Municipal composts reduce the transfer of Cd from soil to vegetables

Cadmium (Cd) is a non-essential trace element that accumulates in agricultural soils through theapplication of Cd-rich phosphate fertiliser. Vegetables can accumulate Cd to concentrations that some-times exceed food safety standards. We investigated the potential of low-cost soil amendments to reduceCd uptake by spinach (Spinacia oleraceaL.), lettuce (Lactuca sativaL.) and onion (Allium cepaL.). Batchsorption experiments revealed the relative sorption of Cd by biosolids, charcoal, lignite, sawdust, twotypes of compost, bentonite and zeolite. Lignite and compost had the greatest ability to sorb Cd and weresubsequently selected for pot trials, which elucidated their effect on Cd uptake by onions, spinach andlettuce in two market garden soils with native Cd concentrations of 1.45 mg/kg and 0.47 mg/kg. Theaddition of 2.5% (dry w/w) municipal compost reduced the Cd concentration in onions, spinach andlettuce by up to 60% in both soils. The addition of lignite gave variable results, which depended on thesoil type and rate of addition. This Cd immobilisation was offset by soil acidification caused by the lignite.The results indicate that municipal compost is a low-cost soil conditioner that is effective in reducingplant Cd uptake.