Managing Irrigation Like You Manage Your Truck’s Fuel Tank

By Travis Goldman
Hortau Grower Support Specialist

When filling up our truck at the gas station we would never think about pumping more fuel than what the tank could hold. We also would never intentionally run our diesel truck out of fuel just so we could spend half the day fixing it. The fuel tank in our truck is a lot like the soil profiles that we farm — and timing is everything when it comes to irrigation management. Without available “fuel” to our “engine,” the daily progress of our crop is stopped and we lose valuable time in a day that we can’t spare.

My territory for Hortau covers a large geographical area that ranges from Dixon, Calif., to Santa Margarita, Calif. If I could, I would only refuel my truck just once a week. This would save myself valuable time and eliminate the need of refueling every other day.

The tank in my diesel truck holds roughly 27 gallons, and I am usually able to drive 450 miles before refueling. If I were to tell a clerk to pump 35 gallons, he would think I was crazy. But I explain to the clerk that I need to travel 580 miles, so he goes along with it. The pump stops at 35 gallons and the extra 8 gallons pumped runs all over the side of my pickup onto the ground. This creates two scenarios:

  • Scenario #1: I’m burned at the stake because I’m in Santa Cruz County.
  • Scenario #2: I just wasted $24.72 in diesel, but I’m happy as a clam because I believe I can drive 580 miles.

While on my 580-mile drive, I run out of fuel after only 450 miles. I tell myself this couldn’t be. Because my tank is a fixed volume. That’s when scenario number two hits me.

In soils, the Available Water Holding Capacity (AWHC) is a fixed volume between Field Capacity (FC) and Permanent Wilting Point (PWP). This fixed volume of water is exactly like our fuel tank. We can’t apply more than the 27 gallons without exceeding our field capacity and we can’t drive further than our PWP of fuel just below the fuel sending unit. Timing is everything, but so is volume.

That said, the example I gave referred to my specific rig, but we all know that soils, like different vehicle fuel tank sizes, have different AWHC.

Hortau precision irrigation for lettuceWhen managing a crop, we have to look at water available to the crop using soil tension as well as volume of water leaving the system through Evapotranspiration (ET). Setting up a Maximum Allowable Depletion (MAD) higher that PWP for a crop is a critical trigger for irrigation management and timing.

In a lettuce field in Salinas, Calif., I know that the AWHC of Lot 22 is 1.8 inches for the one foot root structure and the MAD is 0.75 inches. A 3-hour irrigation will apply enough water to recharge the soil profile back to field capacity by applying 0.75 inches. If the grower were to apply 6 hours of water (1.5-inches H20) at the MAD, he would not gain twice the time before the next irrigation but rather leach .75 inches from the root zone and waste 3 hours on a well that could have been used on a different lot. Equally, if 2.1 inches of water has been used by that crop we have exceeded the PWP and have hurt the crop; simply applying 2.1 inches won’t help.

By maintaining an available source of water to the crop we can maintain the fuel required for plant development. With the right amount of water applied, nutrients will stay within the root zone, reduce leaching and reduce crop stress. Overwatering can result in hypoxic root conditions, leaving the crop handicapped until adequate air/water ratios are met. Equally, if water is depleted lower than the MAD, the plant will reduce transpiration until it meets Permanent Wilting Point.

With Hortau, growers are able to monitor soil tension in real time, helping them to figuratively manage a field’s “fuel tank” in order to reduce stress, keep crops healthy and productive, and ultimately boost the bottom line by eliminating unnecessary irrigation and pumping costs.

If you have any questions in regard to field capacities of your soils, feel free to contact your local Hortau Grower Support Specialist.

About the Author

Grower Support specialist Travis Goldman was born and raised in Watsonville and currently resides in Freedom, Calif. He graduated from UC Davis and has worked throughout the Salinas Valley as a service technician to learn and understand the technology.
As a Grower Support specialist, Travis is committed to ensuring that all aspects of the Hortau platform are supported in the high demands of the territory.

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