Summary
The addition of 2.5 mg/l of polyacrylamide to 0.5M NaHCO3 used as an extractant for ‘available’ phosphate overcame most of the analytical difficulties associated with the use of activated carbon which was recommended in the original procedure of Olsen et al. (1954). No significant differences in the amount of P extracted were found between polyacrylamide and carbon and with the former, technical problems were greatly reduced. Consequently, the modification of the original method to incorporate polyacrylamide has been adopted in the Agricultural Development and Advisory Service of the Ministry of Agriculture, Fisheries, and Food as a standard method for the determination of available phosphate in soils.
Magnesium, extracted from 244 soils using a range of extractants, was determined by (i) atomic absorption spectroscopy and (ii) a number of standard chemical techniques. From an examination of the results, atomic absorption spectroscopy was shown to offer a rapid and reliable method for the determination of magnesium and to have definite advantages over the chemical methods in a number of respects. The atomic absorption technique was distinctly superior for estimating low magnesium levels in the extracts, which correspond to deficiency levels in soils as far as crops and livestock are concerned.Ammonium nitrate appeared to be the most consistent extractant over a wide range of soils. For calcareous soils, acidic extractants produced higher magnesium values than the neutral type, due to the solution of free carbonates. The narrow soil/extractant ratio and a short shaking time for the Morgan's 11 method however, overcame this disadvantage on calcareous soils.
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