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osunpk

osunpk

Since 2008 I have served as the Precision Nutrient Management Extension Specialist for Oklahoma State University. I work in Wheat, Corn, Sorghum, Cotton, Soybean, Canola, Sweet Sorghum, Sesame, Pasture/Hay. My work focuses on providing information and tools to producers that will lead to improved nutrient management practices and increased profitability of Oklahoma production agriculture

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In-season N application methods for Sorghum

Raedan Sharry, Ph.D. candidate under advisement of B. Arnall
Brian Arnall, Precision Nutrient Management Specialist

The data about to be reported is from the study we have fondly named “Burn Baby Burn”, you will see why soon enough.

Grain Sorghum production continues to be an important component of many growers crop rotations in the Great Plains. However, for many growers who focus primarily on small grains production, equipment restraints may impose limits on in season nitrogen (N) management. When producers are able to delay the application until in-season it helps to ensure that N is available to the crop at the time of increased uptake during the reproductive stages of the crops life. Producers often have access to equipment and technologies that may be used to take advantage of improved N application timing, but may worry about the negative effects that nitrogen can have if the fertilizer is inadvertently applied to plant material. An experiment was initiated in Central Oklahoma to evaluate the yield response of grain sorghum to in-season nitrogen application methods.

Trials were placed at Lake Carl Blackwell near Stillwater, Perkins and Chickasha Oklahoma and included 9 in-season fertilization methods and a 0 nitrogen control. Treatments are listed in Table 1 below.

In total 120 lbs of N was applied to all treatments receiving in-season applications. 60 lbs was applied at planting to all treatments including the “Zero N Control”. The remaining 60 lbs. of N was applied according to application method in-season. The urea was applied by hand and the liquid treatments a push cart with adjustable boom height (Figure 1) was used to apply the UAN. Applications were made mid day at V8 growth stage. The temperature at the time of all applications was about 90 F and humidity below 75%. Nozzle position for 30″ and 60″ was set for between rows.

Figure 1 In-season nitrogen application using the T-bar 20″ treatment.

At two of the three locations (Stillwater and Perkins) the addition of 60 lbs. of N in-season increased yield above the control treatment. At the Stillwater (Lake Carl Blackwell) location there were no statistical differences (α=0.05) between in-season fertilized treatments except the T-Bar 20” treatment (Figure 2). The Perkins location (Figure 3) provided a similar result in which again there was no statistical difference between fertilized treatments, excluding the T-Bar 20” treatment.

Figure 2. Grain yield (bu/ac) in a grain sorghum N application study located near Stillwater, OK.
Figure 3. Grain yield (bu/ac) in a grain sorghum N application study located at Perkins, OK.

The Chickasha location differed in that additional in-season nitrogen did not improve yield (Figure 4). While we want a response to applied N, in the case it allows use to solely evaluate the impact of burn associated with N application. The T-bar 20” treatment statistically negatively impacted grain yield and the FlatFan-20″ did at α=0.10, which means we are only 90% confident the yield lose was due to treatment. This response has been consistent across all three locations, on average decreasing yield approximately 21 bu/ac relative to the individual site grain yield average.

Figure 4. Grain yield (bu/ac) of a grain sorghum N application study at Chickasha, OK.

Even though it was mentioned for Chickasha, it is also important to note that while it was not statistically significant (α=0.05) the FF- 20” treatment (Flat Fan nozzles above canopy on 20” spacing) trended towards decreasing yields at all 3 locations and is likely detrimental to crop performance. At all locations substantial damage to leaf material was observed, similar to that pictured in Figure 5 below. Several of the treatments damaged leaf material on the plant through burn injury, but most were not negatively impactful on grain yield in the 2021 growing season. Grain sorghum yield did not benefit from moving the application point below the canopy using drop attachments, nor did adjusting nozzle spacing from 30 to 60”.  Source was not a significant factor impacting grain yield regardless of it application method.

Figure 5. Aerial image of plots located at Perkins, OK in a grain sorghum in-season nitrogen application study.

The observations from this study show that many of the in-season nitrogen application methods that are available to growers will not negatively impact yield. This however does not apply to tools such as the T-Bar. Similar tools that concentrate large amounts of N to leaf material are also likely to produce similar results.  It is important to note that the T-bar was used on 20” spacings and not tested otherwise. Moving the spacing of the T-bar may lead to different results.

Growers who are looking to move N applications in their grain sorghum crop to in-season to capture the benefits associated will likely be able to with equipment that is already available to them. While leaf damage may be observed under sub-optimal application methods, damage is unlikely to contribute to significant yield loss. However, growers should keep in mind that environmental conditions may have a significant impact on the results seen from these types of application as growers should always look to limit stress to the plant when possible.

We of course will be putting out a second year of this study and will share the results when we can.

For more information or questions contact
Brian Arnall b.arnall@okstate.edu 405.744.1722

Can Grain Sorghum Wait on Nitrogen? One more year of data.

