EFFECTS OF RESIDUES OF LIME AND PHOSPHORUS-FERTILIZER ON CADMIUM UPTAKE AND YIELD OF POTATOES AND CARROTS

Potatoes (Solanum tuberosum L.) and carrots (Daucus carota L.) were gr own in the field on Tasmanian ferrosols (humic eutrudox) which had bee n limed either 2, 3, or 5 years earlier, and where tuber cadmium (Cd) concentrations in potatoes grown a few months after the liming had sho wn no lime response. In the current crops lime decreased potato tuber Cd by about 30% and carrot root Cd by about 50%. We attributed the dec rease to more even and deeper mixing of the lime with the soil by the harvest of the first potatoes. Phosphorus (P) fertilizer residues from the earlier potato crops did not significantly affect tuber or root C d, but there was a positive effect at 1 site where some high Cd P fert ilizer had earlier been used. Neither lime nor P fertilizer residues a ffected potato or carrot yields. Analysis at one site of potato tubers from the upper part of the soil ridges showed that they had slightly higher Cd concentrations than did deeper tubers near the fertilizer ba nd, whether P fertilizer was in the band or not. This suggests that ei ther the Cd in the fertilizer band was relatively unimportant as a Cd source for the current crop, or that Cd was redistributed within the p lant during the season, or both. Liming may be a suitable medium to lo ng-term strategy for decreasing Cd uptake by root crops, but site to s ite and seasonal variation can still be great, and knowledge of other major influences is needed for assurance of produce quality. Our obser vations need to be extended to sites which gives rise to higher Cd con centrations in agricultural produce, and to other soil types. Potato c ommon scab was severe in the limed plots at one site. However, this si te had grown 3 potato crops in 5 years, which probably exacerbated the disease. Potato processors in Tasmania demand a minimum of 5 years be tween successive crops which should slow any build up of scab due to l iming, but more work on possible interactions between lime and rotatio n length on scab incidence is needed before liming can be recommended as a Cd control measure for potatoes.