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(NO,)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 kgsoil) 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. I is NOW generally accepted that soil solution concentrations of micronutrients (and hence availability to plants) are probably controlled by sorptiondesorption reactions at the surfaces of soil colloidal materials (Swift and McLaren, 1991). However, although numerous studies have examined the sorption of Cu by soils and soil components (McLaren and Crawford, 1973; Jarvis, 1981; McLaren et al., 1981; Raikhy and Takkar, 1981; Sanders, 1982), very few studies have examined the reverse process, i.e., desorption of Cu into solution. As noted by Swift and McLaren (1991), this is in some ways a paradox, since it is desorption processes that are likely to control the amount and rate of release of Cu (or other micronutrients) into solution for plant uptake. Of the few Cu desorption studies reported in the literature, most have examined desorption from individual soil components, e.g., McLaren et al., 1983; Padmanabham, 1983a. An exception to this is the study reported by Lehmann and Harter (1984), who examined the desorption kinetics of Cu applied to an intact soil. However our understanding of the factors affecting desorption of Cu from soils is far from complete. Such information is required in order to improve our ability to predict the release of both native Cu from soils and the release of Cu added to soils as a fertilizer or as a pollutant. The objective of our study was to examine the deD.S. Hogg, Woodward-Clyde Consultants, 1550 Hotel Circle North, San Diego, CA 92108; R.G. McLaren, Dep. of Soil Science, Lincoln Univ., Canterbury, New Zealand; and R.S. Swift, Dep. of Soil Science, Univ. of Reading, London Road, Reading, England. Received 20 Apr. 1992. 'Corresponding author. Published in Soil Sci. Soc. Am. J. 57:361-366 (1993). sorption of both native and applied Cu from a range of New Zealand soils. In particular, the effects of soil pH and temperature on Cu desorption were studied and, in the case of applied Cu, the effect of contact time between the Cu and the soil. MATERIALS AND METHODS