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

phase: a) anion replacement to enhance desorption, b) complexing of cations binding
P to release P, c) hydrolysis of cations binding P to release P and d) dissolving action
of acids. Therefore, the selection of a soil test for P should depend on extracting P
from certain chemical forms of interest based on the soil characteristics.
Some these methods are reviewed below:
Among the use of different extractants the use of water to extract P from soils is
restricted to soils, to which very high amounts of P have been added and are present
in easily dissolved forms in soils. This is not the case for forest soils in general. Only
small amounts of soil P can be extracted by water creating problems in chemical
analysis. However despite the very low absolute values obtained by water extraction,
these values relate to the intensity parameter of P availability in soils and have been
used in relating them to capacity parameters of soil P obtained by using other methods
(see Ulrich and Khanna 1968). The limitations of its wide scale use are both the
cumbersome extraction procedures to achieve equilibrium solution phases and the use
of such methods for huge number of survey type soil samples.
One of the most common methods used for forest soils is the so called Bray method
(Bray and Kurtz 1945). The extractant is a mixture of HCl and NH4F, where F acts to
complex the Al and Fe and the P bound to them is released. The amount of acid added
is sometimes varied depending upon the form of P to be extracted. The extractant with
low HCl concentration is called the Bray1 method. This method is specifically
designed to be used in acid soils. Bray 1 and Bray 2 (high HCl concentration)
methods are commonly employed for forest soils, especially those with high amounts
of organic matter. This method has been used to show short term changes in the soil
due to treatments such as fire (Romanya et al. 1994), soil heating (Serrasolsas and
Khanna 1995), or fertilizer additions. However, Compton and Cole (1998) reported
that alder stands which had higher P dynamics (uptake, litterfall content, resorption
values) than Douglas fir stands, had lower Bray P value and total P in the soil,
suggesting that the quantification of an available P pool alone does not adequately
reflect P supply in forest stands. When nutrient pools are small, the flux through these
pools, must be considered as well to obtain a realistic picture of nutrient supply from
that pool.
For calcareous and alkaline soils the method commonly used employs 0.5 M NaHCO3
solution at a pH of 8.5 to extract available soil P (Olsen et al. 1954). This extractant
decreases calcium in solution (through precipitation of calcium carbonate), and thus
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