It rained yesterday: Why aren’t my sensors responding?

Jul 6

After heavy irrigation or precipitation, farmers immediately check their Trellis Dashboard expecting a huge spike in soil moisture content, but sometimes receive only a moderate response. In these scenarios, farmers often contact us because they are worried about the accuracy of their soil moisture sensors. This post, written to quell these worries, explains why soil infiltration and application rates are critically important and how they impact your soil moisture content.

What is soil infiltration rate? What about application rate?

Soil Infiltration Rate

Soil infiltration rate is a measure of how quickly soil is able to absorb rainfall or irrigation, measured in inches per hour. The table below lists average soil infiltration rates for various soil textures.

Keep in mind, the soil infiltration rate changes based on how wet the soil is prior to precipitation or irrigation. If the soil is already saturated, its infiltration rate will be lower than the rate shown in the table, regardless of the soil texture.

Application Rate

Application rate is a measure of how quickly water is applied to soil, measured in inches per hour. Sometimes water reaches our fields in a natural, yet unpredictable way - in the form of rain! We were caught in a nasty rainstorm just this week where 2.5 inches fell in two hours. So, the application rate in this case would be 1.25 inches per hour.

If you want to calculate your irrigation system’s application rate in a field, take the amount of irrigation you're applying divided by the time it takes to go over a portion of your field. So, if you're applying half an inch and it takes 30 minutes to walk a portion of your field, the application rate is 1 inch per hour.

Why are these rates important?

Soils can only absorb so much water per hour. If your application rate is higher than your infiltration rate, the extra water is lost to runoff, evaporation, and transpiration through the leaves of weeds.

It’s better for the crop, soil health, and your bottom line to apply less water, but more frequently. Giving crops the amount of water they need, exactly when they need it, means growers can eliminate unnecessary pivot rotations and spare waterlogged crops, saving thousands of dollars in expenses.

How do soil infiltration rates relate to soil moisture management?

Soil’s current moisture content largely depends on the soil’s infiltration rate and water holding capacity. Water holding capacity is the amount of water a certain soil can hold, measured in inches of water per foot of soil. Like soil infiltration rates, water holding capacity values are dependent on soil texture.

Soils with high water holding capacities sustain plant growth and retain nutrients and pesticides better than soils with low water holding capacities.

Be patient with your irrigation. Although it may be tempting to soak your fields when your sensors register as dry after a significant rain event, be mindful of the soil’s water holding capacity. Soils can only hold so much water; clay, for example, can take hours to absorb even a small amount of irrigation. If you overwater your crops, you could be wasting thousands of dollars and inflicting serious damage to your yield. Excess water can wash away essential nutrients and pesticides and make your crops susceptible to disease.

Helpful Resources

Liz Buchen