Environmental Engineering Reference
In-Depth Information
4.2
Methodology
Two pasture blocks that have been under irrigation for the past 22 years were selected for
sampling purposes. One is the sprinkler-irrigated block close to the pump house and
adjacent to the stabilization ponds, which is the one most frequently irrigated. The other
block is under furrow irrigation and is the least frequently irrigated one. The control site
is an area, located at the highest position, and reflecting the upper part of the aquifer flow
gradient. It is located at 1 km from the irrigated area. It has never been irrigated and
previous land use for the control site was maize production without the addition of
fertilizers. Other factors that were considered in site selection include accessibility,
similar soils and the availability of wells for groundwater monitoring.
Sampling was done from November 2000 to April 2001. For both the irrigated and
control sites, 4 replicates of soil samples were collected from each site, following
recommendations of Henning et al. (2000). As shown in Figure 10.3, points C, S and F
indicate sampling locations for soil samples from the control site, sprinkler and furrow
irrigated sites respectively, and Gw indicate sampling locations of ground water samples.
During the sampling procedure, areas that showed evidence of localized ponding, cow
dung, erosion and other visible disturbances were avoided. Samples were collected at 0-
30 cm, 30-60 and 60-90 cm depths using a 50 mm diameter steel hand auger. For the
control sites, sampling was done on land that had the minimum disturbance from
cultivation. To avoid contamination, soil sampling started from the control to the irrigated
sites following the recommendations outlined in Rubio & Vidal (1998). When sampling
the irrigated sites, the auger was washed with distilled water to minimize cross-
contamination. Samples were collected in clean polythene bags and labeled with prefixes
“C” to indicate control, “S”, sprinkler and “F”, furrow irrigated sites. The following
parameters were tested: pH, EC, nitrates, ammonia, phosphates, K, Ca, Cd, Cr, Cu, and
Zn. Samples for nutrients were analyzed in the Analytical Services Laboratory (ASL), the
Department of Soil Science and Agricultural Engineering, University of Zimbabwe, and
for metals in the Institute of Mining Research (IMR) laboratory.
Parameters that are sensitive to pre-treatment such as ammonia, nitrate and pH were
analyzed first before air-drying the samples. For parameters that are relatively insensitive
to drying, soil samples were air-dried in a well-ventilated room in wooden boxes. The
soils were ground, using a mortar and pestle, passed through a 2 mm diameter steel sieve
and kept at 4
o
C until analyzed, following recommendations of Page et al. (1982). All
analyses were done within 72 hours of sampling.
The soil characteristic parameters were tested according to Page et al. (1982).
Ammonium and nitrates were extracted from soil using 2N potassium sulfate (KSO
4
)
followed by steam distillation. Ammonia and nitrate were then determined by the
Salicylate method using an ultra violet/visible (uv/vis) spectrophotometer (model: PYE
UNICAM PU 8600, Philips Pvt. Ltd). Phosphorus as ortho-P was determined by the Bray
1 method followed by ultra violet/visible (uv/vis) spectrophotometer (model: PYE
UNICAM PU 8600, Philips Pvt. Ltd). Exchangeable bases (Ca, Mg, K and Na) were
extracted by acidified ammonium acetate. Ca and Mg were then determined by an atomic
absorption spectrophotometer (AAS) (model: PYE UNICAM SP9) employing
nitrous/acetylene as fuel. Na and K were determined by a flame photometer (model:
