Zn deficiency promotes Cd accumulation by lettuce from biosolids amended soils with high Cd:Zn ratio

Before regulations for biosolids utilization were developed, some highly contaminated sewage sludges were land applied on farmers fields. In one case, with a soil which had been amended with a high Cd:Zn ratio sludge, making the soil calcareous did not reduce Cd uptake by lettuce as raising pH had achieved in many earlier tests. We conducted an experiment to examine metal uptake by lettuce with different treatments (2% limestone, Zn, peat, or 10% addition of a biosolids compost enriched in Fe, Mn and/or Zn) would reduce the potential for Cd uptake by Romaine lettuce from soils which had been amended with high Cd biosolids in the 1960s-1970s. An unexpected result was observed: liming two of the soils induced Zn deficiency in lettuce and raised lettuce foliar Cd concentrations. The high soil Cd appears to have interfered with Zn uptake, and foliar Zn was below the deficiency diagnostic level. Added Zn or peat+Zn prevented the Zn deficiency and greatly reduced lettuce Cd levels, while compost and Zn-amended compost were not quite as effective as the ZnSO(4). No apparent effects of the added Fe and Mn oxides on lettuce Cd were observed, perhaps the oxides also adsorbed added Zn, reducing its ability to inhibit Cd uptake by lettuce. We conclude that application of excess limestone plus added Zn will reduce soil Cd risk to feed-and food-chain for soils with high Cd:Zn contamination such as biosolids amended soils at St. Marys, PA.