Technology for irrigation scheduling

Growers have more — and better — options than ever before. What do they need to know to choose the top tool for their orchards? By Anna Mouton.

Understanding your soils is the starting point for accurate irrigation scheduling, says Dr Eduard Hoffman, chair of the Department of Soil Science at Stellenbosch University. “You need a soil-water retention curve for each of your soil types,” he advises.

Hoffman explains that a soil-water retention curve plots the relationship between the amount of water in the soil and the tension a plant has to exert to extract that water. This tension is expressed as negative pressure — think of it as how hard the plant needs to suck to draw the water into its roots.

Plants operate best when soils are neither too wet nor too dry. As soils dry out, plants need to work harder and harder to draw in water. Eventually, even though water is still present in the soil, the plant isn’t able to extract it. This is called the permanent wilting point — a polite term for the point at which the plant starts dying.

The relationship between the amount of water in the soil and the negative pressure exerted by the plant is not a straight line. It also depends on soil type. “Let’s say there is the same amount of water in both a clay soil and a sandy soil,” says Hoffman. “That doesn’t mean the water is equally available to plants.”

Clay soils have a higher matrix potential — they hold water more strongly — than sandy soils. Therefore, plants need to exert more negative pressure to draw water from clay soils, and may suffer drought stress in clay soils that are comparatively wet. In contrast, plants can access most or all of the water in sandy soils. However, sandy soils dry out faster because they hold less water.

If growers don’t know their soil types, and don’t take their soil-water retention curves into account, they lack the basic tools to manage their irrigation, cautions Hoffman. “Growers are usually irrigating too much, because they’re scared of stressing their trees, but too much water is just as harmful as too little.”

Scheduling according to soil water content

Soil is said to be saturated when all the air has been displaced by water — most plants drown in saturated soil. Soils at field capacity contain both air and water. Field capacity is defined as the amount of water the soil can hold against gravity. This water is readily available to plants.

Ideally, soils would always be at field capacity, but in the real world, growers aim to maintain soil water content at a level that supports optimal plant performance. They decide on a refill point — the minimum acceptable soil water content. The challenge then becomes to schedule irrigation so that soil water content remains between the refill point and field capacity.

Soil water content can be assessed indirectly by using a tensiometer to measure the negative pressure required to draw water from the soil. The tensiometer can only measure up to -85 kPa. “We refer to easily available water in soils when the tensiometer reads between 0 and 85,” says Hoffman, while clarifying that the tensiometer reports positive values, even though the pressures are negative.

Water content can also be measured directly using any of several instruments, the oldest of which is the neutron moisture meter. “It doesn’t matter which instrument you use, as long as it’s calibrated for that specific soil,” cautions Hoffman. “The problem is that the producers rely on the factory calibration, but the factory doesn’t do a calibration — they do a normalisation.”

The factory normalisation ensures that every instrument will produce the same reading under the same conditions. A calibration adjusts the instrument reading based on the soil characteristics. Hoffman recommends a five-point calibration. “Depending on how well the instrument is designed, that calibration curve can change over time,” he says. “For example, because the battery in the instrument runs down.”

If there are different soil types within an orchard, soil water content must be measured for each of these. Hoffman believes that production blocks should be divided into irrigation blocks based on a soil survey, so that irrigation can be precisely tailored to the soil type and profile. He has already seen fantastic results when using this approach in citrus.

Optimise by combining data sources

According to Hoffman, there are essentially three sources of information for irrigation scheduling — the soil, the plant, and the weather. Meteorological models are based on the weather. They don’t take soil water content into account. Instead, they work exclusively with the atmospheric evaporation demand, adjusting for crop type.

“Meteorological models measure climatic variables such as temperature, wind speed, humidity, rainfall, and radiation, and predict water use based on those parameters,” says Hoffman.

Plant reaction is trickier, and subjective assessments are unreliable, warns Hoffman. “The most effective way to determine plant reaction is to measure stem water potential and leaf water potential. The ratio between the two gives you a stress index that tells you what the plant is experiencing.”

Hoffman prefers to use meteorological models to examine long-term trends, and to plan irrigation scheduling according to the average atmospheric evaporation demand. He then monitors soil water content to see whether it stays on target between the refill point and the field capacity. If growers want to be sure that their refill-point and field-capacity estimates are correct, they can measure stem and leaf water potential to check the stress levels of their plants.

The future of irrigation scheduling, according to Hoffman, is creating small irrigation blocks based on accurate soil surveys and soil-water retention curves. This ensures that plants are irrigated optimally, and allows growers to fine-tune water delivery according to the stage of production and the availability of water. The potential for greater control of nutrient delivery through fertigation is an added benefit.

“Growers will have to practice more precision agriculture — maize farmers already do this on a large scale,” asserts Hoffman. “Just sitting down and pressing a button and irrigating an entire 80 hectares simultaneously — those days are over.”