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

Table 8 : Sequential extraction procedure to define soil P fractionation and their
lability with respect to P uptake (Modified from Hedley et al. 1982)
Extraction step Lability and fractions
1. Resin Pi Labile - soluble and
exchangeable
2. NaHCO3 Pi Labile - adsorbed
3. NaHCO3 Po Readily mineralisable
4. NaOH Pi Fe and Al – associated
5. NaOH Po Slowly mineralisable
6. IM HCl Pi Ca – associated
7. Conc. HCl Pi Occluded P and secondary
mineral P
8. Residual P – Pi and Po Primary and secondary
mineral P and organic P
Table 9: Commonly employed soil test methods to determine available P
Soil group Soil pH Most common
P forms
Available P methods
Highly acid to 7.2 Ca-P, Mg-P Anion exchange resin,
calcareous
Olsen, lactate methods
3.2 Factors affecting P status of forest soils
3.2.1 P concentration in soil solution : the controlling component of P uptake
Concentrations of about 1 mM P are commonly required in soil solution to supply P
in adequate amounts to plants, however, P concentrations rarely exceed 10 µm
(Bieleski, 1973). Moreover, P concentrations can vary over a wide range depending
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