Zinc Fertilization Plus Liming to Reduce Cadmium Uptake by Romaine Lettuce on Lockwood Soil.

Lockwood shaly loam (Pachic Argixerolls) and similar Cd mineralized soils derived from marine shale in California contain high Cd levels and high Cd:Zn ratios, and produce leafy vegetables with considerably higher Cd than is normal for US lettuce. Previous work by Burau et al. suggested that, in contrast with Zn-mine waste contaminated or biosolids amended soils, liming the soils had little effect on lettuce Cd. Our previous studies found that when soil Cd:Zn is high as for the Lockwood, liming does not reduce crop Cd but liming plus Zn fertilizer can strongly reduce crop Cd. A greenhouse pot trial was undertaken with addition of 0-100 mg Zn kg^-1 from either ZnSO₄ or ground rubber (contains 1.5% Zn) to Lockwood soil (5.3 mg Cd and 54 mg Zn kg^-1). All pots were made calcareous with reagent CaCO₃ (harvest pH_W 7.8). The control treatment produced lettuce with 13.2 mg Cd kg^-1 DW and only 10.5 mg Zn kg^-1 DW; adding 100 mg Zn kg^-1 soil produced lettuce containing 2.53 mg Cd/kg and 35 mg Zn kg^-1. If we had wanted to reach the normal Cd:Zn ratio for soils, we would have had to add more than 500 mg Zn kg^-1. The ground rubber increased crop Zn and reduced crop Cd, but was less effective than ZnSO₄; but use of the ground rubber could provide substantial savings for increasing soil Zn. The key finding is that liming these soils can be highly effective in reducing crop Cd, much like research has found for other Cd+Zn enriched soils, if Zn is added to compete with Cd in crop uptake and regulatory control of plant Zn relationships. Soil Cd:Zn ratio plays a significant role in risk of excessive plant uptake of soil Cd.