Checking Soil Moisture in Avocado Groves

Irrigating California avocado trees can be challenging for several reasons. Avocado trees are heavy users of water but they have a shallow feeder root system located primarily in the top six inches of soil that are prone to drying out. The feeder roots also have very few root hairs, thus making them inefficient at absorbing water. Hillside groves with decomposed granite drain well, but they drain rather quickly. Groves with high clay content can suffer from poor drainage that leads to root rot.

For the above reasons, monitoring soil moisture in avocado groves is important.

Soil moisture can be checked by hand for a general assessment. To do so, gather a sample about 8 – 16 inches deep in the root zone area and form a ball of soil. If the soil has about 50% of available water remaining, it will form as follows:

• Coarse soil will form a ball that does not hold its shape and it will appear almost dry.
• Loamy soil will form a ball and have a dark color.
• Clay soil will be slightly sticky and form a good ball.

Using a tensiometer.

A tensiometer is one of the best ways to measure soil moisture. It consists of a water-filled tube with a pressure gauge at the top and a porous clay cup at the bottom. The tensiometer measures the amount of energy a plant needs to extract water from the soil.

Here’s how to use a tensiometer:

• Fill the tube with water and place it in a bucket overnight. Use a portable vacuum pump to draw air bubbles out of the clay cup and have water fill the clay.
• Place tensiometers about 2-3 feet away from the sprinkler in the wetted area of the root zone.
• Use an auger or other tool to create a hole for the tensiometer.
• Pour a cup of water into the hole and place the tensiometer at the correct depth. Do not pound on the tensiometer — the clay cup is fragile.
• Pack loose soil around the tensiometer and tamp it down.
• Run the irrigation system to help settle the soil around the tensiometer.
• Protect the tensiometers from pickers by covering them with a bucket or garbage can.
• If the soil becomes too dry (80 cb), the clay cups will lose suction and draw in air bubbles. If this happens, the tensiometer must be removed, put in a bucket of water and a pump used to draw out the air bubbles.
• Annual tip changes and gauge checks will help ensure the tensiometer is working properly.

In a sandy loam soil, irrigation should take place when the tensiometer reads about 20 cb. In loamy floor soils irrigation should take place when the tensiometer reads between 40-45 cb.

Tensiometers can also be used to help growers determine when to turn the irrigation system on and off. Place one tensiometer at a shallower depth (about 8 inches for shallow hillside soils, 12 inches for loamy floor soils) and one at a greater depth (about 2 feet for shallow hillside soils, 3 fee for loamy floor soils).