How to Check Soil Moisture With a Probe: Our Nine-Inch Test Under a Fuerte
How do you know your avocado tree is actually getting enough water? You look. A soil probe pulls up a thin core of soil from the root zone, and one glance tells you whether the last watering soaked down to where the roots are or stopped short near the surface.
In this video Kirk runs the test under Becca’s favorite tree, a Fuerte, the morning after a watering.
What is a soil probe?
A soil probe is a narrow metal tube with a handle. You push it into the ground, pull it back out, and it brings up a column of soil in the order it sat in the ground. Unlike digging a hole, it barely disturbs the roots, and you can check several spots around a tree in a couple of minutes.
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How to do the test
1. Pick a spot. Find a nice spot under the canopy, where the feeder roots are, rather than right against the trunk.
2. Push down and pull up. Drive the probe into the soil, then pull it straight back out with the core inside.
3. Read the core. What you want is soil that is nicely moist all the way down to the bottom of the sample. Dry soil at the bottom means the water is not reaching the roots; soggy, muddy soil suggests too much. Both matter. Avocado Production in California: A Cultural Handbook for Growers, Book Two, notes that avocado roots begin to die from lack of oxygen after about 48 hours of flooding, even without root rot, and waterlogged soil is where the root rot organism thrives. More in why avocados hate wet feet.
How deep should you check?
Avocado roots are shallow, and the handbook adds that they have few root hairs, which makes them inefficient at pulling in water; they need soil that stays evenly moist without being waterlogged. The ones doing most of the work sit roughly six to twelve inches down, so that is the zone you care about. Kirk’s sample came up about nine inches long, right in the middle of that range. The handbook’s second volume backs that up: 80 to 90 percent of an avocado’s feeder root length is in the top 8 to 10 inches of soil, so a nine-inch core covers nearly all of it.
We had watered the night before, and the core was moist from top to bottom. In other words, just the right amount of water. Those shallow roots are also why we do not plant anything beneath our trees; we explained that in why you should not grow plants under the canopy.
When is it time to water again?
The same handbook’s rule of thumb is to water when 30 to 50 percent of the soil’s available water has been used, leaning toward 30 percent on coarse, fast-draining hillside soils. Its feel test works a lot like the probe: take soil from 8 to 16 inches down in the wetted zone and squeeze it. If coarse soil will not hold together in a ball, it has already gone drier than avocados like. Soil type changes how often you water, not how much the tree uses overall; that is set by the weather. And do not stretch it too far: missing irrigations for one to two weeks can set off fruit drop.
What the probe can and cannot tell you
The probe tells you whether a watering reached the roots. It does not tell you how much water the tree will need next week; that depends on weather, tree size and your sprinklers. For that side of the question, our free Avocado Irrigation Calculator estimates weekly needs from your local forecast, and our guide on how much water an avocado tree needs covers the principles. Used together, the calculator gives you a plan and the probe checks whether the plan worked.
Why we are moving to sensors
A probe works well for a handful of trees. It is a lot less practical across ten acres. At the commercial grove we are in the process of upgrading everything, including putting sensors in the ground, so we will be able to see how far down the irrigation went every single time we water, without walking out with a probe. For newly planted trees, the handbook recommends a similar idea on a small scale: put young trees on their own irrigation block, with a tensiometer at the edge of the root ball.
Until then, the probe is cheap, fast and honest. If you have only one or two trees, it may be all you ever need.
Becca’s favorite may be a Fuerte, but most of what we ship is Hass. Our 4lb Hass box and 6lb Hass box and our 4lb Fuerte box ship free in season, with a waitlist on each page for when the trees are between harvests. Until the trees are picking again, the avocado scoop, the Hawaiian shirt and our hats ship year-round.
