SATI Beyond The Bunch 1st Quarter 2026

First page Table of contents 1 Next page Last page

SATI Research Booklet

Quarter 1

BEYOND

DEWALD KIRSTEN | LUCENTLANDS

2 0 2 6

FOCUS ON: Irrigation

DEWALD KIRSTEN | LUCENTLANDS

BEYOND

A SATI PUBLICATION

About Beyond the Bunch This quarterly publication aims to serve as a science-based resource for the South African table grape industry. Each edition explores a topic or questions raised by industry, with links to related additional reading. Submit your topic or questions Producers and industry stakeholders are encouraged to suggest scientific or technical questions or topics of interest. We will strive to address as many of these as possible.

Email tarryn@satgi.co.za to submit your request.

Content Disclaimer SATI does not take responsibility for

accuracy or validity of information supplied by advertisers. Views expressed are those of the partner and may not reflect our organisation’s views. Contact denene@satgi.co.za to feature your brand.

DEWALD KIRSTEN | LUCENTLANDS

PRECISION IRRIGATION AND NUTRITION

What can table-grape growers do to attain homogeneous fruit yields and quality given variable soils and erratic weather?

BY ANNA MOUTON

Picture the perfect vineyard of identical vines. They all have the same vigour. They ripen the same crop load at the same rate, yielding bunches of the same quality and storability. This vision is the goal of precision agriculture: to achieve uniformity and predictability by managing spatial and temporal variability. Spatial variability manifests as non-uniform vines within blocks. It typically emerges from soil differences, but can also have other causes, such as errors in irrigation system design. Temporal variability reflects changes between and within seasons. It arises naturally from interactions between vine phenology and weather, but is significantly influenced by viticultural practices. This issue of Beyond the Bunch focuses on variability in the context of irrigation and nutrition, including why variability occurs, the tools and technology to measure it, and practical tips for achieving uniformity.

BEYOND THE BUNCH • 4 • QUARTER 1 • 2026

A case study of variability

the shallow soils extended only to 30 cm below the surface, whereas those in the deeper soils extended to 80 cm. All blocks were irrigated three times per week, but the shallow soils dried out within a day. Chronic moisture stress was stunting the vines in the shallow areas. Switching to shorter, more frequent irrigations alle viated the water stress in the weak vines without stimulating additional vigour in the vines with deep root systems. Follow-up NDVI images (Figure 2) and visual assessments demonstrated significantly improved uniformity within two seasons.

Satellite images of a commercial table-grape farm in Robertson highlighted non-uniform vigour, as evidenced by highly variable NDVI (normalised difference vegetation index) within most blocks (Figure 1). Visual observations in the vineyards confirmed this. Although the situation seemed to call for differential nitrogen application and vine manipulation, these practices wouldn’t have solved the problem. The issue was irrigation, not nutrition, with variable soil depth as the root cause of variable vigour. The non-uniform vineyards had a patchwork of shallow and deeper soils. The root systems of vines in

VIGOUR

Low

Average

High Cloud Cover

FIGURE 1. The NDVI map of the vineyards in January 2024 shows highly variable vigour. Image taken from the IMPI platform and supplied by AgriMotion.

FIGURE 2. The NDVI map of the vineyards in January 2026 shows a significant improvement in uniformity. Image taken from the IMPI platform and supplied by AgriMotion.

BEYOND THE BUNCH • 5 • QUARTER 1 • 2026

DEWALD KIRSTEN | LUCENTLANDS

Ridging can increase the effective rooting depth in shallow soils.

Spatial variability

Prepare the ground Establishing vines in a uniform planting medium lays the foundation for uniformity in the vineyard. The first step is mapping the soil by sampling a 50 x 50-metre grid. Soil samples are discussed in detail in the 2025 Q2 issue of Beyond the Bunch. Soil maps and GPS-enabled variable spreaders allow precision chemical soil preparation. Keep in mind that many chemical corrections (such as addressing acidification and phosphorus levels) can only be made before planting. Once vines are established, deeper soil layers (below 20 cm) are no longer accessible. Soil maps also reveal physical problems such as waterlogging and abrupt textural transitions that repel roots. Impeding layers due to the natural soil structure or compaction likewise block root growth and hinder water infiltration. When properly executed, physical soil preparation provides favourable conditions for deep (at least

60 cm) root development. It includes loosening and mixing soils, breaking up compaction, and incor porating ameliorants. However, injudicious mixing of shallow soils, for example, using a delve plough, can permanently damage the topsoil by bringing subsoil to the surface. Depending on the soil type, ripping too-dry soils could result in coarse clods, and ripping too-wet soils can cause smearing. In waterlogged soils, installing drains can alleviate wetness and salinisation. Shallow soils can be ridged, with the added benefit that ridged soil warms faster in spring, promoting an earlier root flush, which may be helpful in early cultivars. If a block contains radically different soil types, and drainage and ridging are insufficient to bridge the gap, then drier patches could be variably mulched. Larger areas (more than 0.5 hectares) with contrasting soils could also have their own irrigation taps.

