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

For the older compiled information on the use of soil test methods for forest soils
refer to Ballard (1980). In their recent study (Mendham et al. 2002) compared
different methods to distinguish fertilized and unfertilized sites for determining
available P and suggested that the use of CaCl2 soluble P provided the best results
with respect to the growth responses in Eucalypt plantations. Khanna (1994)
evaluated various indices for measuring P status of forest stands by using both plant
based and soil based parameters from Pine and Eucalyptus sites. Plant based
parameters were P concentrations in the foliage, fine roots, and leaf litter, and plant
bioassay ( 32 P uptake by roots) and phosphatase activity of fine roots. Soil based
parameters included P extraction, anion exchange and Fe-impregnated paper,
microbial P, phosphatase activity in soils, and P mineralisation under laboratory
conditions. He concluded that P content in the fine roots, and Fe-impregnated paper
may be most useful to assess P status of forest stands.
The soil test methods were developed by involving some basic soil chemical
principles. Thus P extracted by these methods does not provide any arbitrary value but
a defined fraction of soil P. Some of these methods have been used for forest soils,
mostly to describe the P status in comparative situations, or to describe the immediate
effect of fertilizer or other silvicultural treatments (fire, removal of harvesting slash,
windrowing, addition of thinning slash). Absolute available soil P values obtained by
any of these methods are difficult to calibrate for forest stands using growth or other
responses and thus cannot be used to describe the long-term P supplying status of
these soils.
There are two other main issues which should be addressed:
1. As forest soils contain high amounts of organic P, many of these methods which
use high pH solutions will extract excessive amount of organic matter, creating
problems in the measurement of inorganic P.
2. Many forest soils are highly acid and it is possible that the amount of Al
complexing agent, e.g., F in Bray’s method may be too low to release P from
soils. However repeated addition of NH4F solution in sequential extractions may
overcome this problem, which arises because such methods were not developed
for forest soils.
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