Zinc Fertilization Plus Liming to Reduce Cadmium Uptake by Romaine Lettuce on Cd-Mineralized Lockwood Soil
Lockwood shaly loam (Pachic Argixerolls) and similar Cd mineralized soils derived from marine shale in California contain higher Cd levels and higher Cd:Zn ratios than uncontaminated US soils, 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 geogenic Zn+Cd enriched soils, liming the Cd-mineralized soils did not effectively reduce crop Cd concentration. Our previous studies found for high Cd:Zn soils, liming may not reduce crop Cd but liming plus Zn fertilizer can strongly reduce crop Cd. Greenhouse pot trials were undertaken with addition of 0-500 mg Zn kg-1 to Lockwood soil (5.3 mg Cd and 54 mg Zn kg-1). All pots were made calcareous with reagent CaCO3 (harvest pHW 7.8), or adjusted to a range of pH without Zn fertilizer. The zero Zn calcareous 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-1 and 35 mg Zn kg-1; adding 250 mg Zn kg-1, 6.3 mg Cd kg-1 lettuce; and 500 mg Zn kg-1, 3.9 mg Cd kg-1 lettuce. For the 0 Zn treatment, acidification or liming increased lettuce Cd. The key finding is that if Zn is added to raise Cd:Zn ratio, liming these soils can be highly effective in reducing lettuce Cd, much like found for other Cd+Zn enriched soils.