Environmental Engineering Reference
In-Depth Information
60-90
F4
4
102
401
2
0
Average 4.98
142.75
341.50
1.78
0.25
60-90
S1
4.7
110
313
2.7 0
60-90
S2
5.9
88
636
3.2
0
60-90
S3
5.5
138
496
3.4
0
60-90
S4
5.6
119
447
3
0
Average 5.43
113.75
473.00
3.08
0.00
Ammonium concentrations in soils at the irrigated sites were higher than those at the
control. There was a significant difference between the control and the irrigated sites at
all depths. This can be attributed to anaerobic conditions that inhibit the nitrification
process. For each site, highest concentrations were found in the topsoil (0-30 cm). This
could be expected because according to Reemtsma et al. (2000) the ammonium cation is
adsorbed on the negatively charged sites of the soil by cation exchange reactions.
Ammonium concentrations in the sprinkler-irrigated site were significantly higher than
those of the furrow-irrigated site at all depths. However, in the sprinkler irrigated site
there was a higher level of ammonium at 60-90 cm depth than at 30-60 cm depth,
evidencing the movement of the NH
4
down the soil profile.
In general, the nitrate concentrations are low. Only the furrow-irrigated site had a
significantly higher nitrate concentration than the control and the sprinkler-irrigated sites
at 0-30 cm depth. In general, this could be explained with the low rate of the process of
nitrification, together with the intensification of the denitrification processes, due to
anaerobic conditions in the soil created by a high hydraulic load, together with high
organic and nutrients loadings. It could be argued that the frequency of irrigation and its
attendant effects on aeration of the soil had a more profound effect than the method of
application (sprinkler or furrow). For all the sites and soil depths, partial co-relation
between the ammonium and nitrate levels in the soil could be observed. For example at
the 60-90 cm-depth profile in the sprinkler-irrigated site, the low levels of nitrates in the
soil are supported by a corresponding higher concentration of ammonia (Table 10.6).
Although conditions are mainly aerobic at the control site, the low concentration of
nitrates was due to limited amounts of ammonia and organic nitrogen.
The control point showed very low concentrations of ortho-P. Similar results have
been reported by Grant (1995), who noted that the sandy soils of Zimbabwe are generally
deficient in P. A marked difference in ortho-P concentrations between the control and the
irrigated sites was found. At the 30-60 and 60-90 cm depth profiles, there was a
significant difference among the three sites. This accumulation of P in the soil was
consistent with data obtained by Oloya & Tagwira (1996) and Thompson (1968) in their
separate studies on the effect of effluent irrigation on soil properties at Aiselby and
Goodhope farms in Bulawayo. For the irrigated sites, there was evidence of P movement
down the profile. This can be explained by the saturation of the P adsorption capacity of
the soil over time. This downward movement was enhanced by over-irrigation, which
was reflected in the significantly higher ortho-P concentration in the sprinkler-irrigated
site at 60-90 cm than the furrow irrigated. Nyamangara & Mzezewa (1999) also observed