BEYOND THE BUNCH • 6 • QUARTER 1 • 2026

A single dripper line is unable to provide uniform soil moisture in this vineyard, with the result that roots only occupy about half the ridge.

A double dripper line is able to provide uniform soil moisture, promoting full utilisation of the ridge by a large root volume.

Too-wide dripper spacing has led to poor root development in this vineyard.

All three images courtesy of Karen van der Westhuizen.

Irrigation system design Effective irrigation systems are designed to eliminate spatial variability in water delivery. Good designs, including correct system pressures, apply water uniformly across the block to attain homogenous horizontal and vertical soil moisture within the intended wetted area. Good scheduling is also essential for uniform soil moisture, but it will be discussed under temporal variation. Spatially imprecise irrigation can result from several issues. As mentioned above, soil variability often leads to relative over- and under-irrigation of different areas within a block. Failure to account for site topography can do the same. In addition, excessive wetting of low-lying areas can occur when water drains from the irrigation system after a scheduled irrigation event ends. The risk increases with more frequent irrigation events (for example, when implementing pulse irrigation) and long dripper lines. Whereas non-uniform irrigation across the vineyard manifests as non-uniform grapevine vigour and fruit yield and quality, non-uniform wetting of root zones impairs root development of individual vines. A small root system limits the plant’s ability to extract nutrients and water, and increases its drought sensitivity. To uniformly wet the root zone, the emitter delivery and spacing must match the soil type, and the rest of the irrigation system must supply sufficient water at the correct pressure. For example, in free-draining soils, double dripper lines and narrower emitter spacing can compensate for poor lateral water movement, reducing the risk of alternating wet and dry areas in the vine row. On the other hand, consider a scenario in heavier soils where low-volume drippers necessitate long irrigation

sessions to apply sufficient water to infiltrate to a depth of 60–80 cm. The soil directly under the drippers will become saturated, drowning roots and eventually reducing overall root volume and health. In addition to delivery and spacing, height matters for microsprinklers, as it affects the wetted width. Lower microsprinklers may be more prone to obstruction by weeds, but higher microsprinklers lose more water to evaporation and wind. If fertigating, higher microsprinklers also lead to more misplaced nutrients.

DEWALD KIRSTEN | LUCENTLANDS.

The height of microsprinklers affects the wetted width and water lost through evaporation and wind.

BEYOND THE BUNCH • 7 • QUARTER 1 • 2026

Temporal variability Soil preparation and irrigation design diminish but don’t eliminate the effects of soil variability, which will influence scheduling and fertilisation decisions throughout the vineyard’s lifespan. Therefore, although this section focuses on changes over time, spatial and temporal variability are intertwined.

Cumulative changes Temporal variability occurs at scales of hours to years. Much of this is caused by weather and vine phenology, but irrigation and nutrition requirements also change during vineyard establishment and maturation. For example, while irrigation should always maintain an oxygenated profile to promote deep, healthy root development, achieving this needs special attention in young vines. Appropriate scheduling involves juggling the lower water demands of smaller leaf areas and root systems with the higher evaporative rates from unshaded vineyard floors. Without (and sometimes despite) interventions, grape vine variability is cumulative over the lifetime of the vineyard. For example, if uncorrected soil variability or poor system design leads to over-irrigating parts of a block, the affected vines may become more vigorous, especially if the farm uses fertigation. Conversely, over-irrigation can also reduce vigour by impairing root development on heavier soils and leaching soil nutrients from free-draining soils. In the short term, vigorous vines require additional actions to control growth, which increases labour costs. In the long term, these vines may develop denser canopies that restrict light penetration, reducing bud quality. Meanwhile, excess nitrogen accumulates, further stimulating vigour. Over several seasons, such a vineyard has higher input

costs (labour), wastes resources (electricity, water, and fertiliser), and produces less and lower-quality fruit. Soil and season In any given season, a precision irrigation and nutrition strategy is built on plant phenology, and the implementation is adjusted according to weather. Throughout the season, refill lines based on soil probes should be regularly tweaked in keeping with plant phenology, with consideration of soil variability. Keep in mind that setting refill lines relative to the block’s average soil moisture may leave some areas too wet or too dry. The impact of missed irrigations due to power outages or system failures will also depend on soil variability. When aiming for slightly lower overall soil moisture, a missed irrigation could result in significant stress for vines on rapidly drying patches of soil. Precision irrigation during the season can support particular viticultural goals. For example, grapevine roots typically flush in spring and after harvest. During these periods, allowing soils to dry slightly more before refilling to optimal soil moisture will stimulate root growth. In contrast, readily available moisture from flowering to véraison promotes fruit set and development. From véraison to harvest, depending on the cultivar, soils can dry slightly more between irrigations, to enhance sugar and colour formation and discourage vegetative growth.

