Increasing competition on supply of fresh water for irrigation by agricultural, domestic, sports and industrial users demands water use efficient irrigation methods and compliance with environmental regulations. Drip irrigation (DI) and subsurface drip irrigation (SDI) are advocated for improvements in water use efficiency (WUE) and are increasingly being adopted by horticultural industries in Australia and overseas. Greater flexibility for automation and versatility of application of drip irrigation technology encourage wider-scale adoption by these industries. However, the higher initial investment for installation and lack of significant yield gains due to drip irrigation compared to conventional irrigation methods are somehow deterrents for broad-scale adoption.

Ways to optimise the use of DI and SDI will have multiplier effects on water savings for irrigation in agricultural and other industries and minimize environmental impacts associated with traditional irrigation methods. One of the significant areas where greater optimization of DI and SDI is realized is through the use of aerated water for irrigation (oxygation). Sustained wetting fronts around emitters associated with DI/SSDI impose hypoxia in the rhizosphere. This impedes root respiration leading to sub-optimal plant performance. As irrigation water exits an emitter, it purges soil pores of soil air (containing up to 20% by volume of oxygen) with water that contains less than 10 ppm oxygen, a quantity we have shown is used up quickly by roots and soil microbes. Rising soil temperatures, salinity, and soil compaction will exacerbate this effect, as may disease such as Phytophthora of pineapple. Plant roots and soil microbes require oxygen for respiration.

In soils with inadequate aeration the lack of oxygen results in reduced plant growth and diminished productivity for many reasons, including: reduced root growth and root size; reduced root ability to absorb minerals and water; reduced photosynthesis and plant growth due to stomatal closure; loss of soil N due to the in-activity of microbes; adverse changes in soil chemistry; increased susceptibility to disease, and an alteration of the balance and supply of plant growth regulators.

Aeration of the irrigation stream, a process termed ‘oxygation’, overcomes this constraint. Oxygation is a new innovation in irrigation technologies. Aerated DI and SDI by different methods, such as venturi for air injection, allows for the simultaneous application of water, air and other agro-chemicals directly to the crop root zone. Therefore, it can potentially improve crop yield and water use efficiency. Conventional irrigation methods such as flood irrigation have large inefficiencies due to run-off, drainage and evaporative loss. SDI can significantly improve the WUE over that of flood irrigation, and oxygation can significantly improve WUE of SDI.

Oxygation involves mixing air with water using a venturi and delivering via a surface or subsurface drip irrigation system. An oxygation system can be installed as part of a new SDI system or may be retrospectively fitted to any existing SDI system. A venturi air injector is installed within the pipeline and draws air directly into the water stream. A single venturi can be installed immediately after the pump outlet and the air distributed through the main line to sub mains and lateral drip lines, or a single injector may be fitted to the beginning of each drip line. The amount of air ingress depends on the pressure differential across the venturi and the motive flow through the venturi.

Mazzei or Netafim Air Injectors improve soil aeration by entraining air (in the form of micro-bubbles) into irrigation water. The added air improves growing conditions, increasing root respiration and microbial activity. These improved soil conditions have resulted in significant increase in yields. All NPSI funded project activities in this report utilized Mazzei air injectors.

System requirements include drip/subsurface drip irrigation, water flow must be 3.8 LPM - 30.3 LPM per drip line (for MI 384, 584 and 1583 injectors) and the terrain must be level to moderately sloped. We are also evaluating alternative approaches for super saturating irrigation water with air using twin vortex, oxysolver and Seair diffusing systems and plan researching benefits on furrow and sprinkler irrigation. We also present our research progress on diversifying the use of oxygation in landscape (lawn) and sports industries (sport grounds) to improve the WUE of these industries and to minimize the offsite movement of pesticides and nutrient from such hidden landscapes.

A number of controlled environments studies in pots and the glasshouse showed positive response to oxygation in medium and heavy textured soils. With this recent innovation of aerating the irrigation stream (oxygation), returns, yields and water use efficiencies (WUE) of SDI crops all increase (see Advances in Agronomy 88: 313-377 (2005)). This preliminary research clearly highlighted the opportunities of harnessing the potential benefits of oxygation for yield, quality and crop water use efficiency in Australian horticultural industries across diverse crops, soil types and irrigation water qualities. On large-scale field trials with SDI and surface drip, yield increases in cotton of 19% and in cucurbits of 12-60% were achieved, with significant improvements in product quality as measured by increase in ºBrix percentage of the fruit (sweetness). We have undertaken trials on heavy clays and lighter soils and for surface trickle under mulch, and trickle above the ground, showing positive and beneficial effects of aerated water irrigation. In this report we summarize the outcomes of oxygation research carried out by CQUniversity Australia in collaboration with Australian primary industries in a range of annual and perennial crops, and suggest the approach for large-scale adoption by irrigation industries in Australia.

Tendifferent crop industries (cotton, pineapple, lucernes, capsicum, strawberry, fig, table grapes, melons, vegetables and apricot), plus crops consultants and irrigation businesses in QLD were involved in testing the benefits of oxygation in field scale research. Data collected over 2- 4 seasons on yield and water use efficiency suggested that yield benefits of 4 – 19% were achievable with oxygation. Oxygation involves installation of an air injector (pressure differential venturi) in-line for mixing air with irrigation water. The installation cost of air injector can be AU$ 600-1000 per hectare depending on size of air injectors and requirements of accessories and fittings. Air injectors can be installed into new irrigation installations or retrofitted into existing drip irrigation systems.

The response to oxygation varies with crop and soil types, quality of irrigation water and type of drip irrigation. Horticulture industries in Australia span the range of these variables, therefore there is need for collaborative research, industry engagement and involvement of multidisciplinary research teams in the field of oxygation research to harness the full potential benefits of this technology to the industry.The project has resulted in significant benefits to cotton, with an average lint yield increase of 14%. Large cotton areas in Australia are furrow irrigated, hence, adoption of oxygation within the realms of existing cotton irrigation practices is currently limited. Future research is therefore suggested on use of aerated water with furrow irrigation, the primary method for irrigation of cotton. Increase in yield (6% in industry yield and 26% in total yield) and suppression of Phytophthora has been recorded on pineapple. In other crops (capsicum, strawberry, grapes) yield increases by 4-10% have been recorded. In apricot and fig the crop is still in the juvenile stage, and will be ready for harvest in 2012/2013 season only. Data will be collected from these crops beyond the funded project duration.

Oxygation as a tool delivers air into the crop root zone. Oxygen limitations can be significant in compacted, saline, and water logged soil, and with high BOD effluent irrigation water. Therefore, potential applications of oxygation can go beyond the improvement of water use efficiency and increased yields with ordinary drip and subsurface drip irrigated crops, into amelioration of other conditions that impede the diffusion of oxygen in the rhizosphere.Air within the irrigation water is a two phase flow fluid, hence, it imposes challenges for uniformity of air distribution along the irrigation line. This situation may be severe particularly when the drip irrigation is run over long row distances. Development of monitoring tools for measurement of air fraction and ways to minimize the heterogeneity of air bubbles distribution are currently underway. A number (7) of refereed journal articles have been published, postgraduate and undergraduate students have been involved (8), active collaboration with irrigation business, crop and irrigation consultants has been developed, and more field testing by independent crop consultants is underway, suggesting a gradual dissemination of the technology beyond the project timelines and resources. The following pictures highlight industries under collaboration for oxygation research in Queensland, Australia, showing diversity in terms of crops and focus in terms of soil aeration.