Evaluation of the electro‐ultrafiltration technique (EUF) to determine available P in neutral and calcareous soils

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Abstract
The electro‐ultrafiltration (EUF) method has been used to evaluate the K supplying power of North‐American soils but has not yet been used for P. The objective of this study was to compare the capacity of EUF with other extraction techniques to predict the plant availability of soil P and P fertilizer responsiveness by three cuts of Ray‐grass (Lolium multifolium L.) growing over an 8‐wk period. Increasingly higher average concentrations of soil P were extracted by 0.01 M CaCl2 (0.24 mg kg−1), EUF at 50 V and 20°C (1.82 mg kg−1), water for 16 h (1.86 mg kg−1), EUF at 200 V and 20°C (12.0 mg kg−1), EUF at 400 V and 80°C (12.7 mg kg−1), Olsen (13.6 mg kg−1) and Mehlich 3 (33.5 mg kg−1). There was a very close relationship between the amount of P desorbed by EUF at 50 V and 20°C and 0.01 M CaCl2‐extractable P (r = 0.86**). Very significant relationships between the maximum P buffering capacity, as determined by batch equilibration, and the amounts of P desorbed in EUF‐fractions were observed (r = ‐0.65** to r = ‐0.77**). The presence of sparingly soluble Ca‐phosphates resulted in EUF desorbing more P than other chemical methods in 8 of 28 neutral to calcareous soils. The amount of P desorbed by EUF between 30 and 35 min at 400 V and 80°C was best among the extracting methods in predicting the P uptake and the relative yield of ryegrass over 3 cuts when the eight soils containing sparingly soluble phosphates were not included in the calculations. Modifications of the extraction procedure such as decreasing the soil to solution ratio are necessary to maintain constant voltage at 200 V and 400 V in some of these calcareous soils in order to avoid excessive current produced during the extraction.

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