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Mark Krasnow, Karen Peterson, and Stewart Field
As a “water guy” in the viticulture space, I’ve spent a good chunk of my career monitoring vine water status for trials and clients. While I am a strong proponent of using vine water status to determine irrigation requirements, the reality is currently soil probes are ubiquitous and convenient tools that many growers have and "use" to irrigation scheduling. Many of you will know that I have a few many soapboxes rooted in sustainable practice and informed decision making, and my stance on soil moisture probes falls into the later category of preaching.
Investing in soil probe calibration and placement analysis helps ensure probe data is meaningful. I know growers know the importance of calibration, and the importance of “inspecting what they expect.” It’s clear because we calibrate sprayers and herbicide rigs, seed drills, and even machine harvesters. But what I’ve observed is that same importance is not always applied to new tech, “old” tech, and especially new data. So, before the irrigation season starts here in the Southern Hemisphere, this is my annual reminder to calibrate your soil probes.
Garbage in garbage out
We live in the age of data availability, and advancements in the agriculture space are producing some really promising tech. However, it is easy to get caught up in the data these services provide, but the old adage “a little knowledge is a dangerous thing" comes to mind. Having data does not mean people know how to use it. Data is a powerful tool, but without the wisdom to extricate good data from garbage data, you’re equally likely to end up with garbage data influencing decisions, resulting in garbage outcomes. This reminds me a of a saying that’s making the rounds on social media of a clip from the Irish rugby player Brian O’Driscoll:
"Knowledge is knowing a tomato is a fruit. Wisdom is knowing not to put it in a fruit salad."
Knowledge is knowing what your soil probes say, wisdom is understanding what it means and how to use it.
How they “work”
There are various types of soil probes that measure using different means, but basically soil probes measure the water content in a cylinder of soil around the probe relative to its “full point” (i.e. field capacity). There are probes that aggregate water content over various depths (i.e. 0-300 mm, 300-600 mm, etc.), other models give readings at 100 mm intervals as well as an aggregate total. Most probes for grapes go about 0.8-1.2 m deep. Longer/deeper probes are available, but get pretty pricey-not in small part because installation becomes much more challenging. Most modern probes offer real time monitoring of soil moisture. Older models required manual readings, but there are fewer of those every year, so I’ll focus on the modern tech and how to best use it.
Placing the probe—an important decision that should not be taken lightly
Regardless of whether you’re installing new probes or are just now getting around to calibrating the probes you have, there are 5 key questions that will help determine how you’ll use the data from the probes.
1. What’s the total area are you trying to “monitor” with each probe?
2. What’s the soil/vigour variability across that area?
3. Are there any varietal, rootstock, or stylistic changes within that area?
4. What is your overall irrigation strategy based on fruit intended use?
5. Are there any well-known site characteristics that may influence the monitoring depth or irrigation strategies?
Answering these questions is just a jumping off point meant to calibrate the user’s expectations of how probe data will be used, and where to place the probe if a new one is going in. At a bare minimum, NDVI or EM38 maps should be consulted before placement and/or in season calibration. Since I can make an NDVI from a satellite image at homne, it's my standard go to. If soil probes are used to assist in irrigation decisions, I strongly recommend digging soil pit(s) and collecting soil samples to ensure vigour across the area you’re monitoring is not a result of nutritional differences or soil property anomalies. More on this later.
A Trial is Born A couple years ago I was brought into to the inner circle of a project led by my good mate
Dr. Stewart Field, from NMIT (the local polytechnic) in collaboration with the Marlborough Grape Growers Coop. This project, and its intentions, are such a great example of 3 things: the power of grower-led applicable research, calibrating your tools and expectations, and the importance of good data. With their blessing, I’ve been allowed to include some figures and findings in this post.
Over the 2022-23 and 2023-24 seasons, Stew worked with the Coop to install and calibrate soil probes that were placed in vineyards in five “representative” soils across Marlborough. The goal of this trial was to serve as an early warning for other growers in similar soils that their vines might be getting near stress, so that irrigation could be initiated at the correct time. 2022-23 was a rainy season, so none of the vineyards irrigated the deficit area. In 2023-24, a much drier year, irrigation was initiated if vines came to stress levels as measured by midday stem water potential. This work perfectly illustrates the importance of knowing about your site and soil to best interpret soil probe data.
