Woods County Agricultural Newsletter, November 2021
Monday, November 1, 2021
Improving Nitrogen Management
Precision Nutrient Management Extension Specialist
The basics for nitrogen (N) fertilizer rate determination can be described in a mechanistic approach by the Stanford Equation NFert = ( NCrop – NSoil ) / Neff. This equations states that the N fertilizer rate is equal to the amount of nitrogen taken up by the crop minus the amount of nitrogen supply by the soil, divided by the efficiency of the nitrogen fertilizer used.
There are nitrogen application approaches which utilize averages collected over diverse environments to create accurate N rate recommendations. The best example of this is the yield goal rules of thumb such as wheats 2.0 lbs N per yield goal bushel minus soil test nitrate. This method and others like it provide an accurate N rate with a slight probability of yield loss. However, the recommendation is often highly imprecise. Meaning that if I apply the method to 100 fields the average will be spot on, however if I look at the performance of the recommendation on a single field, I will likely be disappointed.
When we apply nitrogen based on these broad recommendations, the rate prescribed will maximize yield 90-95% of the time. For example, take the data presented in Figure 1. Over fifteen years of the long-term winter wheat fertility study near Lahoma, Oklahoma the average pounds of N per bushel to reach economic optimum nitrogen rate (EONR) was 1.6, however if 2.0 of N was applied per bushel yield would have been maximized 13 out of the 15 years. While 2.0 lbs. of N per bushel would have been quite accurate for maximizing yield, it would be highly imprecise as over the 15 years optimum pounds of N per bushel ranged from 0.0 to 3.2.
The trick to improving your N rate recommendation closer to a precise and accurate system is to obtain representative site-specific values for the Stanford Equation NFert = (NCrop – NSoil) / Neff.
Looking at the 15-year long-term data above the yields range from a low of 27 to a high of 88 bushels. Of those 15 years, I personally planted multiple years, usually sometime in October, and many of those years while sowing I could have guessed a range of 55-60 bushel, which just happened to be just above the 15-year average. It was not until February and March when the yield potential really starts to express itself. Why, well there is a lot of weather between October to March, with a lot of environmental positive and negative impacts on that final grain yield. February-March is the best timing to go out with approaches, models, or techniques to estimate yield potential for N rate recs.
While soil testing is valuable, pre-plant soil samples for N are just a snapshot in time. The nitrogen cycle, will roar-on well after the soil sample is collected. Organic matter (OM) is the central component of this cycle and drives availability of NH4 and NO3 in the system. For each 1% OM in the top 6″ of the soil there is approximately 1000 lbs of organically bound N. The amount of N going into and out of OM pool is driven by C:N ratio of residues, soil temperature and soil moisture. Our inability to predict long term weather patterns is the greatest factor limiting our ability to predict future availability of NSoil.
Consider how the approach follows Stanford’s mechanistic approach to N management. First the Yield Potential component is related to NCrop. In effect researchers have built models that can correlate the Normalized Difference Vegetation Index (NDVI) collected from a sensor, such as the GreenSeeker, with the crops biomass and chlorophyll content. If given the number of days the crop has been growing it is possible to use the NDVI collected from the crop as a tool to predict final grain yield.
The closer the wheat gets to hollow stem, or the corn gets to tassel, the better the prediction becomes because more “environmental influence” has happen. While post sensing stresses that can bring down final grain yield, the models do quite a good job on predicting final grain yield in-season. This results in a much more site specific value for NCrop.
The final factor in the equation is NSoil, which I will argue is at least as important as NCrop. It would be nice to have a weather station on every field, but we have found the use of a reference strip, high N or low N, really provides an site specific estimate the of nitrogen the crop has access to. If the high N reference (N-Rich) strip is showing up that means the remainder of the field is N deficient. This may be due to losses or lack of mineralization, either way more N is needed. If the N-Rich strip is not evident then the crop is finding enough N outside of the reference strip to support its current growth. This could be that residual N or mineralization is high, or it could mean that crop growth and therefore N demand is low. Having the N check strip in each field allows for a season long evaluation. We can use the NDVI to characterize how big or little of a response we have to N. We call this the Response Index (RI). An RI of 1.8 means that we could increase yield by 80% if we add adequate N, if the RI is 1.05 then we are looking at a potential increase of 5%.
Spike in Fertilizer Cost Impacting Farmers
Area Ag Economics Specialist
Henry Hub Natural Gas futures traded at $5.410/MMBtu concluding a spike in prices that reached as high as $6.500/MMBtu. These high natural gas prices are troubling for producers due to the increased input cost of creating nitrogen fertilizer. Anhydrous Ammonia has been quoted at more than $1,000/ton which is over 3 times as high as it has been in previous years. The USDA fertilizer index (using a 2011 Base) in August increased to 84 from 82.4 in July. A year ago that value was 65.3.
The graph to the right paints a clear picture of increasing fertilizer input costs to farmers. Crop prices remain good so be critical of fertilizer purchases and ensure that application is efficient and based on soil tests to minimize financial impact.
OSU Winter Crop School
Student Union, 2nd Floor Ballroom on December 15-16, 2021.
- Early Registration Fee $150
- After November 29, 2021 Registration fee is $225
- No Refunds after November 29, 2021
- Full refunds of online payment will be given if conference cancelled
- Registration includes: Breaks, Thursday Breakfast at the Ranchers club, 2 lunches, and Wednesday night reception.
Updates will be sent via PSS Extension News List-serve - email
Brian Arnall to be added.
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The Woods County Agriculture Newsletter is printed by the Woods County Cooperative Extension Service, Phone: 580-327-2786. This is one way of conveying educational information to agriculture producers and interested citizens. This publication is printed and issued by Oklahoma State University as authorized by the Vice President, Dean, and Director of the Division of Agricultural Sciences and Natural Resources and has been prepared and distributed at a cost of $11.24 for 281 copies.
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