Abstracts Category : Other

Cadmium management in New Zealand's horticultural soils : a thesis presented in partial fulfilment of the requirements for the degree of Master of Environmental Management, Massey University, Palmerston North, New Zealand

by Hadee Thompson-Morrison

Cadmium (Cd) is a heavy metal trace element which presents risks for the horticultural industryin New Zealand (NZ). This element is added to soils through phosphate fertiliser application, andonce there may be available for plant uptake and food chain transfer. When food products exceedinternational standards for Cd concentrations, these products may be excluded from internationalmarkets upon which NZ relies to maintain its economy. This presents a reputational risk for NZshorticultural exports. Soil pH and organic matter (OM) content are the two key drivers influencingCds bioavailability, and field trials are currently being undertaken in four horticultural sitesthroughout NZ Pukekawa, Manawatu, and two adjacent sites at Lincoln to test the efficacy ofthe use of lime and compost amendments to influence these soil variables and thus reduce Cdplant uptake from soils. Potatoes are grown at all sites while Lincoln also includes wheat. Thisresearch aimed to characterise these soils, including total and plant-exchangeable Cdconcentrations, pH, OM content, cation exchange capacity, total and plant-exchangeable Znconcentrations, aluminium and iron oxide content, total phosphorus and total nitrogen content.Findings indicated that total Cd concentrations varied among sites, with the highest (0.52 mg kg-1) at Pukekawa, followed by Manawatu (0.26 mg kg-1) and Lincoln Wheat and Potato sites (both0.13 mg kg-1). Exchangeable Cd concentrations were low at all sites (0.01-0.02 mg kg-1) indicatinglittle risk of plant uptake from these soils.The mitigation strategy tested in this work focuses on pH as a key soil variable that canbe readily changed to restrict Cd uptake. However, the effectiveness of amendment rates to effecttarget pH values is dependent on soil chemistry and rates will vary across sites. Incubationexperiments were conducted to determine amendment rates for lime and sulphur, and to comparethe pH of amended soil in a laboratory situation with the in-field situation. Incubation and fieldsituations were found to be similar, with no significant differences between pH values after aperiod of 274 days in the incubator and 169 days in the field. The accuracy of the calculatedamendment rates at achieving target pH values was assessed with extended incubationexperiments. The results here varied between soils, with the sulphur application rate proving moreaccurate in the Pukekawa soil, however too high for the Manawatu and Lincoln potato soils. Thecalculated liming application rate similarly resulted in a higher-than-target pH, however after aperiod of 231-274 days the pH reduced and approached the target value.A cost-benefit analysis was undertaken to determine the economic viability of theproposed mitigation strategy at each potato site. Results proved the strategy to be a viable option,which would remain viable in the face of varying uncertainty and reductions in potato yields.Practical considerations including timing and weather conditions, and compost