Agriculture Reference
In-Depth Information
concentrations that were obtained by the VMS with those obtained from
the sediment extract shows that nitrate concentrations in sediment sam-
ples from the upper part of the vadose zone under the organic greenhouse
gradually increase from an average value of 8.8 mg kg
−1
in the root zone
to 85.8 mg kg
−1
at a depth of 3m (Fig. 6). However, nitrate concentrations
in sediment samples obtained from the conventional greenhouse were sig-
nificantly lower, with an average value of 14.7 mg kg
−1
over the entire
measured profile. The difference between the mean nitrate concentration
under the conventional and organic farms at depths of 1 to 3m was found
to be significant (P = 0.03 for a two-tail t test). Apparently, the nitrate con-
centration pattern that was obtained through the sediment samples is very
similar to the nitrate concentration pattern that was observed by the VMS.
For convenience of comparison, nitrate concentration in water samples
that were obtained by the VMS (Fig. 3) were converted in Fig. 6 from
concentrations in pore water to concentrations in dry soil.
To extend representativeness of the fi ndings, the survey was extended to
six greenhouses, three organic and three conventional. All six greenhouses
were established at the same time, in the same area (within a distance of up
to 5 km from each other). To avoid potential bias due to farmers' specifi c
working methods, the extended survey included farms that were all owned
and operated by different farmers. The top 3m of the sediment in each
greenhouse was sampled in two boreholes with a depth sampling resolu-
tion of 0.5m (total of 72 sediment samples). The nitrate concentrations in
the sediment samples from the extended survey showed ambiguous results
(Fig. 7). Two organic greenhouses exhibited nitrate concentrations that
increased with depth, from 27.1 mg kg
−1
at 0.5m to 133 mg kg
−1
at 3 m. On
the other hand, the nitrate concentration patterns under all the convention-
al greenhouses exhibited lower average concentrations, ranging between
34.2 to 44.1 mg kg
−1
. This difference in nitrate profi le is very similar to the
pattern that was observed by the VMS over a long period of continuous
monitoring (Fig. 2). Nevertheless, nitrate concentration profi les under one
of the organic greenhouses exhibited a rather low concentration profi le
with an average value of 29.9 mg kg
−1
(ranging between 27.4 to 32.9 mg
kg
−1
). These results, however, did not match the results obtained from the
other organic greenhouses. A closer examination of the fertilization meth-
ods practiced by the farmers showed that in the two greenhouses where ni-