| As availability of irrigation
water decreases due to a combination of drought and legislation, many
cotton growers have considered using alternative sources of water for
irrigation. One such alternative is ‘grey’ water or treated
sewage effluent.
Treated sewage effluent contains large amounts of nitrogen and phosphorus
which can be used by the crop. It can also contain moderate to high
amounts of salts — particularly sodium and chloride salts.
So salinity and sodicity could increase in a field irrigated with treated
sewage effluent if not managed carefully. In this article we present
results of measurements made at ‘Federation Farm’, near
Narrabri — a cotton farm established by the Narrabri Shire Council
and share-farmed by the Narrabri Educational Trust. The farm is irrigated
with treated sewage effluent produced at Narrabri Shire Council’s
sewage treatment plant.
It started operating in May 2000. Prior to this the land was under rainfed
pasture.
We measured water quality and soil changes from shortly after the start
of the project until January 29, 2002.
Soil was sampled to a depth of 1.8 metres from paired plots, which had
either been treated with 2.5 tonnes per hectare of gypsum in June 2000
or remained untreated. These treatments were repeated in three adjacent
fields at ‘Federation Farm.’
The soils in the three fields were alkaline, self-mulching gray clays
with excellent structure, with clay content differing between fields.
Clay content in the surface 60 cm was 49 per cent in Field 1, 60 per
cent in Field 2 and 54 per cent in Field 3. Wheat was sown in June 2000
and cotton in October 2001.
Water quality during the
2001–02 cotton season
Treated sewage irrigation water was alkaline, and initially, moderately
saline and sodic. But as the season progressed, alkalinity, salinity
and sodicity increased markedly (Table 1).
This is probably because treated sewage water was diluted with winter
rainfall during the early part of the cotton season, whereas undiluted
treated sewage effluent was used to irrigate the cotton later in the
season. The results also show that relatively high amounts of sodium
(Na) were added in irrigation water.
The amounts of calcium (Ca) and magnesium (Mg) were much lower than
that added in bore and river irrigation water typical of the lower Namoi,
whereas Na was higher. Potassium (K) was also higher (between two and
three times) than in bore and river irrigation water.
Treated sewage effluent may, therefore, be a potential source of additional
K. Nitrate-N in treated sewage water was also high — between two
and eight times higher than that measured in other commercial farms,
even allowing for the urea applied as fertiliser.
In summary, compared with traditional water sources, treated sewage
effluent appears to be a good source of N and K but is more saline and
sodic.
As cotton seedlings are highly sensitive to salinity and the mature
crop is relatively insensitive, either river or bore water of low salinity,
or stored rainfall should be the preferred source of water for early
season irrigation. As the crop matures and becomes more tolerant of
salinity, treated sewage effluent can be used for irrigation.
Soil salinity, sodicity and nitrate-N
Relative changes in soil salinity, sodicity and nitrate-N differed between
the three fields sampled and were mainly due to differences in clay
content. But the general trends were similar.
Both sodicity, measured as ESP (Figure 1), and nitrate-N (Figure 2)
increased markedly in the subsoil between June 2000 and January 2002.
Sodicity almost doubled in the depths greater than 60 cm, whereas nitrate-N
increased between two and six times in the depths greater than 20 cm.
A distinct nitrate-N bulge is also present at about a depth of one metre.
EC1:5, an indicator of salinity, while decreasing in the surface 10
cm, did not change significantly in spite of the observed salt loads
(approximately four tonnes per hectare during 2001–02) in irrigation
water. But this is not surprising when deep drainage rates are considered.
Average deep drainage out of the 180 cm depth in gypsum-treated plots
was 64 mm, and in untreated plots was 42 mm and suggests that most of
the salts which entered the root zone were leached out. Similar differences
existed throughout the soil profile. The relatively high EC1:5 in the
surface 10 cm during June 2000 is probably due to a high nutrient load
after the pasture.
Gypsum application did not have any effect on sodicity and salinity,
but resulted in significantly lower nitrate-N in the depths below 60
cm. The lower nitrate-N in gypsum-treated plots is a reflection of their
higher drainage. The higher drainage, in turn, is due to the better
soil structure which results from gypsum application.
But overall, the effects of gypsum application in 2000 were small or
not measurable by January 2002, and suggests that more frequent and/or
higher rates of gypsum are required at ‘Federation Farm’.
Alternatively a combination of lime and gypsum may result in longer-term
effects. Other ameliorants such as polyacrylamide (PAM) should not be
ruled out as well.
Conclusions
• Treated sewage irrigation water was moderately saline and sodic,
and in comparison with bore and river water, had higher concentrations
of Na, nitrate-N and K, and lower concentrations of Ca and Mg.
• Irrigation with treated sewage effluent caused nitrates to accumulate
in the subsoil, increased soil sodicity but not salinity.
• Salinity did not increase because of salt leaching out of the
root zone.
• A nitrate ‘bulge’ is present at a depth of about
one metre.
• Gypsum applied in 2000 had no obvious effect on soil salinity
and sodicity, and only a small effect on nitrate-N.
• Some possible management options, such as ‘shandying’
of effluent with rainfall or river water, more frequent application
of gypsum and use of polyacrylamide (PAM), may be required.
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