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Fig. 18.1 Relationship
between exchangeable acidity
and N fertilizer rates in long-
term fertility plots. Error bars
indicate standard deviation of
the mean. Means indicated by
the same letter are not
significantly different at the
5 % level by ANOVA. Cubic
spline drawn through
treatment means (Barak et al.
1997 )
Fig. 18.2 Relationship
among exchangeable Ca 2+ ,
Mg 2+ , and K + +Na + +NH 4 + ,
and N fertilizer rate in long-
term fertility plots. Mean
comparisons are valid for the
same measure across
treatments. Means indicated
by the same letter are not
significantly different at the
5 % level by ANOVA (Barak
et al. 1997 )
cations and exchangeable acidity, showed strong correlation to the N fertilizer rate.
Long-term soil acidification due to nitrogen fertilizer input has also been con-
firmed through similar results obtained in other field trials in North America,
reported for Kansas silt loam (Schwab et al. 1989 ), Saskatchewan loamy soil
(Bouman et al. 1995 ), and Kentucky silt loam soil (Blevins et al. 1977 ).
Phosphorus forms include organic and inorganic fractions, consisting of com-
pounds characterized by different solubility and availability. Calcium phosphates,
which are the predominant stable compounds in soils, are characterized by various
solubilities as follows: H 2 PO 4 [ H 2 PO 4 2- [ PO 4 3- . Stable phosphate compounds
such as tricalcium phosphate and apatite are favored by high pH, while Al- and Fe-
phosphate are formed at low pH. In a field experiment testing phosphate application
on a red-yellow podzolic soil from North Carolina, Lutz et al. ( 1966 ) observed that P-
treated plots were moist, loose, and easy to plow, or had a lower bulk density and a
higher moisture content, compared with untreated control plots. Later studies (Lutz
and Pinto 1965 ;Lutzetal. 1966 ) on the effect of phosphorus on soil water retention
and soil hardness provide more specific information on this phenomenon.
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