Nj. Nuernberg et al., EVALUATION OF AN ANION-EXCHANGE MEMBRANE FOR EXTRACTING PLANT-AVAILABLE PHOSPHORUS IN SOILS, Communications in soil science and plant analysis, 29(3-4), 1998, pp. 467-479
Anion-exchange resins (AER) have been used to determine plant availabl
e phosphorus (P) since the fifties and their results have shown strong
relationships with plant growth and P uptake irrespective of soil pro
perties. However, this procedure is still not widely used by laborator
ies because of difficulties in handling resin beads under routine cond
itions. New kinds and different shapes of resins are being produced ea
ch with specific characteristics that must be evaluated before use in
laboratory procedures. Thus the objective of this work was to evaluate
an AER manufactured in membranes reinforced with a Modacrylic fabric.
These anion-exchange membrane (AEM) sheets are commercially available
, making them suitable for soil testing. The membranes were cut in pie
ces (1.0x7.5 cm) identified as AEM-strips. The AEM-strips were soaked
in 0.5M HCl for a few days and transferred, after being rinsed with de
ionized water (DI), to 0.5M NaHCO3 to convert them to HCO3- form. The
AEM-strips and resin beads in nylon bags recovered 98.4 and 98.0% of t
he P content in an aqueous P solution, respectively. Three eluent solu
tions were evaluated with different shaking times. The 0.1M H2SO4 and
1.0M NaCl in 0.1M HCl were equally suitable for the molybdenum blue co
lor development without any pH adjustment, while the pH of the 0.5M HC
l was too low. The elution of P from the AEM-strips was independent of
time with a 15-min shaking being adequate for removal of all P from t
he strips. A comparison of soil sample preparation demonstrated that i
t was not necessary to vigorously grind or sieve the soil to improve t
he repeatability of the results. The AEM-strips were compared with oth
er methods (Pi impregnated filter paper, Mehlich I and Bray 1) using 3
2 soils from Guatemala with widely varying physico-chemical and minera
logical properties. Phosphorus extracted by the AEM and Pi procedures
(similar principle) were highly correlated and gave similar results ir
respective of soil type. The acid extraction (Mehlich I and Bray 1 met
hods) attacked soil components (apatites) resulting in higher and inco
nsistent amounts of P extracted which may not be available to plants;
the correlation between these methods within soils of similar properti
es was good, but when all soils were considered together the relations
hip was not significant. This demonstrated that the acid extraction me
thod for P is not suitable for soils containing apatites, while those
based on a sink for P (AEM and Pi) can be applied irrespective of the
type of soil.