Isotopic exchange studies were applied to the laboratory and glasshouse measurement of labile zinc in 25 soils from nine Great Soil Groups. The laboratory equilibration procedures worked well with acidic and most near neutral soils, but may overestimate labile zinc values for the lateritic podzolic soils. The values for some acidic soils were also compromised because of lack of isotopic equilibrium. The laboratory method gave erratic and unrealistic data when applied to alkaline soils due to fixation of the added zinc. The procedures based on the specific activity of zinc absorbed by plants from soils equilibrated with carrier-free 65Zn gave reproducible values of the total amount of plant available zinc for all soils. These values agree well with the corresponding laboratory data for acidic soils. Furthermore, the specific activity data showed that magnesium chloride and EDTA extractions had equilibrated with the same chemical form or forms of zinc as that absorbed by the plants.

The effect of applied cobalt on the cobalt content of pasture plants has been studied in pot, field, and laboratory experiments. The generally poor agreement between pot and field experiments may be partly due to the noted variation of cobalt status of pasture with season and the influence of waterlogging. Cobalt treatments were effective for at least 2 yr on the humus podzol soils, but there was only a residual effect for 1 yr on other soils. Alteration of the soil pH did not prove a satisfactory means of increasing cobalt status of pasture on soils where it was most required. The efficiency of cobalt fertilizers as a means of alleviating problems of cobalt deficiency depends on the manganese content of the soil. Plants grown on soils containing > 1000 p.p.m. of total manganese are unlikely to benefit from application of cobalt fertilizer to the soil, necessitating foliar application by sprays or the use of cobalt oxide pellets for stock.

Representatives of nine soil groups were extracted with reagents that have been used to predict zinc deficiency. The amounts of soil zinc removed were discussed in terms of specific and non-specific bonding in relation to the reagent used. The desorption of natural zinc was also described in terms of the quantity/intensity (Q/I) relation and an equilibrium zinc concentration (ZnQ) at natural pH. The ZnQ values varied from 1 to 4 �gI. for the alkaline soils and 8-190 pg/l. for the acidic soils. The Q/I ratio was derived by radioisotopic and chemical isotherm procedures. Log Q/I (I = total soluble zinc) approximated closely a linear relation of unit slope with pH. This was ascnbed to a common reation of zinc with all soils by specific sorption dominated by ZnOHA ions such that Q/I� = constant where I' = (ZnOH+aq). Deviations from this relation are discussed. The relations between soil and plant (clover and wheat) variables were studied by simple and multiple regression analysis. Single values of intensity variables, and, to a lesser extent, Q/I variables, correlated well with plant data but not the quantity variables. The improved correlations of Q variables when combined with Q/I variables accorded with published work. The problem of predicting zinc availability on alkaline soils which are dominated by very high Q/I values, is discussed.

The experimental conditions and precision of alternative methods of measuring isotopically exchangeable cobalt (Coi.e) were investigated. The values of Coi.e varied from 0.16 to 5.4 p.p.m. for 25 soils and were highly correlated with the sum of cobalt extracted by ammonium acetate and subsequently by quinol, and the relationships between these forms are discussed. The amounts of cobalt sorbed in the presence of calcium chloride by different soils varied markedly between and within soil groups. The bonding energy of specific sorption of cobalt did not vary significantly between soils after the first sites were filled. Cobalt sorption capacity for all soils studied was highly correlated with cobalt content and surface area and to a lesser extent with manganese and clay content and pH, but not with organic matter. When soils of high montmorillonite content were excluded, highly significant correlations were obtained only with manganese and cobalt contents and surface area. The initial slope of isotherms of isotopically exchangeable cobalt 60Co(soi1)/60Co (solution) and Coi.e may provide measures of the intensity and capacity factors, respectively, of cobalt availability to plants.

Glass-house experiments were made to measure the effect of soil waterlogging on the cobalt and copper status of subterranean clover and ryegrass. The cobalt content of the plants was much increased by soil waterlogging. Very high plant cobalt contents were obtained when soil was waterlogged for 2 months at room temperature, or for shorter periods at higher temperatures, before planting; or when it was waterlogged while the plants were growing rapidly. The effect of waterlogging on the cobalt content persisted when soils were stored air-dry. Soil waterlogging raised the copper content of the plants much less than it did the cobalt. The copper content was significantly increased only when plants had been severely stunted by waterlogging. Waterlogging raised the plant cobalt content more than it reduced the yield. Thus the total uptake of cobalt, but not of copper, from the soil was increased by waterlogging.