The specific adsorption of divalent Cd, Co, Cu, Pb, and Zn on goethite is measured as a function of pH. For each mole of heavy metal adsorbed approximately two moles of H+ ions are displaced from the interface. Using these results the heavy metal adsorption data are expressed as functions of the solution concentrations of both H+ and metal ions, and the interfacial reaction is described by the equation,The adsorption data are consistent with an electrochemical model of the simultaneous adsorption of H+ ions and divalent metal ions on to the oxide. The intrinsic affinities of the metal ions for the oxide surface increase in the order, Cd < Co < Zn < Pb < Cu. However, besides the affinity of the metal ion for the surface, the adsorption curves are considered to be influenced by surface charge, the adsorption density of the metal ions and their size.The analysis of the data in terms of H+ and M2+ ion adsorption is considered to be complementary to the hydrolysis model for heavy metal adsorption.

No abstract

Organic farming in its various forms is seen by many as a sustainable alternative to conventional farming. This review considers and compares aspects of soil and environmental quality associated with organic and conventional farming systems under New Zealand conditions. The sustainability parameters considered include soil quality, nutrient dynamics, nutrient budgets, trace elements, and pesticides. The review used information from appropriate comparative studies conducted in New Zealand and overseas. However, because of the shortage of data on nutrient dynamics under organic systems in New Zealand, we also used a nutrient balance model (OVERSEER) and a nitrogen leaching estimation model to assess the comparative sustainability of typical model systems. Interpretation of the measured data coupled with the results of the modelling exercise suggests that organic farming carried out according to the Bio-Gro New Zealand production standards can be sustainable if sufficient amounts of nutrient are returned to match removal and losses. Biodynamic farming may be unsustainable because nutrients removed in farm produce are not adequately replaced. Soil organic matter content and A00013

The 0-7.5 cm depth of soil from 29 pastures on yellow-brown pumice soils contains more H+/H202-and EDTA/NH4Cl-extractable Co, Cu, and Zn than the corresponding 7.5-15 cm depth. The differences in H+ /H202-extractable quantities between the two depths ranges from. 0.1 to 2.3 ppm Co, 0.1 to 2.2 ppm C~, and 0.5 to 9.8 ppm Zn. H+/H202 is known to dissolve orgamc matter, any manganese oXides, and some Al and Fe in these soils and the above increments in extractable Co, Cu, and Zn are shown to be consistent with thei~ . acc~mulation. from t:ertiliser additions. and. plant and faecal deposition. Fertiliser additIOn IS the major contnbutor to changes In sOlI Co, and Cu in plant and faecal residues is the major contributor to changes in soil Cu. For Zn, both factors make significant contributions. Laboratory experiments support the conclusion that these elements derived from fertiliser, accumulate in the surface 7.5 cm of soil. At a solution concentration of 1O-5M the percentages of di,vale?t Co, Cu, and Zn, sorbed from O.OlM NaN03 are greater than 90% at pH 5 to 6, which 18 the normal field pH range. The similarity of the sorption reactions of hydrolysable divalent cations is examined for Co and Cd, where the amounts sorbed at pH 3.7 are highly correlated (r = 0.97*** for 0-7.5 cm and 0.94*** for 7.5-15 cm samples) . The amounts sorbed appear to depend on the organic matter content of the soil as well as the pH, but the influence of manganese oxides is probably small. 153

The relationship between topdressing rate and Cu concentration in lucerne grown on Hinemaiaia sand was investigated using dressings of 0, 0.5, 1, 1.5, 2, and 4 kg Cu/ha. The changes in Cu concentration in the foliage over two years, with three harvests in each, are reported. Differences between years, and among treatments, were significant. In the first year, 4 kg Cu/ha increased the average harvest concentration from 3.6 to 6.7 ppm Cu. In the second year, the effect from 4 kg Cu/ha applied only in year 1, was as good as that from 2 and from 4 kg Cu/ha applied in both years. Cu application did not increase yield. The annual DM production was 11 960 kg/ha in year 1, and 11 780 kg/ha in year 2.