Geoscience Reference
In-Depth Information
18.1.1 Fertilizers
Nitrogen forms detected in soils include NO
3
-
,NO
2
-
, exchangeable and mineral-
fixed NH
4
+
, and nitrous oxide (N
2
O
2
). Mineral-fixed NH
4
+
is a function of soil
type and environmental conditions, comprising 10-50 % of the total N. When
large amounts of NH
4
+
are applied, it can, theoretically, become a dominant
exchangeable cation and, like Na
+
, may favor dispersion on soil colloids. How-
ever, in most situations, NH
4
+
is rapidly nitrified to NO
3
-
and may cause dis-
persion of soil colloids only under unusual conditions of low pH and low moisture
content (Haynes
1984
). In early agricultural management schemes, when this
process was ignored, NH
4
+
-induced dispersion of soil colloids caused reduced
infiltration rates and crust formation. For example, Pilsbury (
1947
) reported a
significant decrease in the soil infiltration capacity of an irrigated Californian soil
after application of (NH
4
)
2
SO
4
. This resulted in a pronounced decrease of soil pH,
a drastic decrease in nitrification, and a large accumulation of NH
4
in the soil.
A decreased nitrification process under low temperature conditions caused soil
colloid dispersion, crust formation, and erosion following broad application of
NH
4
NO
3
on soil in Nebraska (Fox et al.
1952
). Soil dispersion and crusting after
continual application of (NH
4
)
2
SO
4
on an irrigated soil were also reported by
Aldrich et al. (
1945
). They concluded that under irrigation, dispersed soil colloids
were transported at depth, clogging the subsurface pores, and irreversibly changing
the infiltration rate of the NH
4
+
applied to the soil. Moreover, soil dispersion and
crust formation following NH
4
+
accumulation can stimulate soil erosion, which is
also an irreversible process.
Analysis of long-term fertility trials has revealed changes occurring in soil
following continuous application of nitrogen fertilizers. For example, Barak et al.
(
1997
) examined long-term soil acidification due to nitrogen fertilizer in fertility
trial plots at Arlington Agricultural Research Station, Wisconsin. The trial plots,
located on Plano silt loam soil (Argiuoll), were initiated by L. A. Paterson in 1962
and subsequently cultivated under various crops; over a 30-year period, the plots
received annual amounts of nitrogen (as ammonium nitrate or urea) of 0, 56, 112,
and 168 kg N/ha.
Exchangeable acidity of soils was strongly dependent upon the rate of N fer-
tilization applied during the course of the 30-year fertility trials (Fig.
18.1
).
Exchangeable acidity of the 112 and 168 kg N/ha/yr treatments averaged 0.48 and
1.58 cmol
c
/kg greater than the control plot, which did not receive N fertilizers. The
increase in exchangeable acidity following N fertilization led to a decline in the
soil exchangeable base cations and is related directly to the amount of N applied.
The relationship among exchangeable Ca
2+
,Mg
2+
and K
+
+ Na
+
+ NH
4
+
,
and N fertilizer rate in the long-term fertility plots is presented in Fig.
18.2
. Under
the higher N treatment, the declines in the soil contents of Ca
2+
and Mg
2+
were 31
and 36 %, respectively. No statistically significant response of summed mono-
valent cations to N fertilizer levels was observed. The cation exchange capacity of
the Arlington long-term fertility plots, determined as the sum of exchangeable base
