Growers and consultants often ask us, “How did you determine the ‘Wet,’ ‘OK,’ and ‘Dry’ zones on the soil moisture chart?” and “How do I know when to irrigate?” With the growing season gearing up, we thought it would be a great time to dive into that topic.

Where do the moisture zones come from? 

The Trellis Dashboard shows growers and consultants soil moisture and temperature data collected by sensor stations installed in their fields. A chart like the one shown in Figure 1 below displays each sensor station’s moisture and temperature data. Each sensor station collects moisture data at two depths. The solid line shows the moisture readings of the top (shallow) sensor and the dashed line shows the moisture readings of the bottom (deep) sensor (see Figure 1, below).

Bottom line, the soil texture in which you install your soil moisture sensors determines the values of the “Wet,” “OK,” and “Dry” zones that show up on the soil moisture chart.

Each Trellis sensor station uses two Watermark sensors to measure moisture; these sensors work best in clay, loam, sandy loam, fine sandy loam, and loamy sand soil textures. Figure 2 (right) shows these textures and the Available Water Depletion curve associated with each. It’s widely accepted that if 50% of available water is depleted from a field, it is now considered “Dry.” If you draw a vertical line from 50% Available Water Depletion down to the corresponding soil texture in your field, and then view the corresponding Soil Suction (potential) value, you’ll arrive at the “OK”/”Dry” Threshold that you’ll see in your soil moisture chart.

Similarly, if only 10% of the available water has been depleted, the field is considered “Wet”. If you draw a vertical line from 10% Available Water Depletion down to the corresponding soil texture in your field, and then view the corresponding Soil Suction (potential) value, you’ll arrive at the “Wet”/”OK” Threshold that you’ll see in your soil moisture chart.

The table below shows the “Wet”/“OK” and “OK”/“Dry” threshold for each soil texture.  

When should I irrigate? AND HOW MUCH?

So, now that you understand what the "Wet”, “OK”, and “Dry” zones mean and where they come from, how do you use this information to decide when to irrigate and how much?

1. Look at how fast your moisture levels are changing to determine when to irrigate. The drier the soil gets, the faster the soil moisture readings will change. For example, it might take three or four days for the readings to change from -10 kPa to -15 kPa, but it might take only one day for the readings to change from -25 kPa to -30 kPa. 

2. Look at readings from both sensors to determine how much to irrigate. If the top (shallow) sensor is dry but the bottom (deep) sensor has adequate moisture, your field only needs a shorter irrigation cycle. If the top and bottom sensors are dry, your field needs a longer irrigation cycle to replenish the entire root profile.

If you're looking for a more precise way of deciding when to irrigate, use the predominant soil texture in your field and irrigate when the shallow sensor reaches the trigger levels shown in Table 2 below.

Key Takeaways

• Look at how fast your moisture levels are changing to decide when to irrigate and use both sensor readings to decide how much to irrigate

• Using the field's predominant soil texture helps determine your soil's "trigger level"

• With the Trellis Dashboard, you can set your thresholds, decide how you want to be notified, and then receive notifications via email and/or text when your field reaches critical moisture levels.

Click here to learn more about our Dashboard.

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