Fjölrit RALA - 15.06.2004, Page 120

supematant solution for each extraction was determined by the AAB method. Desorbable P
was estimated as % of sorbed P remaining after 8 successive extractions.
Results showed that P adsorption isotherm pattems of the studied soils differ widely in
relation to their extractable A1 and Fe, and allophane contents. P adsorption values in the
surface horizons from lowest to highest order were: 0.002-1.01 g P kg"1 (Greece), 0.39-2.08 g
P kg'1 (Hungary), 0.19-6.70 g P kg"1 (Italy), 0.95-12.18 g P kg"1 (Tenerife), 4.22-16.88 g P kg'1
(Iceland), 6.80-18.31 g P kg'1 (France), 8.89-20.71 g P kg'1 (Portugal). Similar order was
shown in the subsurface horizons with values: 0.01-0.04 g P kg"1 (Greece), 0.19-2.60 g P kg"1
(Hungary), 0.05-17.60 g P kg"1 (Italy), 1.02-10.57 g P kg"1 (Tenerife), 2.11-16.35 g P kg"1
(Iceland), 6.27-37.39 g P kg 1 (France), 9.52-48.43 g P kg'1 (Portugal). P sorption isotherms of
studied soils fitted the Langmuir equation. Values of Langmuir P adsorption maxima (Ads
max) were strongly correlated with Al0 (r = 0.91, p < 0.05) and allophane (r = 0.86, p < 0.05)
contents, and values of Al0 + Vi Fe0 (r = 0.91, p < 0.05) and of ferrihydrite + allophane (r =
0.87, p < 0.05). Positive correlations between Ads max values and contents of Ald, Fe0, (r =
0.77, p < 0.05) and Alp (r = 0.49, p < 0.05), and P retention (PR) (r = 0.73, p < 0.05) were also
found. Ads max values in highest to lowest order were: 10.81-51.95 g P kg'1 (Portugal), 10.06-
27.54 g P kg"1 (France), 4.59-15.29 g P kg'1 (Iceland), 3.17-13.23 g P kg"1 (Tenerife), 0.02-
12.82 g P kg"1 (Italy), 1.61-2.27 g P kg'1 (Hungary) and 0.02-0.50 g P kg"1 (Greece). Within
andisols, the highest Ads max were observed in Bw horizon of Acrudoxic Hydmdands and
Alic Hapludands. The lowest were determined for Vitrixerands and non-andisols
(Xeropsamments and Xerothents).
Percentage of P sorbed in the soil after eight successive extractions with 0.01 M CaCl2
showed strong negative correlation with both Fe0 and ferrihydrite (r = -0.61, p < 0.05), Al0 +
Vi Fe0 (r = -0.60, p < 0.05), ferrihydrite + allophane (r = -0.61, p < 0.05) contents and values
of PR (r = -0.76, p < 0.05). They also showed weak negative correlation with Al0 (r = -0.58, p
< 0.05), Ald (r = -0.54, p < 0.05), Alp (r = -0.44, p < 0.05), Fep (r = -0.39, p < 0.05) and
allophane (r = -0.58, p < 0.05) contents. Samples from Italy and Iceland (22-100% and 73-
100%, respectively) showed the highest percentage of P desorbability. Other samples from
Portugal (6%-75%), Spain (Tenerife) (16-77%), Hungary (29-42%), Iceland (7-24%) and
France (13-20%) showed low percentage P desorbability. The lowest values were observed
for Acmdoxic Hydrudands. Differences in the amounts of P desorbed by the soils suggested
that the critical P levels needed for P management must be different.
We thank all members of COST ACTION 622 for the selection of the reference proftles,
Otto Spaargaren for soil classification and Isabel Meireles for technical assistance.
References
Fox, R.L., and E.J. Kamprath. 1970. Phosphate sorption isotherms for evaluating the
phosphate requirements of soils. Soil Sci. Soc. Am. Proc. 34: 902-907.
Villapando, R.R., and D.A. Graetz. 2001. Phosphorus sorption and desorption properties of
the spodic horizon from selected Florida spodosols. Soil Sci. Soc. Am. J. 65: 331-339.
96