The Right Nitrogen

 Dr. Josh McGrath, soil management extension specialist at the        
 University of Kentucky, spoke about in-season nitrogen                      
 management in corn and gives some advice about nitrogen rates in wheat.  

 Photo by John LaRose, Jr.

Many Tools Available To Check Nitrogen Needs In Plants

MidAmerica Farmer Grower

   In-season nitrogen management in corn was one of the topics presented by Dr. Josh McGrath, soil management extension specialist at the University of Kentucky. He also gave some advice about tweaking nitrogen rates in wheat.
   “We have several tools that have been available for a long time to help us manage nitrogen fertilizer in season,” he said. “We also have some other new tools that I think are really exciting; they are going to move us along to being more efficient with our nitrogen fertilizer.”
   Whether it’s increasing cost of nitrogen fertilizer or decreasing grain prices, it’s really important to be mindful of that efficiency with fertilizer usage. Nitrogen is tricky to manage because it’s so mobile and the nitrogen cycle is so complex. Therefore any tool that can help farmers become better informed and make better decisions with nitrogen applications is going to boost profitability. With increasing scrutiny environmentally on agriculture, water quality, ground and surface water, farmers have to be good stewards; using these tools puts forward a good face for agriculture. McGrath first discussed a nitrogen reference strip.
   “I think the nitrogen reference strip is critical for in-season management of nitrogen in corn,” he said. “A lot of people say ‘well, I don’t need to manage my nitrogen in season, I can put it all down preplant or fall anhydrous.’ However, that in-season nitrogen makes you smarter. You have 30 or 40 days of knowledge in your back pocket; you know what the weather has been, what the growing conditions have been since you’ve planted that crop and that makes you smarter. So, using a nitrogen reference strip, you can watch how it compares to the rest of the crop that has a normal rate and that strip is going to have excess nitrogen on it.
   “You can go out in that strip, maybe it’s 100 feet maybe it’s 200 feet, and double down on your preseason nitrogen; maybe put an extra 100 pounds out there,” McGrath suggested. “Then you can see how it compares to the rest of the field and as this starts to separate you’ll learn you’re growing out of the nitrogen that you put down at or before planting.”
   This can be seen visually but some tools are available that are much better than a visual examination. Trimble’s Greenseeker, the AgLeader’s OptRX Sensor, Yara’s N-sensor, are active optical sensors that measure NDVI which is the ratio that involves red and near infrared light. They are active so they put out their own light; it doesn’t matter what the atmospheric conditions are. Trimble has a nice sensor that is small, hand held and inexpensive, and you can sense that reference strip, sense the normal area in the field, collect that data and see how it compares to that reference strip. There are algorithms that many of the land grant universities have put out across the country that have been proven to work very well at providing a nitrogen rate based on those sensors, whether they are hand held, or mounted on a tractor. They vary that nitrogen rate in season.
   “A number of land grant universities have done that, they have really good algorithms that convert that reading from that sensor into a nitrogen rate,” he said. “The two key things are the reference strip and the algorithms that have been developed for your local growing conditions.”
   Another tool available for a long time is the leaf chlorophyl meter. Commercially most people know it as the SPAD meter. You clip it on a leaf, it passes light through the leaf, looks at the absorption, and measures chlorophyll content compared to a reference strip. Again, the reference strip is needed, and it helps form a decision on in-season nitrogen rate. Farmers can learn whether a little more or a little less nitrogen is needed to catch up to that reference strip.
   “For the amount of nitrogen we need, we typically must consider that which comes from the interaction of climate and soil, determining the amount of plant available nitrogen that the soil is going to provide,” McGrath said. “That’s really hard to predict and that in-season knowledge management makes you smarter; you have 40 days of climate, you know at that V4, that V6 rapid growth stage, how much mineral nitrogen the soil pumped to the plant, and that sensor or SPAD meter will help you get that variation across the field. Farmers can learn what they have gotten out of the soil, considering maybe a really warm spring, and determine whether there already is plenty of soil nitrogen.
   “If, however, it was a little cooler, maybe you need to add a little extra kick,” he added.
   Where manure has been used is also a great place for these tools. “I’ve got manure I put down over the winter, but in the spring it’s hard to predict what is available. We can sample that manure all we want, but it’s difficult to know how much organic nitrogen is going to mineralize and these tools I mentioned help to narrow in on that.”
   McGrath also mentioned the in-season nitrate tests for corn, typically called the PSNT pre-sidedress soil nitrate test.
“That’s a fantastic test, and most states have recommendations built around it,” he said. “It’s great when you have manure in the system to help adjust that in-season nitrogen rate.”
   McGrath also recommends the corn stalk nitrate test. It’s not an in-season tool, it’s postseason, postmortem. After black layer, after the corn is matured, farmers can get the corn stalks and, according to local university recommendations, determine whether there was too much nitrogen in the system.
   “That’s a good thing for checking nitrogen supply from manure,” he said. “After two, three years of collecting corn stalks you can put your finger on what your fields are going to do; because if you’re over about 2,000 parts per million in that corn stalk, chances are you had more nitrogen than you needed for the yield you made. So you need to bring your yield up or you can dial back your nitrogen rate, but that is something you need a couple of years of data to determine.”
   A lot of phone calls come in questioning nitrogen content in tissue samples at V4, hoping that will help determine the sidedress rate.
   “I am not a fan of managing my in-season nitrogen in corn with a tissue test of V4,” McGrath said. “That plant is just hitting that rapid growth stage, that nitrogen in that tissue just changes so fast as the plant grows because you’re putting more carbon into that plant, so the concentration of nitrogen might go down and might go up; so I don’t really look at that as a tool that can effectively predict the rate needed for sidedress in corn.”
   McGrath said farmers need to be efficient with fertilizer resources.
   “Nitrogen costs money, we want it to produce yield, and as farmers we don’t want it in the waterways either,” he said. “So managing nitrogen in-season, split applied, put some up front, some at sidedress is about the best practice we have. Even if it doesn’t decrease the rate that we end up putting down, we’re smarter when we sidedress, it makes us more efficient. We have tools, we have active sensors like the Greenseeker, the OptRX or the Yara N-sensor. We have leaf chlorophyll meters, in-season soil nitrogen tests like the PSNT, and we have post mortem tests like the corn stalk nitrate tests, all these give us information that, with the right recommendation system, are going to make us more efficient with our nitrogen management.” ∆
   BETTY VALLE GEGG-NAEGER: Senior Staff Writer, MidAmerica Farmer Grower
MidAmerica Farm Publications, Inc
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