This tip was provided by:
Calvin Trostle, Extension Agronomy, Lubbock, 806-746-6101, email@example.com
Still “Free” Nitrogen for Grain Sorghum Production: Revisiting Soil Profile N
It has been almost five years since I first mentioned this topic in Sorghum Tips. What has changed?—Nothing! Across Texas crop consultants, producers, AgriLife staff, etc. continue growing awareness of the potential for accumulating nitrate-nitrogen (NO3-N) in the soil. This nitrogen has value for your sorghum and other crops. “Profile N” is readily available nitrogen accumulating below standard soil sampling depths (most often 6”). This accumulation is due to over fertilization (or underutilization in years when production is sub-par) with N, and the majority of the time producers are not aware of the presence of this N in the soil. Historically this N is not accounted for in supplying crop nutrient requirements, but it should be—all of it!
The level of N accumulation can vary greatly due to fertilization practices, downward percolating moisture from rains which carries the soluble and mobile nitrate, soil type, etc. Sometimes substantial N is found even below 3’ in the soil, but only deep rooted crops can tap that N. Texas A&M AgriLife’s Soil, Water, and Forage Testing Lab has updated their “Profile Soil Sample Information Form” (SP17) for 2017. This form pairs your standard 0-6” soil sample (analyzed for multiple nutrients including N, P, etc.; routine analysis is still only $10, the same as in 2012) with a second soil sample from below 6” to as deep as 24”. This paired soil sample is analyzed for nitrate-N only, $4/sample. The submitter marks the depth so a proper calculation of nutrient requirements can be made by the soil test lab
Is profile nitrate-nitrogen down to 24” deep 100% available to grain sorghum?
Yes. Even slightly deeper N is largely available. The Profile Soil Sample form for N credits all nitrate N at 6-24” to your crop requirement thus reducing fertilizer costs. Extension recommends that producers include at least some profile soil N sampling to establish whether there might be deeper N present.
Some producers disagree with crediting all nitrate-N in the 6-24” range to grain sorghum. In Texas across the state for different soil types, and yes, different crops, for all practical purposes you can credit all measured nitrate-N to your crop requirement. Many producers have never heard this. When I conduct Extension educational programs on this topic, I ask for a show of hands “How many of you think you can fully credit 100% of soil nitrate-N in the top 24” to your crop requirement?” (I deliberately emphasize ‘fully’ and ‘100%’). Normally about 10% of attendees say you can, and 20-30% say you can’t, and the rest are sitting on the fence. Yes, as noted above, “for all practical purposes…” Perhaps a producer is truly uncomfortable with this idea; if so, then credit 2/3 or ¾ of you soil test profile N—this is still a potentially significant cost savings. What do other states do in their soil testing labs do? They generally credit 100%, and in fact several states including Kansas now recommend a 24” soil sampling for all nutrient analyses.
The soil test lab I prefer doesn’t offer a profile soil test for N. What do I do?
Most labs in fact don’t have a separate form for nitrate-N only soil profile test. If you like the concept you can still submit your samples to that lab, just fill out one form for each sample then you will need to total the sum of nitrate-N. for all samples (above 3’) by your own hand.
The current Texas A&M AgriLife Extension Profile Soil Sample form is found at http://soiltesting.tamu.edu/files/profilesoil.pdf