Agriculture Reference
In-Depth Information
Figure I.32c presents the concentrations of total phosphorus in the A1 horizons of
1788 Australian soils. It can be seen that most of these soils have very low concentra-
tions; continents with younger landscapes and wider distributions of basic igneous rocks
are likely to have a greater proportion of their soils in higher concentration classes.
Much of the phosphorus in non-agricultural soils is organic and this form of phospho-
rus may assume very high proportions of total phosphorus in highly-weathered soils.
Singh
et al.
(1991) reported that organic phosphorus comprised 98
per cent.
of total
phosphorus in the highly weathered soils of an Indian savanna environment. In a con-
trasting climate‚ Walker and Adams (1958) found that‚ over 22 different New Zealand
soils and at depths to 53 cm‚ organic phosphorus was highly correlated with carbon
(
r
= 0.79) and comprised a large part (median 67 %‚ interquartile range 52-80 %) of total
(organic plus inorganic) phosphorus. Not surprisingly‚ the lowest ratios were found in
a few soils containing phosphorus-rich limestone. Further‚ the amount of organic phos-
phorus declined from a median value of 78
per cent.
in the surface 18 cm to 59
per cent.
