Agriculture Reference
In-Depth Information
taken during the rainy season and four samples for the dry season
(corresponding to 12 months of the year). Water destined for use in the
market garden should have a pH of 6.5-8.4 (FAO, 2003). The chloride
concentration should be approximately 600mg/l, and the maximum electrical
conductivity should be 2500 mS/m (Niang, 1996). The results in Table 8.4
indicate that most of the water sampled met these criteria. Monthly
variations in pH and temperature proved to be slight. Irrigation water was, in
most cases, acidic and at optimum levels for aquatic species (Nisbet and
Verneaux, 1970; Arrignon, 1991). Average values for total dissolved solids
indicate that the water is not very saline.
Turbidity and suspended solids of the irrigation water. Turbidity
measures the clarity of water. Suspended solids are any particles larger than
2 mm. The turbidity determined for the two seasons studied ranged from 10 to
79mg/l for all water samples (Table 8.4). The results for turbidity and
suspended solids indicated the irrigation water was polluted and, although
pollution will not affect the porosity of the soil, it affects aquatic life. For
example, in water where there are high levels of turbidity, fish exhibit
thickened skin cells in the gills, reduced productivity and changes in behaviour
(Nisbet and Verneaux, 1970; Rodier, 1984).
Chemical oxygen demand of the irrigation water. Chemical oxygen
demand (COD) is an expression of the water's organic load and testing
involves determining the total amount of organic matter in the water. The
irrigation water of the Centre Agricole de N'djili was found to be rich in
organic matter: values ranged from 294 to 444mg/l (Table 8.4). The highest
concentrations at most sampling locations were registered in the dry season,
suggesting a change in farming activities or concentration affected by
dehydration.
Total alkalis and sodium contents of the irrigation water. Total alkalis
content is related to the presence of bicarbonates and especially to sulphates,
phosphates and silicates (Rodier, 1984). The alkalinity varied noticeably
according to the month, but seasonal variations were not remarkable (Table
8.4). The research found, however, that sodium (Na) concentration for all
samples was very high (Table 8.5). High concentrations of sodium in the
irrigation water are likely to increase the alkalinity of the soil through
leaching and reduce its permeability, particularly at the surface, where
crusting will be observed (FAO, 2003). The literature recommends adding
calcium, such as gypsum or calcium carbonate, to amend sodium-rich soil
(FAO, 2003).
Major fertilizer elements in the irrigation water. Wastewater carries
nutrients that fertilize the soil; however, in excess, macronutrients can damage
the environment. The average concentrations of N
NH
4
,N
NO
2
,N
NO
3
,
PO
3
4
and K
รพ
we found in our study are depicted in Table 8.5. These results
indicated high concentrations of nitrate and potassium, low concentrations of
phosphate and even lower concentrations of nitrite. The presence of NH
4
in
water indicates organic pollution by micro-organisms, especially of fecal
origin. Exposure to high doses of nitrates is accompanied by an increase in the
