The blue dye doesn’t lie

by Bill and Mary Weaver
“We have to manage soil nutrients and irrigation together, because the two are intertwined,” stated Dr. George Hochmuch, Professor at the University of Florida, whose research focuses on developing Best Management Practices for plant nutrient use to protect water quality. “A grower can do a visual demonstration of this simply by injecting blue dye into his irrigation system along one row. Turn on the irrigation and watch where the dye goes.” If you’re over-irrigating, or irrigating too long at one time, you can clearly see the blue dye (along with any nutrients you’ve added to the water) ending up below the root zone, where they will be of no benefit to your crop plants. Any leached nutrients can eventually contaminate the ground and surface water. This eye-opening demonstration has been done widely on Florida farms by Extension agents, leading to the common saying among agents and growers, “Blue dye doesn’t lie.”

“Florida soils are mostly very sandy, and Florida weather conditions can be very different from the Mid-Atlantic” [where this presentation was given] or other areas struggling with nitrate contamination of ground water,  according to Hochmuth, “but the principles I’ll be presenting are very similar in many areas, and hold across the country. “But I do encourage you to consult the irrigation section your state or region’s production guide also. The Mid-Atlantic guide has much very good information. The irrigation section echoes what we do in Florida. Work with your Extension agent. Use the information I’m giving you as guidance to review with your Extension Service.”
The State of Florida is very serious about managing soil moisture and nutrients as a single package. For example, one agricultural area in northern Florida has several thousand center pivots. “We now have a resource that goes around to check each of those center pivots every year for leaks and for uniformity,” commented Hochmuth.
In addition, the Florida Department of Agriculture has provided grants to farmers to buy each farm a weather station. The idea is to provide each farm with real-time evapo-transpiration data that growers can use to manage their irrigation, which in turn can help to prevent nutrient loss through leaching. “In Florida, irrigation management has become so precise that we can dial up and find out the evapo-transpiration rate for a given crop under certain soil and weather conditions,” added Hochmuth. Why is evapo-transpiration being stressed so much? “Ninety-eight percent of the water the plant takes up goes up through the plant and passes out through the leaves in transpiration,” continued Hochmuth. But that is not water loss. That’s a very important use of water, because it moves nutrients from the soil up to where they’re needed in the plant, and also helps to cool the plant. “Evapo-transpiration (ET) is a summary of the amount of water that’s being evaporated out of our fields from the soil surface, plus the amount that’s being used and transpired by our crops.” Hochmuth uses the ET for a given crop under the given weather conditions and soils to set up a kind of “checkbook approach” to timing irrigations.  “You can type in ‘checkbook irrigation approach,’ and a number of websites come up that have spread sheets where you can plug in your data,” he added. ET will vary with the stage of the crop. Small plants with few leaves will lose less than large plants with many leaves and a large root system. But if you know the ET for your crop and know your soil texture, you don’t have to do a lot of math to figure all this out yourself.
Soil scientists have already figured it out, under varying weather conditions, and can give you that amount for the “output” in your irrigation “checkbook.” You make “deposits” to the checkbook in the form of irrigation when the “water reservoir” you are managing in the plants’ root zone is 30-60 percent depleted. But when you’re irrigating be careful not to fill up your root zone water reservoir completely. If you get a rain, then that zone will be overfilled and leaching or runoff of nutrients and water can occur. “We do get Mother Nature,” Hochmuth chuckled. Your water holding capacity in the root zone is related to your soil texture. In regions like much of the Mid-Atlantic, where the soil has less sand than Florida and more silt and clay, the finer soil particles will hold moisture better. “A soil with finer particles could hold 3 times as much water in the root zone.”
Also, remember that wilting doesn’t mean there is no water in the soil. Some of the water in the soil is tightly bound to soil particles, particularly when you have finer soil particles, and unavailable to plants. So by taking our soil texture into consideration, we can find out more about the available water in our soils and the water holding capacity. Organic matter also influences water holding capacity. If you’re unsure, ask someone familiar with soil types to check your soils. Another factor in determining how large a “deposit” of water to apply to your “checkbook” is irrigation efficiency, Hochmuth continued. For example, if your irrigation system has leaks, you could be applying less to the crop than it appears. With black plastic and drip, you can apply less total water, but more frequently and in smaller amounts per irrigation, than with center pivot, because it’s more efficient and more accurate, since water is being applied to the root zone primarily.
Double-checking your irrigation management with a soil moisture sensor is a good idea. “The better sensors are becoming more financially available,” advised Hochmuth. “Tensiometers may be one of the best approaches. They’re simple to use. Know where the needle needs to be on the gauge for your type of soil. (Check the irrigation section of your Region or State’s Production  handbook.) If you use two tensiometers, a short one in your active root zone to schedule irrigations, and one in your deeper soil, when you see the deeper one below the root zone getting wet, you might want to back off on irrigation amounts.”
Good irrigation management will allow you to be a good manager of your nutrients. “Learning the ‘checkbook water budget’ approach is a good goal. In the irrigation guide for the Mid-Atlantic States of PA, Maryland, and New Jersey, for example, you have a table of irrigation run times. That would be a good place to start. Depending on your soil texture, the table tells you how long to run your irrigation.” Other states and regions may have similar resources.