DEWALD KIRSTEN | LUCENTLANDS

Irrigation and nutrition affect vine growth cumulatively over time.

BEYOND THE BUNCH • 8 • QUARTER 1 • 2026

DEWALD KIRSTEN | LUCENTLANDS

Irrigation must be adjusted throughout the season to match the plant phenology.

A daily decision Weather drives short-term fluctuations in water demand. Theoretically, growers would track soil mois ture (tools for this are discussed below) and irrigate as needed to stay within field capacity and the refill line. In practice, pump capacity, system delivery, and water availability are often limiting factors. The goal is uniform soil moisture throughout the profile, but the root zone (top 40 cm) typically dries faster than the buffer zone (lower 40–80 cm) due to evaporation and a greater root density. Therefore, irrigation will be some combination of short events targeting the root zone and longer events targeting the buffer zone. Soil moisture must be checked in both zones to achieve the right weekly irrigation schedule. However, daily scheduling decisions can’t be based solely on soil moisture, as many growers discovered during the past season when sudden hot weather led to skyrocketing evaporation and transpiration. By the time soils had dried, the irrigation infrastructure couldn’t replenish soil moisture or keep up with plant demand, resulting in reduced berry growth and, ultimately, fewer export cartons at harvest. To avoid such losses, growers can use weather stations and evapotranspiration-demand estimates to schedule irrigation proactively ahead of hot spells. Of course, irrigation may also be adjusted in anticipation of cold spells or rain. Irrigation scheduling is a big topic, and the details are beyond the scope of this article. More information is available in the resources listed under Further reading. Feed according to need Precision grapevine nutrition addresses spatial and temporal variability. Nitrogen applications are tailored to the phenological stage and observed vigour, which

is usually assessed during flowering, when berries are pea-sized, and after harvest. Where appropriate, differential vigour within blocks can be managed by variable applications of nitrogen, compost, or mulch, keeping in mind that nitrogen is not a panacea for weak growth. With fertigation, be aware that one block may receive nutrients mixed for another block due to residual fluid in the irrigation lines. Production figures, yield targets, and soil and leaf samples must also inform fertiliser programmes. Soil and leaf samples are discussed in detail in the 2025 Q2 issue of Beyond the Bunch. Even when not fertigating, irrigation and nutrition are inseparable. Everyone is doubtless aware that over irrigating leaches soil nutrients. What is likely less well known is that nitrogen availability decreases in oxygen depleted, waterlogged soils because soil microbes convert nitrogenous compounds into forms that are lost to the atmosphere.

DEWALD KIRSTEN | LUCENTLANDS

Weather stations can help growers irrigate proactively ahead of hot conditions.

BEYOND THE BUNCH • 9 • QUARTER 1 • 2026

Tools The following tools can provide spatial or temporal information, or both. They range from the bare basics to blue-sky technologies.

For growers who prefer traditional methods, rest assured that new technology isn’t required for precision irrigation and fertilisation. And for those who can’t wait to send armies of satellite-guided robots into their vineyards, be warned that new technology isn’t a substitute for doing the right thing to the right degree at the right time.

Seeing is believing Manual assessment of soil moisture (the squeeze test) is old-school but highly effective. Soil samples are collected at three depths (0–20 cm, 20–40 cm, and 40–60 cm). A handful of soil is squeezed, and the moisture content is rated on a scale of 1 (too dry to hold together) to 5 (water drips out). Samples can be taken with soil augers or from profile holes. Soil augers are quick to use, but manual augers are not suitable for very rocky soils. Profile holes take a little longer but present an opportunity to evaluate root development, which should be assessed monthly in all blocks. Moisture must be assessed at least weekly in all blocks, just before scheduled irrigation events. Where blocks have highly variable soils, evaluate moisture separately in each soil type and in areas of different vine vigour. Keep in mind that weak vigour can result from too-wet or too-dry soils. Manual assessment of soil moisture primarily provides temporal data relevant to irrigation scheduling. However, soil augers and profile holes are also valuable tools for collecting multiple samples across a block to evaluate spatial variability.

