Cotton is mostly grown on self-mulching,
cracking clay soils. In the past, it was widely assumed that under irrigation
there was no deep drainage in these soils. Recently, several researchers
have disagreed with this view.
Measurements made in southern Queensland and northwestern New South
Wales show that deep drainage rates between nine and 151 mm per year
are possible. At Warren, in the Macquarie Valley, deep drainage rates
of 17 mm per year in a soil with 53 per cent clay, and 202 mm per year
in a soil with 35 per cent clay have been measured under irrigated cotton.
Substantial amounts of salts and nutrients can also be transported with
deep drainage. But detailed studies of nutrient and salt leaching are
few, with most reporting only nitrates and chlorides. These studies
suggest that under saturated or near saturated conditions, such as under
irrigation, nutrient and salt leaching can be high.
The large amounts of nutrients which can be leached from the crop root
zone means, in turn, that the costs to the farmer can be high. For example,
nitrate-N leached in an irrigated field in south east Queensland was
227 kg per hectare per year. This is equivalent to 277 kg per hectare
per year of anhydrous ammonia which has an estimated market value of
$194. On the other hand, salt leaching out of the cotton root zone is
beneficial in that it allows a crop to grow unhindered. But leaching
of salts with deep drainage may increase groundwater salinity.
Trial Sites
Nutrient and salt movements were monitored on several grey, self-mulching
clay soils during the cotton seasons of 2000–01 and 2001–02.
The first site was near Wee Waa, the second in a sodic soil near Merah
North and a third near Narrabri. The Wee Waa site was sown with a cotton–wheat
(N-fertilised) rotation with stubble incorporation. At the Merah North
site, which was bore-irrigated, two treatments were studied —
back-to-back cotton and a cotton–wheat rotation. The site near
Narrabri was irrigated with treated sewage effluent and sown with a
cotton–wheat (N-fertilised) rotation with standing stubble retained.
Measuring nutrient and salt leaching
Ceramic cup samplers (Figure 1) were used to measure salts and nutrients
in drainage water under irrigated cotton crops. These samplers are a
cheaper alternative to field lysimeters, and relatively simple to install.
They also cause little disturbance of the soil profile and allow continuous
monitoring at different depths in the same profile.
Ceramic cup water samplers were installed under cotton crops at depths
of 0.6, 0.9 and 1.2 metres, and water sampled at frequent intervals
during both cotton seasons. But in this article we only report the results
from the 1.2 metre depth.
The water samples were analysed for chloride, nitrate-N, potassium,
calcium, magnesium and sodium concentrations. We also estimated seasonal
deep drainage using a chloride mass balance model. The seasonal drainage
was used to convert these concentrations to seasonal nutrient and salt
leaching in kg per hectare.
Nutrients and Salts in Drainage Water
Seasonal drainage estimates during 2000–01 were 98 and 76 mm for
back-to-back cotton and cotton–wheat, respectively, at Merah North
and 117 mm for the cotton–wheat rotation at Wee Waa. During 2001–02
drainage estimates in three separate fields at the Narrabri site were
22, nine and 89 mm, and 29 mm at Merah North.
Seasonal leaching of nitrate-N was high (Table 1). But soil and plant
N measured after a wheat crop was sown at both Wee Waa and Merah North
during the following winter showed that it was able to recover most
of the leached N.
The seasonal losses of calcium, magnesium and potassium (Table 1) were
far less than that supplied in irrigation water. Irrigation water can
add 20–30 kg per hectare of potassium, 100–140 kg per hectare
of calcium and 70–110 kg per hectare of magnesium during a cotton
season. At all three farms, deep drainage was able to leach excess salts
out of the root zone.
About one tonne per hectare of chloride was added in irrigation water
during the 2000–01 season at Wee Waa, and about six tonnes per
hectare at Merah North.
During the 2001–02 season, about four tonnes per hectare of chloride
was added in irrigation water at Merah North and 7.6 tonnes per hectare
at the site near Narrabri.
Conclusions
Leaching of salts out of the root zone took place under irrigated cotton.
If this did not take place, and given that salt loads in irrigation
water can be high, soil salinisation would take place very quickly.
So salt leaching with deep drainage is essential to maintain a sustainable
cotton production system.
Nutrients also leached out of the cotton root zone. This is costly and
requires suitable management strategies.
Deeper rooting crops like wheat can recover the nitrate-N that leaches
below the cotton roots. If salts and nutrients pass beyond the reach
of crops such as wheat, they may eventually leach into ground water
systems.
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