Soils overfertilized with phosphorus (P) are widespread in the Europea
n Union and there is consensus among soil scientists to better explore
their potential to release phosphate. In this work we report the prin
cipal physical and chemical properties of twelve overfertilized benchm
ark soils of contrasting agricultural areas in Italy, Germany, Great B
ritain, and Spain. The criterion used to consider them as overfertiliz
ed was that the available P amount, measured by the regional soil P te
st, was at least twice as large as the accepted critical level for an
average crop. The soils could be usefully divided into three groups, c
alcareous, slightly acidic and acidic based upon their basic chemical
properties and reactions and proportion of the major P fractions (NaOH
- plus citrate-bicarbonate-, citrate-bicarbonate-dithionite-, and HCl-
extractable P). Six extraction procedures commonly used to evaluate po
tentially plant-available P (Olsen, anion-exchange resin (resin-), ani
on plus cation-exchange resin (resin+/-), Ca acetate-lactate (GAL), wa
ter, and Fe oxide-impregnated paper strips (strip) were compared. The
extractable P values by each method were correlated but the amount of
P extracted varied and differed in the order r-P<Olsen-P<CAL-P<strip-P
<resin(+/-)-P<resin(-)-P. Olsen-P and strip-P extracted a proportion o
f the more available P fraction (NaOH- and citrate-bicarbonate-extract
able P), which increased with increasing pH, and decreasing amounts of
active Fe and Al forms in soil. Consequently, pH can be used in conju
nction with simple soil P tests to provide a first evaluation of poten
tially releaseable P.