Drip irrigation has little effect on structure
Environmental and management benefits from drip irrigation outweigh effects on soil condition and the method could be made even more efficient by addressing the naturally poor structure of Australian soil which is made worse by compaction under tractor wheels.
This is a general finding from a study funded by the National Program for Sustainable Irrigation and the Grape and Wine Research and Development Corporation.
Investigations into the sustainability of drip irrigation were appropriate considering its rapid uptake. In Australia drip irrigation could be found on just a few properties in the 1960s, yet by 2006, when the study started, around 80% of the 150,000 hectares of vineyards had precision systems. The vast majority of these systems were drip, with some micro-spray. Although the work concerned vines, there has also been wide adoption of precision systems in citrus, stone fruits, vegetables and other crops.
Some grapegrowers thought change was occurring to soil structure, so it was important to see if this was the case and if so, what was driving any change to physical and chemical properties of soil under the vines.
Early research did find some instances of minor structural decline and a build-up of salts around the wetted pattern created by drip irrigation. But it was recognised that a bigger contributor to structural change was compaction caused by vineyard traffic and that salt accumulation might not necessarily be problematic given a normal flush of winter rain.
Team leader Rob Murray from the University of Adelaide explains that as far as soil properties are concerned the big picture is dominated by the naturally poor structure of soils at depth, a fact reinforced by the study of structure under drip lines. It posed the question of what can be done to make structure better, and so enhance the efficient distribution of water through drip irrigation, whether above-ground or sub-surface.
“New vineyards may start well with pre-planting treatments to improve structure and root penetration,” he said.
“Unfortunately deep-ripping benefits are only temporary, normal mulching and cover-cropping have a positive effect on structure in the upper part of the soil but do not have a deep enough effect, and cultivation to treat compacted soil along wheel lines is only useful until the next tractor pass.
“This means we have to consider other ways to improve structure at depth and limit re-compaction.
“Over-row machinery, where wheel loads are in the centre of the space between rows, will ultimately improve the extension of vine roots by having the compacted wheel lines away from vine rows.
“The options are limited, however, for making real improvements to structure beyond 30 to 40 cm, although there is the potential for planting species of plants which can push their roots deeply through the hard soil layers. By penetrating further through the profiles, carbon will be added, and structure and water-infiltration should improve, enabling a bigger ‘root friendly’ volume for vine roots to explore.”
In summary, Rob Murray says that drip irrigation does not compromise soil quality but the study has emphasised the natural limitations of most agricultural soils and the need to look for new ways of improving structure at depth.
“Drip remains an efficient irrigation system which will be even more effective if ways are found for extending the spread of roots,” he said. “This will also add to the resilience of vines in extremely hot, dry, seasons.”
An example of drip irrigation efficiency can be found on Steve and Jinky Nicholls’ property at Dareton in NSW, where 27 hectares of vines have been under drip irrigation for two seasons. Previously served by undervine sprinklers, the vines are producing slightly better yields while still meeting the required quality specifications for winegrapes and dried vine fruit. The family has recorded a third less water needing to be applied under drip and less time being required to check the system. Last summer was hotter than normal yet the total water applied was only 6 megalitres per hectare.
“The management and water-saving benefits are clear and the environmental benefits are there too because we are not taking the spray tank through the vineyard causing compaction when the rows are still fairly moist from sprinkler irrigation,” Steve said. “Another sustainability advantage with drip systems being taken up in the district is that the water table had been lowered and outflows have been reduced.”
Local NSW Department of Primary Industries irrigation officer Jeremy Giddings said conversion to drip systems across the district was the main factor responsible for 80% less water going into evaporation basins since 1998.