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Agronomics of Over Irrigation – Part 2

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Agronomics of Over Irrigation – Part 2

In our last blog, our Midwest RSM, Scott Speck provided some great insight on the Agronomics of Over Watering. Now Scott follows up with more information on additional effects to over Irrigation.  

I mentioned above about other detrimental effects agronomically to over irrigation. One of the big topics in management is fertility, and the nutrient that is in the spotlight is Nitrogen. Nitrogen is a unique nutrient for crops, and the ever guessing game of placement, uptake, and form is one that continues to be studied. One thing that I find crucial with proper N management ties directly to being a mobile nutrient affected by water. The quantity of Nitrogen that is lost with over irrigation or large precipitation events is a puzzle comprised of soil types, soil temperatures, fertilizer types, placement of the fertilizer, and timing of the application. Nitrate loss is most significant form of Nitrogen loss with improper irrigation or rainfall events. Urea in the urea form is similar to nitrate loss and fertilizer placed as urea prior to rainfall events must also be watched. Soil temperatures at 70oF take 1 week to convert N fertilizer into nitrate form in Midwest soils. Soil temperatures in the Midwest in the top 4” of the soil are higher than 70oF allowing for nitrification in our soils. In sandier soils, leaching is a predominant factor in N loss. Every 1” of water onto a sandier soil can result in nitrate movement of 6-8” through the profile. By over irrigating early in our sandier soils, we can see movement down below our active root zone quickly! In our heavier soils, leaching is still a factor but we also run into denitrification and runoff. In the Midwest, with our soil below 4-6” being wet, irrigations can cause saturation of the profile resulting in denitrification. When the soil is saturated, microorganisms in the soil steal oxygen from Nitrate to survive, resulting in N2 gas leaving the soil and robbing our fertility from the crop. Denitrification is significant, and can cause a 2%-5% loss in nitrate per day of saturation. With higher temperatures and longer saturated fields, this number can climb higher for nitrate loss. If we continue to irrigate when it is unnecessary, we can be causing large losses of nitrate in our field. At $0.40/lb of Nitrogen, a field with 200# applied to date, with an irrigation event saturating the soil for 5 more days could be a cost of $20.00/ac just from nitrate loss alone, regardless of what it costs to run our irrigation system or lost yield at harvest time. Each soil is different based on the Water Holding Capacity (WHC) determined through soil texture and organic matter. Understanding our soils and how to manage them is key to properly timing applications in the right place with the right amount. CropMetrics VRI can adjust water rates to impact these soil types and run Runoff Control or Fertigation Prescriptions, tailored to each field. CropMetrics Certified Dealers identify areas of different soils in a field, and then determine the WHC through texture and organic matter sampling, identifying the true WHC of each different zone.

Another component of overwatering and improper timing of application is chemical movement. Chemical is similar to nitrogen in that chemical moves with our water application. If we are over applying, chemical can seep through our profile to depths unavailable to be active against weed pressure. Not only is this financially a waste of money in economically deterred times, but also is unsafe to our environment. Leached chemical, like Nitrogen, can end up in our groundwater or be ran off in slopes into off-target locations. If placed too far in the profile due to over irrigation, the chemical can become useless to oncoming pressures.

Each field, crop, and soil is different, but it is key to ask ourselves some questions first before answering the question of “Should I start irrigating yet?” Reach out to us if you have questions!

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