Soil Moisture Sensors are Not All the Same

My soil moisture measurements for the University orchard were important. I used manual tensiometers and the Watermark Soil Moisture sensor. For a fast and fairly accurate idea of how wet your soils were these inexpensive soil moisture sensors are not bad. They are not accurate enough for publishing in scientific journals but for knowing if time to irrigate, its not bad.

I have been mentioning quite often how to know when to apply water by using a soil moisture sensor. By sticking the probe in the soil near the middle of where the roots are located and taking a moisture sensor reading, you can roughly estimate the level of moisture in the soil. Here's how.

Chose a moisture sensor.

This is an inexpensive soil moisture sensor you can buy from Amazon, Lowes, Home Depot and any local nursery. Its less than $10 and made for soft soils that are easy to push into but with some care how you use them you can make them last a couple of years of occasional use.

 

This one had a recognizable name from their compost thermometers but this one didn't hold up in desert soils. The tip fell off after using it for three months.

This is what happened to the Reotemp Soil Moisture Sensor after three months of occasional use. Maybe ten times. Maybe I got a lemon.

2. Adjust the sensor.

The Lincoln and Reotemp both have an calibration screw on the backside. I stick the tip of the probe in a jar of water, let it get wet for about 30 seconds and adjust the meter so it reads "10". I need a very small tipped screwdriver, like one a jeweler uses, to make the adjustment. I do this every time I go out and take measurements.

Here I am inserting the tip of the probe into a glass of water, waiting about 30 seconds and then checking to make sure it reads "10". If not, which is often, I adjust the sensor to read 10 by turning the adjustment screw on the back until it reads "10". 

Some of the inexpensive plastic types may not have anything to calibrate. In cases like that, I make a note of what number it reads when the tip is inserted in the glass of water and make a mental adjustment to my measurements in the field.

3. I always take three measurements near the same spot. This is because the soil is not the same due to the variability of the soil in the field and rocks. I insert the tip of the meter into the soil, slowly, until it reaches about halfway of the depth of the roots of the plants. Sometimes I have to do this four or five times to get a good measurement due to rocks. For instance on new fruit trees the depth it is inserted is maybe only four or five inches. But I watch the needle move as the tip is inserted deeper. For established trees I will measure the soil moisture at about 6 to 8 inches. Average readings of about "5" tell me its time to irrigate.

4. I use rebar to tell me how deep the irrigation water is draining.

I use 3/16 inch rebar, four feet long, to tell me if I watered deep enough.

The irrigation depth of plants should be 8 inches for annual vegetables, lawns and annual flowers. For small shrubs and perennial vegetables like artichokes and asparagus it is 12 inches. For medium sized shrubs and small trees, 18 inches deep. Medium sized trees 24 inches deep. Large trees like many pines, 36 inches deep. by pushing on the rebar into wet soil it will be hard to push where the soil is not wet. For large trees you need a four foot long rebar.

To change the depth of irrigation on the same valve, change the size and number of emitters to the plants not getting enough water.

5. Irrigation water should be applied to AT LEAST half the area under the plant canopy. If the plant is three feet wide then four emitters placed 12 inches from the main stem is enough. If the spread of a tree is ten feet, then place a circle of four emitters 12 inches from the trunk and a second ring of emitters 18 inches beyond that. I doubt people will water the entire area under the canopy.

The area under theis tree needs more drip emitters and the emitters spaced closer to get enough water applied and to the right depth.

6. Mulch slows loss of soil water from evaporation and makes plant roots more competitive. Mulch will add one to two days extra between irrigations during summer months.

Apply woodchip mulch 3 to 4 inches deep to soils beneath plants that are not native to desert areas like these fruit trees. Rock mulch can be used 2 to 3 inches deep to cover soils beneath desert plants. But my experience is that even desert plants like woodchip mulch.