Most lucerne producers spray irrigate soon after mowing and baling, but other factors such as lack of rain, heatwaves and drying winds can mean additional irrigation is needed. The appearance of the crop is the most widely used guide to the need for irrigation. Visual symptoms such as a general darkening in crop colour, tendency to wilt and cessation of growth, and drying and cracking of surface soil all indicate that the optimal irrigation time has passed.
For optimal production, soil water levels should be maintained above 500/0 of water available to plants. There are an increasing number of scheduling tools available. If properly calibrated, these can be used to monitor the soil water status of the lucerne paddocks and assist in making irrigation decisions.
A steel probe can also be used to determine evenness of application and the depth of moisture immediately after irrigation.
Drainage
The life of a lucerne stand is very dependent on adequate drainage. Obvious hollows where irrigation water will lie should be levelled before planting. Provision should be made for excess irrigation water to drain off the paddock as quickly as possible.
Water quality
Crops vary in their sensitivity to root zone salinity. The average root zone salinity (ECse) refers to the salt content of the soil - water in the crop’s root zone. Table 7.1 shows the placement of a range of commercial crops into salt-tolerance groups — for more detail on the effect of irrigation water salinity upon crop growth refer to the NR&M Waterfact Irrigation water quality: salinity and soil structure stability.
Table 7.1 Tolerance of plants to salinity in irrigation (in increasing tolerance within each group — this is a general guide only).
Table 7.1 Tolerance of plants to salinity in irrigation
The tolerance of lucerne to saline irrigation water varies with irrigation management practices as well as the stage of growth, soil type and climate. The amount and intensity of rainfall during crop establishment and growth also has a marked influence.
Where saline water is used to irrigate lucerne use the following guidelines:
• Each irrigation application should be as much as the soil type will take without wasteful run-off and within the guidelines previously outlined.
• Intervals between irrigation should be as long as possible — avoid frequent light irrigations.
• Whenever possible, irrigate at night, particularly during hot periods.
• To prevent scalding, avoid irrigating under windy conditions.
Management of saline water is particularly important during the seed germination and establishment phase. Producers with saline water supplies tend to use high planting rates (e.g. 20-30 kg/ha).
In parts of the Lockyer Valley where very saline waters are applied to lucerne (in excess of 2000 mg/L chloride and up to the electrical conductivities of 5 dS/m) seedlings can, and do, die if good management practices are not followed. It is imperative to:
• plant in April - May, into a well-prepared seedbed containing sufficient moisture following wet-season rains;
• withhold irrigation for as long as possible (ideally up to first cutting) to enable best possible root development;
• avoid frequent light irrigations;
• select deep, well-drained soils in which rainfall will leach salts from the main root zone.
In some parts of Queensland, alkaline irrigation waters can present problems. These waters are usually high in sodium bicarbonate and/or sodium carbonate, which are indicated by a high residual alkali figure in water test results. These salts combine with calcium and magnesium in the soil, removing them by precipitation, with a resultant increase in the relative sodium concentration.
In lucerne continually irrigated with alkaline water, soils become hard and cloddy, and structure deteriorates. Eventually, water penetration is reduced, leading to surface ponding and the ultimate death of the stand. If use of alkaline water is unavoidable, high application rates should be used at widely spaced intervals.
Where poor quality water is used in which the chief salts are sodium salts and the sodium absorption ratio (SAR) is greater than 5, gypsum applications can result in improved soil structure and water penetration. Gypsum should be applied before planting at the rate of 5-7 t/ha and worked well into the soil.
Further reading:
Irrigation Water Quality: Salinity and Soil Structure Stability, NR&M Waterfact, available from Queensland Dept Natural Resources and Mines website http://www.dnr.q1d.gov.au , or DNRM Free Faxback Line 1800 200 691.
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