Geoscience Reference
In-Depth Information
1.2
2000
No cover
1.0
0.8
0.6
0.4
1.2
2001
1.0
0.8
0.6
0.4
1.2
2002
1.0
0.8
0.6
0.4
1.2
2003
1.0
0.8
0.6
0.4
90
120
150
180
210
240
270
300
Day of Year
fIGURe 19.4
Distinctive shapes of the apparent soil conductivity (EC
a
) mean values shown with chrono-
logical events for each season.
manure and compost at the phosphorus rate (which had not been applied since 1998). Sequential
measurements of profile weighted EC
a
effectively identified the dynamic changes in available soil
N, as affected by animal manure and N fertilizer treatments, during the corn growing season. The
sequential measurements also clearly identified the effectiveness of cover crops in minimizing lev-
els of available soil N before and after the corn-growing season, when soluble N is most subject to
loss. Ferguson et al. (2003) reported that use of a winter cover crop was effective in reducing NO
3
-N
levels at depths below 0.5 m. Nitrate levels from 0.5 m to the surface tended to be higher under the
cover crop, indicating that the cover crop is releasing NO
3
-N in the upper portion of the soil profile.
Sequential EC
a
graphs indicate that the immobilized NO
3
-N is released when the +CC and -CC
curves converge. This 4-year study supports the initial findings of a 1999 study that soil conductiv-
ity appears to be a reliable indicator of soluble N gains and losses in the soil under study, and may
serve as a measure of N sufficiency for corn early in the growing season, as well as an indicator of
N surplus after harvest when N is prone to loss from leaching and runoff.
