their products.