MAGNESIUM DESORPTION STUDIES ON CALCAREOU S SAND

In a PK build-up fertilisation trial set up on calcareous humous sandy soil, soil samples were taken build-up from rye fields to examine mag nesium desorption using a fractionation technique. 10 mg fractions wer e removed a five-minute intervals from a soil column set up in a Schac htschabel tube using a peristaltic pump to maintain a constant (2 ml/m in) rate of liquid flow, with identical volumes of liquid being introd uced and removed and with a minimum of mixing. The soil samples were t aken from the ploughed layer on ten occasion during the vegetation per iod. The results can be summarised as follows: the quantity of availab le KCl-soluble magnesium rose as the result of N, P and K fertilisatio n. At higher rates of NPK there was a relative decrease compared with the other treatments (Table 1); in an aqueous solution the desorbed Mg content increased significantly in the N, NP, NK and NPK treatments o ver all the samples compared with the control (Table 2); even in the f irst fraction the intensity of magnesium desorption was very great, af ter five minutes more than half the total amount desorbed over the 25- minute period was already in solution (Table 2); the seasonal dynamics of aqueous desorption could be statistically proved, maxima were meas ured in early spring when the first soil sample were taken. A decrease was generally observed during the later stages of vegetation. Minima were found in summer, in samples taken in June (Table 3); on this sand y soil magnesium rapidly enters the solution, thus becoming available to plants; the calibration of this desorption process should be extend ed to cover other soil types, using extraction agents more characteris tic of soil suply levels.