NDVI maps were created to assess placement options, and probes were ultimately placed in low vigour areas of the vineyard. While vigour as it appears on an NDVI map can relate to more than just water availability, selecting low vigour area in an irrigated block in Marlborough will generally correlate well with water availability. At install the soil’s refill point was determined, the “stress” zones were set to a conservative 70-80% of full, and during the season soil moisture was measured with Sentek probes to a depth of 900 mm.
Two treatments were imposed: the control, grower standard (generally irrigation every 1-2 days), and soil moisture based irrigation initiation strategy, where irrigation only started after probes were closing in on the “stress point" reported by the soil data. Probes were placed in the centre of a 3 bay 3 row shut off zone, within the trail to be able to compare vines within the same vineyard area. Mid-day stem water potential readings were used to compare vine water status to the preset "stress" points reported from the soil moisture probes. For detailed information on stem water potential data interpretation click here.
While the trail was run for two years, I’ll focus on three sites in their “calibration year”, where preset soil moisture thresholds were in place. These results illustrate the importance of calibration, and how blind reliance on uncalibrated data can go wrong.
The Results...
Location 1 – Typically thought of as a high vigour, low to no irrigation subregion of Marlborough.
The probe data is reporting soil moisture content of nearly 85% on both dates where stem water potential was measured. This is not surprising for the subregion, and based on that anecdotal knowledge of the area and the soil probe data it would be easy to take this data at face value and not irrigate. However, the stem water potential readings tell a different story. Both days where SWP measurements were taken the vines were severely stressed (-10.65 and -10.83 bar respectively). The vines only started displaying visual signs of water stress later in the season.
At the end of the season, this early, mostly invisible stress reduced yield over 20% compared to the growers standard irrigation, even in a rainy year. In a more typical year, the crops loss could have been greater.
So what’s going on here?
At 90% soil moisture and stress values like that there is definitely something going on, and as soon a soil pit was dug it all made sense. After digging the pit the day before, it had filled with water overnight, only allowing about 30 cm of soil to be examined (Figure 2). To make matters worse for the vines, this water was saline (as the crow flies, the ocean is very close to this particular vineyard site). Vine roots were restricted to the top 300 mm of soil due to this saline water infiltration.
That’s why the preset “stress point” was so far off, the probe was measuring down to 0.9 meters, much of which was wet, but the vine’s roots were exclusively in the dry top 30 cm.
Vineyard 2 - A relatively variable subregion with one well known soil feature that can limit water status IF present.
Again, looking at this data, you’d assume your vines were, as the Kiwis say “sweet as,” with no stress. However, the pressure chamber reading shows the vines getting to the edge of stress (SWP -7 bars) well above the 85% soil moisture preset. This grower would also have been costing themselves yield if they blindly followed the soil probe data.
So what’s going on here?
The pit was equally illustrative here, again indicating an impediment to root growth, a compacted layer at 500-600 mm, a feature not unusual in the loess based soils in the area. In this case, the probe again underestimated the vine’s stress based on the preset, but was not as far off because the vine roots were able to occupy more of the area the probe was measuring. Again, following the preset recommendation would be invisibly costing the grower money.
Vineyard 3 - Data from 2024, located in the middle of the Marlborough grape growing region.
A very different beast here, and it's important to reiterate:
The pre-sets are a conservative estimate of the stress point.
When reading these graphs, a short period in the red, or sitting at the green/red line for a period of time is generally acceptable water maintenance - for a calibrated probe.
Extended periods of time below the stress point will be when you start to see signs of stress - for a calibrated probe.
If you were the grower managing your irrigation based on this soil probe, you’d probably be pouring on the water in early December, based on the slope of graph, and proximity to the "stress" point. However, the pressure chamber readings show absolutely no stress, even well below the 70% soil moisture red line.
This is a similar situation to Vineyard 1 in that the soil probe data is likely not representative of the rooting depth of the vine. However, in this case the vine has a greater root zone than the probe measures, and access to more water, which would mean the "stress" point would be much lower than the standard, conservative presets. The soil pit, confirmed this probe was in a deep sandy loam with gravels beneath it. Large roots were present in the midrow, and can be seen in the photo as well. The water table in the area is relatively shallow, around 2.5 meters deep. Presumably these vines' roots have access to that water, and they will likely never require irrigation.
