Abstract
The extraction of cobalt from 3 soils with 0.5 M acetic acid was studied under conditions of varying temperature, duration of extraction and soil-solution ratio. The first two parameters significantly affected the quantity of cobalt extracted, but variation in soil-solution ratio had little effect.
In an examination of 20 soils from south-east Scotland, the total soil cobalt content was found to be positively correlated with the concentrations of iron and manganese oxides. Extractability studies using 0.5 M acetic acid and 0.0b M di-sodium EDTA indicated that cobalt was associated to a greater degree with the manganese oxides than the iron oxides. However, the 0.01 M CaCl^-extractable cobalt concentration was not related to the soil oxide content, but was negatively correlated with soil pH. Isotopic exchange studies showed that the concentration of isotopically exchangeable cobalt was directly related to the soil oxide content. The equilibrium between isotopically exchangeable cobalt on the solid phase of the soil and in soil solution was highly dependent on soil pH.
No relationship between total, extractable or isotopically exchangeable cobalt and soil organic matter was observed.
In an incubation study waterlogging caused mobilisation of both indigenous and added cobalt, as determined by acetic acid extraction. Drying of the soil to 20 per cent moisture caused rapid immobilisation of this cobalt and, over a period of months, the supplementary cobalt became non-extractable.
The specific sorption of cobalt by individual soil constituents and by 18 soils from south-east Scotland was examined. The equilibrium solution cobalt concentrations used were those likely to be found in soil ( 10 ^ M). Amorphous soil oxide material showed the highest capacity for sorption, followed by humic and fulvic acids and then montmorillonite. However, the reaction between cobalt and montmorillonite showed greater irreversibility than with humic acid. The sorption characteristics of the intact soils indicated that sorption took place predominantly on the manganese oxide fraction.
Two pot experiments were carried out to study the uptake of native cobalt by perennial ryegrass and red clover and also the increase in the cobalt content of perennial ryegrass following application to the soil. The concentration of plant cobalt derived from indigenous soil supplies was negatively correlated with soil pH, as was the increase in plant content due to artificial addition to the soil. The latter quantity was also negatively correlated with the soil iron and manganese oxide contents.
Field trials were set up in south-east Scotland to monitor the effect of top-dressing with cobalt on uptake by deficient pasture (mixed herbage). Over a 3 year period, the addition of 6 kg CoS0^.7H^0 per hectare raised herbage cobalt concentrations to above 0.08 pg/g (deficiency level) in less than half of the 15 soils studied. Applying the same amount of cobalt as the EDTA complex was still less effective. Although herbage cobalt concentrations decreased over the 3 year sampling period, there was no concurrent decrease in acetic acid-extractable soil cobalt. Most the applied cobalt was present in the top k cm of soil.
