Agriculture Reference
In-Depth Information
300
30
3
0.3
0.03
0.003
Solubility relationships between
pH and P for P fertilizer
Figure 5.7
reaction products in the soil
4
5
6
7
8
pH
(redrawn from White 1997).
As the pH rises, AlPO
4
and FePO
4
dissolve and hydrolyze to release P. Cal-
cium phosphates, on the other hand, become more soluble as the pH decreases.
The overall influence of soil pH on the solubility of P from fertilizer reaction
products is shown in figure 5.7. This diagram shows that, in a soil regularly fer-
tilized with P, the soil solution P concentration should be highest between pH 6
and 7.
Reactions of Insoluble P Fertilizers in Soil
. Natural rock phosphates are some-
times used as fertilizers in vineyards. In contrast to SSP and TSP, which are gran-
ulated, rock phosphates are best applied in powder form to increase the area of
contact between the fertilizer and the soil. Phosphate rock (PR) is arbitrarily clas-
sified as “reactive” or “unreactive” (hard) on the basis of the PR's solubility in
chemical extractants, for example, M ammonium citrate at pH 7 (Australia and
the United States), or 2% formic acid (European Union). Hard rocks such as
Florida pebble (from the United States) are used for the manufacture of soluble
P fertilizers, but the more reactive phosphate rocks (RPRs), such as from North
Carolina and North Africa, may be used for direct application. The dissolution
of these RPRs depends not only on the chemical composition of the rock itself,
but also on soil and climatic conditions. The general reaction in the soil is
Ca
10
(PO
4
)
6
F
2
12H
6H
2
PO
4
10Ca
2
2F
(5.7)
from which we see that the dissolution requires H
ions. Because H
ions are
consumed as RPRs dissolve, there is a small liming effect.
RPRs can be used on grapevines since they are perennial and do not need a
rapid uptake of P, except during the establishment phase. However, they are not
very effective unless used on acid soils (pH
6 in water) with adequate soil mois-
ture (
800 mm of rain or irrigation, reasonably well distributed through the year).
RPRs are acceptable in organic viticulture (section 5.7), but they should be ap-
plied to the soil only in a finely divided form.
