Fig. 6.32b. Parallel-oriented thin prismatic form is a commonly found character of
weathered apatite in a young Andisol. As the EDX spectrum Fig. 6.32c lacks X-ray
counts for Al and Fe, there is no coating of active Al or Fe materials on the surface of
the weathered apatite, suggesting that weathering of apatite in and around the dashed
square (c) is congruent dissolution of apatite. The reaction of apatite and active Al is
different from the formation of a Ca oxalate coating on the surface of apatite
(Fig. 6.31).
6.4.2 Reactions of Phosphate with Active Al and Fe Materials
Phosphate concentration in soil water is generally low, and it is less than 10
À6 mol L
À1
in mature Andisols, which are rich in active Al materials. This low P concentration
in soil water is less than that at dissolution equilibrium of AlPO 4 Á2H 2 O, variscite.
The reason for the low P concentration in the soil water is that P is sorbed by active
Al and Fe. In general, P sorption by active Al and Fe occurs at lower P concentration
than precipitation of AlPO 4 Á2H 2 O from soil water. With increase in the amount of
P sorption, P concentration in soil water and the amount of plant-available P increase
gradually.
0
2
4
6
8
A
A2
C
2A3
2Bw
a
b
c
d
c
d
Energy (keV)
Ca
P
Fe
Fig. 6.32 Weathering of apatite in an Andisol. (a) Soil profile of Hapludand, (b) SEM image of a
sand particle including partially weathered apatite and allophanic material, (c and d) EDX spectra of
the dashed areas (c) and (d), respectively
168
6 Role of Inorganic Soil Constituents in Selected Topics