Soil probes, stem water potential, and weather stations

Soil probes are usually permanently installed and automatically log moisture. They can provide data to inform scheduling decisions if correctly installed and working properly. Probes are useful for assessing the depth to which a known amount of water will infiltrate. Like any complex technology, probes can fail (for example, due to flat batteries, disturbances, break ages, or vandalism) or produce misleading data (for example, due to calibration errors or the wrong placement within a block or relative to emitters). Growers should never rely on probes alone when scheduling irrigation. The tools discussed so far all measure soil moisture. It’s also possible to measure the plant’s moisture stress directly with stem water potential. The technique is most often used in research, but growers can also employ it to relate plant stress to soil moisture levels measured with probes or squeeze tests. More details on stem water potential are available in the resources listed under Further reading. In the future, stem water potential may become mainstream as technology is developed to enable automated measurements in commercial vineyards.

DEWALD KIRSTEN | LUCENTLANDS

BEYOND THE BUNCH • 10 • QUARTER 1 • 2026

DEWALD KIRSTEN | LUCENTLANDS

LUCENTLANDS As mentioned above, weather stations are crucial to irrigation management. If there isn’t a weather station within 5 km of the farm, growers should consider installing their own. Several companies offer this service. If used to plan irrigation schedules, the service should include forecasts of evapotranspiration demand. Besides facilitating proactive irrigation, weather stations and evapotranspiration-demand estimates also help growers sense-check their water delivery. For example, if conditions have been cool and rainy and growers haven’t applied less water, but a probe indicates drier soils, there is a problem (with that probe or with water delivery in that area of the block). Remote sensing Remote sensing using drone- or satellite-mounted instruments has become a standard tool for measuring vigour in vineyards and orchards, with several companies offering mapping services. Satellite monitoring can provide updated data as often as every four days. Vigour is usually estimated based on NDVI. Remote sensing data must be calibrated using visual observations at least once, but preferably three times per season. Although vigour maps highlight spatial variability, figuring out the cause still requires boots (and usually a spade) in the vineyard. Technology is now also available to map fruit development and crop load within vineyards, for example, by analysing data from vehicle-mounted cameras. Although this is mostly a tool for harvest prediction, it delivers spatial information that can supplement vigour maps and provides clues to problem areas within a block. At the cutting edge, researchers are working on remote sensing utilising multispectral imaging, for example, to identify moisture stress in vineyards. Resources discussing some of these projects are listed under Further reading. Samples and spreaders Soil samples before vineyard establishment are indispensable. Thereafter, soil samples should be

repeated at least every three years, and more frequently when managing a soil problem such as salinisation. Leaf samples should be analysed annually. Both soil and leaf samples inform fertilisation pro grammes, so they need to reflect the vigour variability across the block. Soil and leaf samples are covered in detail in the 2025 Q2 issue of Beyond the Bunch. Variable fertiliser applications are one tool to manage variable plant needs. Spatial maps obtained from remote-sensing platforms are now routinely used to generate GPS maps for variable-rate spreaders, enabling precision nutrition in vineyards. In the near future, growers will likely also have access to GPS-enabled sprayers for precision application of foliar feeds, plant growth regulators, and even crop protectants. Natural and artificial intelligence Artificial intelligence and machine learning could expand growers’ toolkits for precision irrigation and nutrition. For example, artificial intelligence can theoretically integrate data from multiple sources, including soil moisture probes, stem water potential loggers, remote sensors, and weather stations, to estimate irrigation requirements. The same artificial intelligence could then determine the optimal irrigation schedule based on information about water availability, pump capacity, system delivery, and even electricity rates. Should conditions change (for example, a pipe bursts), the irrigation plan could be automatically updated. Such a system would have many advantages, not least freeing growers from daily scheduling decisions. But we’re not there yet. For now, growers’ best bet is to harness the natural intelligence of qualified irrigation and nutrition consultants who can help them map and prepare their soils, calibrate their soil moisture estimates and probes, make sense of the data deluge, and ultimately implement precise yet practical measures that optimise vineyard performance and profitability.

Acknowledgements The following specialists provided technical inputs:

• Eunice Avenant. Training manager at SATI and extraordinary lecturer in Viticulture in the Department of Viticulture and Oenology at Stellenbosch University. eunice@satgi.co.za. • Dr Carolyn Howell. Senior Researcher at the ARC Infruitec Nietvoorbij. howellc@arc.agric.za. • Danie Kritzinger. Soil scientist and horticulturist at AgriMotion. danie@agrimotion.net.

• Karen van der Westhuizen. Soil scientist and irrigation consultant at MuddyBoots. karen@muddyboots.co.za. • John-Murray Visser. Senior research assistant (soil physics and soil-plant interactions) in the Department of Soil Science at Stellenbosch University. visserj@sun.ac.za.

BEYOND THE BUNCH • 11 • QUARTER 1 • 2026