Tuesday, October 28, 2008

Irrigation Notes - UPDATE 28/10/2008

Consider the effects of this seasons lack of Spring rain on your soil moisture levels. As soil moisture dries out, or is used up by the plant, the shoot tendrils first shorten then are not present. As the plant further slows and the soil dries the shoot tips shortens until the leaves overlap the growing tip. The final sign of dry soil is the shoot tip becomes burnt as shown above.

Too much soil moisture can also be a problem. It is well known that over irrigation of grapevines grown in McLaren Vale can lead to the development of large vines that carry heavy crops of low quality grapes.

Initial irrigation experiments involving the application of Regulated Deficit Irrigation (RDI) to grapevines began in the late 1980s and involve reducing the water inputs to vines during the early stages of bunch development after both flowering and fruit set are complete.
RDI can reduce both bunch and berry size, and increase grape flavor, colour and aroma. When applied properly RDI improves fruit quality but has the potential to significantly decrease yield.

Shiraz and Grenache respond well to RDI irrigation. Cabernet Sauvignon does not seem to benefit from deficit irrigation. White grape varieties also are best kept ‘happy’ through flowering as stress can reduce quality.

RDI is effective when we have limited spring rain where soil moisture tensions measured by gypsum blocks in the vine root zone reach and can be maintained with careful irrigation in the range 200-400 kPa for heavy (clay) soils and 50-80 kPa for sands.

New soil moisture monitoring equipment measures soil moisture levels as a volume, mm per metre, of percentage of water. With these readings levels for RDI vary from site to site.

RDI may contribute to salinity problems where soils and irrigation waters are high in salt. It pays to be careful if your bore water quality is above 1500ppm.

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