Key take aways from this trial:
Calibrate your soil probes.
In other vineyards in the study, the preset red line was about right in terms of vine stress, but we’re able to know they are set about right because of the SWP readings from this trial. Blindly following presets can invisibly be costing money.
Calibration is ideally done with vine water status measurement, because it can measure pre-stress or early values long before visual symptoms appear.
But, at the very least, check your shoot tips for signs of water stress, and kick some dirt to compare against the probe data instead of blindly irrigating off a soil probe. Keep in mind visual symptoms manifest slowly and only become obvious after days, if not weeks, of stress.
The value of a soil pit.
For example, had Vineyard 1 dug a soil pit prior to installing the probe, they would have known:
The active rooting depth of their vines is only 30 cm
They could have purchased a shorter soil moisture probe, saved some money, and had less irrelevant data. That’s because it is the top 30 cm that has the greatest impact on the irrigation requirements of the vines in that location.
Have an answer to those 5 key questions.
As they relate to wine style:
For example, Marlborough is in a redevelopment/replanting period – if Vineyard 3 was up for redevelopment it would probably be unwise to plant high end Pinot noir in that exact location, because the vines may end up being too vigourous for the intended use and deficit irrigation may not be an option.
As they relate to probe placement:
For example, a small percentage of the total area may be low and/or high vigour. If the majority of the area you’re trying to monitor is moderate vigour, it is probably of more benefit to place the soil probe somewhere in the moderate vigour zones so that it is hopefully more representative of the greater area.
Know the soil probe’s limitations:
Soil probes are incredibly powerful tools. However, they only measure water around the probe. That's it. How you interpret this information is what makes all the difference. If you know your soil has a barrier to rooting depth, you either need to measure to that depth with the probe, or to figure out what total soil moisture actually correlates to vine stress and re-draw the red line there.
Did you inherit soil probes with little or no information or records on placement?
Don’t worry – you’re not SOL.
Get an NDVI map to understand vigour across the area
Review yield information against historical soil probe data
Whnew possible, review the measurements at specific depths not just the summary graph
Get some SWP measurements during the season to help assess the “stress” line
Dig a soil pit
If you determine the probe is in the “wrong” place based on these assessments, probes can be moved, but have a plan on where you’d like to move it and why – but note it does take a while for a probe to settle in, and even then you should try to move these in winter.
What this all means
Soil probes are a pretty big investment, but spending a bit more to dig a soil pit, review an NDVI map, or at the very least calibrate the stress/slow down point by looking at vine water status, ensures that investment pays off. Soil probes are only as good as your understanding of the site and data they are providing, and it is unwise to believe that probe data is right based on presets as the risk of INVISIBLE yield/profit or quality loss of under or over irrigating is entirely possible.
It’s relatively simple to have someone come in and check the vines over soil probes to help translate soil moisture number to actual vine water status numbers. I do this for many clients to help them fine tune what they do. After all, we’re farming vines, and understanding how they interact with the soil helps make the most thoughtful choices. In the second year of the study, stem water potential measurements from year 1 and observations from the soil pits were used to adjust thresholds and excluded irrelevant data sets for Vineyard 1 and 2. By measuring the top 300 mm and 500 mm respectively, and growers had much better results with predicting vine stress based on the soil moisture data.
Calibration only needs to be done every 4-5 years for younger blocks as root systems enlarge, and probably only once for mature blocks unless you dramatically change management like going herbicide free. Reach out if you’d like to discuss soil probe interpretation and how you might get the best information from your investment for your particular viticultural situation. In NZ, we’ve got boots on the ground in Marlborough, Hawke’s Bay, North Canterbury, and Central Otago capable of measuring vines so you can calibrate your probes and know where the “red line” really is. Those further afield should take advantage of local companies capable of vine based measurements.
The authors would like to personally thank Dr. Stew Field, the Marlborough Grape Growers Coop, Mart Verstrappen, Johnny McMillan, Jim Mercer, Fruition Horticulture, and the participating vineyards.
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