Olive orchard sensors: soil tension for deficit irrigation and water salinity

Olive is one of the most drought resistant fruit trees, and that is exactly why its irrigation is a measurement problem. The tree survives on very little water, yet in dry climates it gives large yield gains when irrigated, and it gives most of them back if water is withheld at the wrong moment. Modern olive irrigation is therefore built on planned deficits: water is cut at the stage the tree tolerates and restored before the stages it does not. A grower running that strategy needs to know how dry the root zone actually is, block by block, and when the deficit has gone far enough.

What an olive tree needs from the soil

A deficit at the right stage. Research in Spain and California agrees on the calendar. The stage least sensitive to water shortage runs from pit hardening to veraison, according to the CICYTEX and IRTA group that automated irrigation in an Extremadura hedgerow grove. The UC Davis extension material summarises the thresholds in midday stem water potential: flowering and fruit set do best near -10 bars and lose flowers around -20 bars, pit hardening tolerates -20 to -30 bars with no significant yield loss once water is restored, and oil accumulation shows no significant effect while the tree stays above about -20 bars. Below -30 to -40 bars during pit hardening, fruit drops and yield falls. In a Madrid trial on Arbequina, Gómez-del-Campo found that water could be saved in July at -2.9 MPa, but stem potential had to stay above -2.0 MPa in August to protect oil production.

Less water than most orchards. The UC Davis material puts the California olive crop coefficient at 0.65 and mature seasonal water use from April to November at about 25 inches, among the lowest of any California crop, and reports full oil yield with 80 percent of evapotranspiration under sustained deficit. Conditions change that number: the Ramat Negev research station in Israel’s desert reports water use of 900 to 1,000 mm a year there, against 400 to 700 mm in regions further north.

Roots that depend on the soil. The Madrid hedgerow had an effective rooting depth of 0.60 m, and a six hour irrigation wetted the soil to that depth. On a deep sandy loam in Mendoza, the INTA team explained why trees under a deficit showed no stress by roots exploring below 90 cm and a likely contribution from groundwater. The deep sensor belongs at the bottom of your own root zone, not at a fixed number.

No standing water. The same UC Davis material warns that olive is sensitive to root asphyxia and that young trees with small root systems must never sit in saturated soil. A deficit strategy that ends with a long recovery irrigation can overshoot into waterlogging, which is what the deep sensor catches.

Some tolerance for salt, not uniform. FAO Irrigation and Drainage Paper 29 rates olive as moderately tolerant to salinity. The Ramat Negev trials show how much variety matters: across 2012 to 2023, Koroneiki, Arbequina, Souri and Barnea passed the station’s threshold of about 200 kg of oil per dunam on both fresh water at 1.2 dS/m and brackish water at 4.2 dS/m, while Picual passed it only on fresh water.

What to measure and where

Soil tension at two depths, close to the dripper

The field trials give a placement that works. Gómez-del-Campo installed Watermark sensors in pairs at 30 cm depth, 30 cm from the emitters next to the trunks of three representative trees. Millán and colleagues put sensors under the dripper at 30 and 60 cm, with one more between drippers at 30 cm. For a commercial block that becomes:

One station per irrigation block, next to a tree that represents the block, is the minimum. Hedgerow blocks on uneven soil benefit from two.

A trigger to start from

In the Madrid trial the fully irrigated control was watered when the 30 cm sensors averaged about 30 kPa (30 cb), until 15 August. After that date the trigger was relaxed to 60 kPa until the end of the season, so the trees would harden before autumn frost. Deficit treatments in the same trial were held far drier during July and August. Those numbers are a starting point, not a recipe: the paper itself notes that soil thresholds are site specific, while stem water potential travels better between orchards. It also found the two strongly linked, with relative extractable water to 0.8 m explaining 77 percent of the variation in stem potential. The practical routine is to set a soil trigger per block, take pressure chamber readings at the key stages, and move the trigger until the two agree.

When the soil dries past a tensiometer

The SR tensiometer reads from 0 to 100 cb, which covers the wet range where decisions for flowering and fruit set are made. During a pit hardening deficit the soil is meant to dry further than that. A Watermark granular matrix sensor reads to 200 cb and needs no refilling, so it keeps recording how deep the deficit has gone and when it is time to restore water.

Irrigation water EC

Where blocks are fed from a brackish well, treated effluent or a blend, the salinity reaching the trees is whatever the source delivered that week. A conductivity probe on the irrigation main records it continuously, so a block that is losing yield can be checked against the water it actually received.

Agrinovo hardware for an olive block

• Irrometer SR tensiometer, 0 to 100 cb, in the 30 cm and 60 cm lengths for the shallow and deep positions, suited to medium and heavy soils. With the manufacturer’s voltage transducer it reads into the controller as an analog input.
• Watermark 200SS, 0 to 200 cb, buried and maintenance free, for the deficit months and for stations that are rarely visited.
• EC-100 conductivity probe, RS485 Modbus with automatic temperature compensation, on the irrigation main. With its standard K=1.0 cell it covers fresh and brackish irrigation water up to 10,000 µS/cm.
• Omni Genesis controller, four sensor ports configured from the cloud, 4G-LTE with a SIM included, rechargeable batteries with a solar input. A pole in the middle of a grove needs no power line.
• Omni Genesis Lite, one port, for a single tensiometer station on a small block.

Each block then has shallow tension, deep tension and, where measured, irrigation water EC on one chart. An app notification goes to the users on the account when the shallow sensor stays past the trigger longer than the time you allow.

Using the readings through the season

Spring, flowering and fruit set. Keep the shallow sensor near its trigger. This is the period when shortage costs flowers and set, so irrigations should follow the readings closely.

Pit hardening. Let the planned deficit run, and watch the Watermark sensor rather than the calendar. The aim is to restore water before the tree passes into the range where fruit drops, which the stem water potential readings confirm.

Oil accumulation. Return to a tighter trigger. The Madrid results tie oil production in August to tree water status, so this is not the month to stretch intervals.

Late season. In climates with autumn frost, the Madrid protocol relaxed the trigger after mid August to harden the trees. Where effluent or brackish water is used, the EC line is the record of what each block received over the year.

The sensors do not replace the pressure chamber. UC Davis calls midday stem water potential the reference for deciding when to irrigate, and notes that it records a single moment and takes labour. Soil tension fills the hours and days in between, and the two together give a schedule the grower and the agronomist can both check.