Geoscience Reference
In-Depth Information
Fig. 18.3 Effect of P on the modulus of rupture of three soils using a H
3
PO
4
, and b Ca(H
2
PO
4
)
2
as sources of P (modified after Lutz and Pinto
1965
). Copyright 1965 Soil Science Society of
America. Reprinted with permission
The effect of phosphorus on soil hardness was determined in laboratory
experiments by measuring the modulus of rupture of three soils with different clay
mineralogy. The effects of phosphoric acid, H
3
PO
4
, and monocalcium phosphate,
Ca(H
3
PO
4
)
2
, on the modulus of rupture of these soils are shown in Fig.
18.3
. The
modulus of rupture decreased with increasing rates of P; both phosphoric acid and
monocalcium phosphate led to greater reductions in the modulus of rupture in soils
containing higher amounts of 2:1 type clay. In all soils studied, phosphoric acid
had a greater effect than monocalcium phosphate.
The effect of P on water retention properties of soils was determined in labo-
ratory experiment by using the same soils and type of phosphate fertilizer
described above. Water retention was measured by determining the percent water
held at increasing pressure, from 1/10 to 15 bars. In all soils studied, P increased
the water-holding properties appreciably (Lutz et al.
1966
). Associated particle
charge determination established that P increased the negative charge of the par-
ticles and that surface charge is the dominant factor in water retention.
