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

upon soil and plant processes and the amount of P inputs. Among the several factors
influencing the phosphorus concentrations in soil solution, soil pH, organic matter
content, clay content, oxides of Fe and Al, carbonate content, uptake patterns of roots
and microbes, and organic matter mineralisation are the most important ones. To
describe changes in P concentration of the soil solution, sorption isotherms are used,
which account for both intensity and capacity factors of P equilibrium in soils. These
two factors and the kinetics of P release and uptake constitute important parameters
for predicting the P supplying characteristics of soils for optimum plant growth. Any
soil test that involves sorption isotherms is considered more accurate than
conventional P analysis methods (Klages et al. 1988) as isotherms may include
several factors mentioned above which influence the phosphorus dynamics in soil
(Monterroso et al. 1996). P sorption isotherms describe the amount of P required to
bring the concentration of P in the soil solution to an optimum level required for
maximum plant growth.
Concentrations of elements in soil solution can be used to calculate the so called ‘ Ppotentials’
or ‘Schofield’s potentials’. These P potentials are related to lime potentials
or pH values in the soil solution to obtain solubility diagrams. Such diagrams provide
information on the predominant form of solid P which may be determining P
concentrations in the soil solutions (Schofield 1955). One such diagram is given in
Fig 12 which was taken from Ulrich and Khanna (1968). This figure is based on 230
forest soil samples from northwest Germany. All the values presented in the figure
(pH, Ca, Al, P) were obtained in soil solutions by preparing saturation extracts of field
moist soils.
The Fig. 12 shows a number of categories, where soils of distinct groups were
combined. The groups were based on their soil pH (lime potential), calcium carbonate
content, organic matter content, and soil depth (water logged soils, and those
receiving fertilization were excluded). In soils of pH >6.8 and calcium carbonate of
>2% of soil weight concentration of P in the soil solution was defined by Ca-P forms,
which were more soluble than apatite but less than octacalcium phosphates. In
contrast, in soils with