Low-flow drip irrigation in apples

A new Hortgro-funded study compares tree performance and root development in low- and ultra-low-flow drip-irrigated apples. By Engela Duvenage.

Continuous low-flow drip irrigation is being used with great success by the South African citrus industry. A Hortgro-funded project launched in 2021 will scientifically evaluate the pros and cons of low-flow drip irrigation for the apple industry. The project — the first of its kind on apples globally — is being led by Dr Eduard Hoffman and Prof. Johan van Zyl of the Department of Soil Science at Stellenbosch University.

“Our ultimate aim is to supply the apple industry with objective knowledge regarding soil-water distribution, water-use efficiency, and tree response in terms of stress, tree growth, yield, and fruit quality,” says Hoffman.

He anticipates not only water savings, but also improved ease of fertiliser application and reduced management inputs. “We hope to provide quantitative evidence to demonstrate to growers what the advantages and disadvantages of low-flow drip fertigation of apple orchards are, and how it compares with microsprinklers.”

The first research on low-flow drip irrigation was done by Hoffman and Van Zyl in citrus in the Worcester area. Low-flow — 1.6 litres per hour — was compared to ultra-low-flow — 0.7 and 0.4 litres per hour — irrigation. The study site, a Nadorcott mandarin orchard, is very sandy, with quite poor water-holding capacity.

In the first year after changing to drip irrigation, only the trees receiving water at 0.4 litres per hour showed a very slight increase in yield. But things changed drastically in the second year. “Trees in all three treatment groups had such great yields in the second year that some of their branches broke off from the load that they carried,” recalls Hoffman.

Water distribution was best with the drippers that delivered 0.4 and 0.7 litres per hour, and showed an impressive lateral distribution. With the 1.6-litres-per-hour drippers, water distribution was generally only vertical.

The new study in apples is being conducted near Grabouw on clay soils containing at least 70% gravel. According to Hoffman, the site was deliberately chosen, because gravelly soil allows little lateral water movement, and is notoriously difficult to manage under drip irrigation.

Three different types of drip irrigation are being compared: low-flow — 1.6 litres per hour — and ultra-low-flow — 0.7 and 0.4 litres per hour. The wetting profile associated with each, as well as root growth, will be evaluated.

To assess root growth, Hoffman previously deployed rhizotrons, in-situ root-inspection chambers that let researchers monitor root growth and distribution in real time. This non-destructive method is already used in the ongoing citrus study mentioned above.

“Other methods are all destructive, and only allow us to analyse root growth at the beginning and end of a season,” explains Hoffman. “This method allows us to determine the daily effect of the different irrigation treatments, and its influence on yield and fruit quality.”

The team will also investigate how fast root systems grow laterally, what influence a particular irrigation system has on soil chemistry, and the rate of acidification of soils.

The project is set to run until 2024.

Image: The rhizotron in action.

Supplied by Eduard Hoffman | Stellenbosch University.