A Challenging Year for Planting Corn – Don’t Over Do the Nitrogen

I had a producer call yesterday with an interesting question. He was a little concerned about whether the poultry litter that he had applied and incorporated right away was going to hold his corn until he could take a Pre-Sidedress Nitrogen Test (PSNT) and apply sidedress nitrogen (N). Our first conversation piece was on the current stage of growth of the corn and suggests that there may be some of you who could use a refresher on how to determine the leaf number stage of corn so let’s start with that.

Up until the tassel emerges and we begin talking about the R-stages (reproductive stages) of corn, we use a vegetative designation to indicate the number of fully developed leaves on the corn plant. If three leaves are fully developed, the stage is called the V3 stage. So, exactly what is a fully developed leaf? A fully developed leaf is one in which the collar region of the leaf has emerged from out of the leaf sheath of the leaf below that leaf as can be seen in Photo 1.

The height of the individual plant or the number of immature leaves visible does not affect the actual growth stage designation of a given plant as that is only determined by the number of fully-emerged leaves. Soil nitrate levels for the PSNT are usually determined when the corn is 10 to 12 inches tall so that there is adequate time to obtain the test results and apply N as a sidedress application before the crop grows too tall to safely drive through it applying UAN.

Showing the collar region of the first two fully emerged leaves


Photo 1. Showing the collar region of the first two fully emerged leaves

After we determined that the corn was still too small to conduct the PSNT, we discussed the critical value for the PSNT and the impact of a value greater than that critical value. Numerous tests around the region have indicated that once the PSNT value reaches 20 or higher there is little chance for an economic response to sidedress nitrogen. Work by Hansen and Blackmer (2000), actually showed that when the PSNT N-concentration was above 24 ppm, there were more test locations fertilized with extra nitrogen fertilizer that saw a reduction in yield than saw an increase in yield (about 35 versus 25 percent of test locations). For irrigated growers who have the ability to fertigate corn, the PSNT possibly can save them money by indicating if there is enough N available from manure to carry them to when they traditionally irrigate the crop. In such a case, the grower could skip the sidedress application, monitor the health of the crop, and apply N if the crop needs more during a regular irrigation event.

This procedure is a very real possibility this year since the cool and almost cold spring weather. Until recently, especially in the northern part of the state, little mineralization was probably taking place so much of the N that poultry litter or other manures would supply is still available in the soil. Keep in mind that the majority of the N in manures and poultry litter is in an organic form and must be mineralized (converted from organic N to an inorganic form—ammonium N) before it can be taken up by the crop. Mineralization is a microbial process and requires not only soil moisture but reasonably warm soil temperatures. I think that the mineralization process has been delayed quite a bit this year with the cold March and April weather we’ve experienced and that the N from manure or poultry litter applied and incorporated some time ago is only now becoming available through mineralization.

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