DESORPTION OF COPPER FROM SOME NEW-ZEALAND SOILS

Availability of soil Cu to plants is dependent on the desorption into the soil solution of Cu from the surfaces of soil colloidal materials. Although there have been many studies of Cu sorption by soils, however, few have examined the process of Cu desorption. The objective of this study was to examine some of the factors likely to affect the desorption from soils of both native Cu and Cu added to soils. Ten New Zealand soils were used in this study and Cu desorption was determined by repeated equilibrations in 0.01 M Ca(NO3)2 solution. Desorption of native soil Cu varied between soils and the proportion of the total labile Cu in the soil (as determined by EDTA [ethylenediamine tetraacetic acid] extraction) that could be desorbed readily was strongly influenced by pH. Below pH 6.5, desorption increased with decreasing pH, and above pH 6.5 increased with increasing pH. When Cu was added to the soils (7 mg Cu kg-1 soil) with an initial contact period between added Cu and soil of 24 h, < 8.5% of the added Cu could be desorbed readily. The proportion of added Cu desorbed was reduced substantially by increasing the contact period to 12 wk before desorption. Desorption of native soil Cu was increased by increasing the temperature at which desorption was carried out. The effect of temperature on the desorption of added Cu differed between soils. The results of this study provide more evidence for the existence of slow reactions between added Cu and soil that reduce the ability of the Cu to desorb back into the soil solution.