Sorption behaviour and plant availability of copper in soils

Abstract

Some New Zealand soils were shown to have a high affinity for low concentrations of added copper. Between 95 - 100% of added copper was adsorbed by these soils. Adsorption isotherm gradients calculated using total solution copper and also ionic copper indicated that the adsorption of copper was highly pH-dependent over the range pH 4 - 6. Solution copper measurements indicated that while in most soils there was little change in total solution copper with pH, the partitioning between uncomplexed (Cu²⁺) and organically-complexed copper was highly pH-dependent. The largest proportion of uncomplexed copper was observed at pH 4 and this declined as the pH was raised to 6. This led to the suggestion that the adsorption of copper was related to the levels of ionic copper in solution rather that total solution copper. When ionic strength was increased, by increasing the electrolyte concentration from 0.01M Ca(NO₃)₂ to 0.1M Ca(NO₃)₂ the adsorption of copper was shown to decrease. This was considered to be related to changes in soil surface potential.

Desorption studies indicated a marked hysteresis effect between adsorbed and desorbed copper. Under the experimental conditions used, less than 10% of copper added to soils could be desorbed after ten desorption periods. As the period between initial adsorption of copper and the beginning of desorption was increased, a greater proportion of both native and added copper was 'irreversibly' held against desorption.

Both the desorption and adsorption of native and added copper was facilitated by increasing equilibration temperature. The net desorption of copper was considered to be a reflection of temperature on both the forward (adsorption) and backward (desorption) reactions.

The distribution of both native and added copper between various soil fractions was compared by means of a sequential copper fractionation scheme. This provided useful information regarding the amounts and forms of copper in soils. Only small amounts of both native and added copper were present in the soli solution and on exchange sites.

Significant amounts of copper were associated with the organic fraction and three oxide fractions. But the largest proportion of total soil copper was present in the residual fraction (i.e. > 25%). Small changes in soil copper fractions due to copper addition and time could not be successfully determined using this method.

Plant availability studies using two glasshouse experiments indicated that changes in soil solution copper concentrations and speciation were considerable, and were influenced by factors such as; soil pH, plant presence or absence and the copper status of the soil.

Adsorption and desorption reactions were shown to be important in controlling soil solution copper levels and labile solid phase copper. Their effect, however, was masked by the levels of total labile soil copper in the system. EDTA-extractable copper was highly correlated to plant copper uptake therefore was considered to give a better estimate of total labile soil copper and hence plant available copper than adsorption and desorption characteristics.