A daily decision

Two soil scientists share their take on irrigation scheduling and systems. By Anna Mouton.

“Irrigation is critical, but it’s difficult to optimise,” observes Karen van der Westhuizen of Agrimotion. “For nutrition, you take leaf and soil samples and make a few adjustments in a season, but irrigation is a daily decision dependant on weather, crop and soil variation.”

According to Louis Reynolds of Fruitful Crop Advice, one positive result of the 2015–2017 drought was a greater focus on irrigation. “It might sound bad to say this, but a drought can be good for teaching us to be more effective with our water. Hopefully, people will continue saving water and not return to their old methods after a wet year like this.”

So, how should growers approach their irrigation? Scheduling and systems matter — but don’t discount the basics.

Successful scheduling

Even though water is scarce in most of South Africa, Van der Westhuizen says that growers tend to default to applying water whenever they think their trees might be under stress. “If the soil moisture is good, but the root volume isn’t enough to take up that water, giving more water will only stress the plant further.”

When asked whether he thinks growers tend to over-irrigate, Reynolds laughs. “Let’s say they irrigate safely,” he jokes. However, one water-saving change he has seen during his 20-year career is the move to a smaller wetted area. Full-surface irrigation is giving way to short-radius micro-sprinklers, and drip irrigation is making a comeback.

“In a young orchard, up to 50% of the applied water can evaporate from the soil surface,” he says. “The smaller the wetted area is, the less the evaporative loss will be.”

Both drippers and micro-sprinklers can wet a small area but require different mindsets when scheduling, explains Van der Westhuizen. All else being equal, longer irrigation with micro-sprinklers will increase infiltration depth, whereas longer irrigation with drippers will increase wetted depth and width.

Slower water delivery using drip irrigation can improve soil infiltration and reduce run-off. Appropriate soil moisture throughout the profile promotes root development. “A big, deep root system is your best insurance policy for load-shedding and heatwaves,” says Van der Westhuizen.

Low-flow drip irrigation — deliveries under 1 litre per hour — offers potential advantages over standard drip if used on the correct soils. One benefit Reynolds and Van der Westhuizen mention is that low-flow drip runs with fewer pulses than standard drip, translating into fewer periods during which the irrigation pipes drain under gravity. Frequent drainage of the dripper lines can overwet some areas of an orchard.

Drippers versus micro-sprinklers

“I think the biggest mistake people make is to install drip irrigation when establishing a new block that shares existing main lines with blocks on micro-sprinkler irrigation,” says Reynolds. “Mixing drip and micro-sprinklers on the same main-line system is asking for trouble.”

One reason to not mix the systems is the scheduling differences, but another is fertigation. With micro-sprinkler irrigation, granular fertiliser is surface-applied and watered in at intervals during the season. With drip irrigation, dissolved fertiliser is applied with each irrigation event.

Reynolds notes that fertigation with micro-sprinklers is possible, but the volume of water applied differs from drip, so the fertiliser concentration would also differ between the two irrigation methods.

There are some limits to fertigation. It doesn’t allow zone management — if growers want to address localised issues in orchards, they will still need to resort to surface application. On the other hand, Van der Westhuizen points out that fertigation avoids the soil compaction and diesel use associated with vehicles in the work row.

During wet periods, especially in spring, growers may have to apply granular fertiliser to feed their trees. “You don’t want to irrigate to fertilise,” says Reynolds. “You want to irrigate when the tree needs water.”

When it comes to load-shedding, arguments can be made for either drippers or micro-sprinklers. “The advantage of drip is that you can irrigate more hectares at a time,” says Reynolds. “If you only have a limited capacity to pump water, you’ll be able to do more with drip because it’s more efficient. But the smaller wetted area with drip can also increase your risk.”

He advises growers to design their systems so that they only need to irrigate for 12–14 hours a day, leaving a buffer for load-shedding or other problems.

Back to basics

Both Reynolds and Van der Westhuizen highlight advances in automation and technology that make grower’s lives easier. But they have far more to say about the basics that they feel need attention.

Van der Westhuizen’s advice to growers is to dig more holes. “There’s a very good baseline understanding of irrigation in the Western Cape, but people sometimes start making things too complicated. They should stop having discussions around a table — dig holes and look at the root distribution and the soil moisture throughout the profile.”

She also thinks more emphasis should be placed on the soil and how water moves in the soil when designing irrigation systems. Just considering evapotranspiration and crop factors and designing a system to deliver a specific volume of water is not sufficient.

Reynolds wants growers to quantify their water use. “It frustrates me when I visit clients, and they tell me that they irrigate six or eight hours a week,” he says. “They need to know how many millimetres or cubic metres per hectare they’re irrigating.”

He has seen water productivity improve but attributes this mainly to horticultural advances, such as high-density planting on dwarfing rootstocks. Nonetheless, he believes that growers are becoming more mindful of efficiencies and want to know how many litres of water it takes to produce each kilogram of fruit.

“Each season, growers are under more pressure for top-quality fruit,” reflects Van der Westhuizen. “So, I think there is more of a focus on zone management, and not just ranching, because growers need every tree to perform.”