assessment of changes in the phosphorus status of forest ...

or Fe-oxide impregnated paper (Menon et al. 1989, Chardon 2000) have also been
used as a P-sink to determine available P in a wide range of soils.
Given the wide range of methods to test for different P fractions in soils, it is very
important to select the right test for specific soils and target fractions. The soil
properties affecting selection of the appropriate P test and recommended methods are
outlined in the Table 9.
Da Silva and Van Raij (1999) reviewed different methods of measuring the available
P content in agricultural soils. The methods considered were: anion exchange resin,
Olsen, Bray 1, Bray 2, Mehlich 1, Troug, Egner, water, 0,01M CaCl2, iron hydroxide
impregnated filter paper (Pi) and the determinations of the E and L values. Details
about these methods are given by Kuo (1996). Average values of the goodness of fit
for regressions between the soil test method and P uptake of plants and corresponding
numbers of articles (in brackets) in which the method was considered were obtained,
showed the following descending order: resin, 70% (34); E value, 68%(16); L value,
65% (8); Olsen, 54% (48); Bray 1, 50% (42); Mehlich 1,46% (25); Egner, 44% (9);
Bray 2, 42% (19); Water, 42% (15); Truog, 38% (13); CaCl3, 36% (13), and Morgan,
32% ( 13). The results show that the anion exchange resin method was superior to
other methods. It has the additional advantage that it can be used for both acid and
alkaline soils, has probably the best theoretical basis, and does not overestimate the
amount of available P as may be the case when insoluble P forms get dissolved by
acids extractants.
Neyroud and Lischer (2003) compared the methods used for soil P availability across
Europe which involved 16 different methods on 135 soils from 12 countries. Each
European country was using its own method for the determination of phosphorus
availability to plants, together with an appropriate interpretation scheme of the P
status and fertilizer recommendations. The amount of extracted P decreased in the
order Ptotal > Poxal. > PAL > PMe3 > PBray > PAAEDTA, PDL, PCAL > POlsen > Ppaper strip,
PAAAc, PMorgan > PH2O, PCO2, PCaCl2. Isotopically exchangeable P was also measured.
Even though all the methods reacted in the same way to increasing amounts of added
P in several trials, there were wide differences between results obtained with different
methods. The interpretation schemes for P status suggested that about 50 % of the
tested soils were P-deficient.
Sibbesen (1983) ranked different P-tests on the basis of data from 29 published papers
into three groups: best group: anion-exchange resin method; intermediate group:
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