Michaela Smith, Ph.D. candidate under advisement of B. Arnall
Brian Arnall, Precision Nutrient Management Specialist

The impressive ability of sorghum to recover from significant N stress with late applied nitrogen was originally reported in the blog “Can grain sorghum wait on nitrogen”. This projected was replicated again in 2021 and these are the results. During the 2021 growing season, frequency of rainfall events and amounts were similar to the 2020 growing season. Figure 1. shows the application dates and rainfall events.

Planting date and date of Preplant N was April 27, 2022. Graph downloaded from the Oklahoma Mesonet.
Table 1. Planting information for the 2021 delayed nitrogen trial. Dekalb 3307 is labeled as a 61 day to mid-bloom cultivar.

Statistically there was no significant difference in grain yield from the pre-plant (0) and any application up to the 56 day application. This data matches up quite well the yield results from 2020.

Figure 2. Grain sorghum yield results for the 2021 delayed nitrogen study.

Looking and the current soil moisture conditions (Figure 3.) and fertilizer price I think this data supports the recommendation to get the seed in the ground and see how the situation plays out. If there are decent rains at and shortly after planting then there is time to apply nitrogen with little risk of yield loss. If fact the weather during late May and early June provide some of the best chances to getting the fertilizer rained in (Figure 4) . If the weather doesn’t cooperate and provide us the much needed rains, then by waiting to apply we are not left with a lot of Expensive nitrogen setting out in the field of a failed crop.

Final Thoughts Heading into the 2021 Sorghum Planting Season.
– Make sure you have a soil sample, knowing P and K will be critical.
– No need to front load N fertilizer, especially if soil moisture is short.
– Utilize in-furrow P if soil pH and or soil test P is low.
– If you skip pre-plant N APPLY N-RICH STRIPS!!!!
– If you apply pre-plant N consider applying 50% of the expected N or less.
– If you are applying any level of pre-plant N, Create Zero-N Strips.

Figure 3. 32-inch plant available water downloaded from the Oklahoma Mesonet on April 6, 2022.
Figure 4. Probably of rainfall for the Breckinridge Mesonet location for the grain sorghum growing season.

For more information or questions contact
Brian Arnall b.arnall@okstate.edu 405.744.1722

Special thanks to EDC Ag Products Co LLC for suppling NH4NO3 used in the delayed N project.

Original Post March 22, 2022.
Data for this post available in thesis Impact of delayed nitrogen application in grain sorghum
Smith, Michaela Lynn (2021-05). Available at https://shareok.org/discover

            Grain sorghum producers in Oklahoma are challenged greatly by their environment and sporadic rainfall patterns, which diminish as the season progresses. These uncontrollable variables influence timing of nitrogen (N) application and nitrogen use efficiency. Using rainfall events as an incorporation method forces producers to apply before the event regardless of its intensity or delay application until field conditions are acceptable while anxiously waiting for another rainfall event. When deciding to delay N application it’s important to know the effects on physiological development and grain yield.

Figure 1. Field trial at Perkins, showing visual heading differences among nitrogen application timings. Timing from left to right were made 49 DAP, CHECK, 63 DAP.

Trial structure and breakdown

            This study was conducted over the 2020 growing season consisting four locations, including one double cropping system following wheat. Ten in-season applications were made using ammonium nitrate (AN) as the N source at a rate of 90 lbs. ac. Using AN as the N source reduced the risk of nitrogen loss through the process of volatilization as the goal of the research was to test the plant not the fertilizer. A pre-plant treatment served as the standard check, while in-season applications were initiated at 21 Days After Planting (DAP) and applications made sequentially at 7-day intervals. A non-fertilized check was included to the study to confirm locations were responsive to N fertilized applications Hybrid, plant date, and seeding rate can be found in Table 1.

Table 1. Planting information or the delayed nitrogen sorghum trials.

Physiological Response to Application Timing

            Two of the four locations demonstrated an effect to physiological development and maturity with the delay of nitrogen application. A delay in heading by a one to two-week period was observed at Perkins and Lahoma for applications made after May 21st (Table 2.). This delay in heading contributed to similar delay in maturity and potential harvest date. At Perkins decreased plant height was observed in the pre-plant plot and was associated with the onset of late season nitrogen deficiency (Figure 2). While this response was unexpected, the impact of nitrogen deficiency experienced early in the crop growth on the root and shoot growth has been well documented in many species. As a plant experiences nitrogen limitations growth changes from above ground to the below ground parts (roots) in an attempt to alleviate nitrogen stress. This increase in root growth could contribute to a more efficient uptake of nitrogen and decrease loss. In contrast to Figure 2, pre-plant application is shorter than compared to later season applications, this could be a result of inadequate N uptake thus leading to N loss by leaching, whereas later applications had increased root growth for efficient N interception and uptake.

Table 2. Delay in Heading for the Perkins (gray) and Lahoma (green). Letters indicate the start and finish of heading. S represent the start of heading while F indicants the finish of heading, SF denote treatments the started heading and finished within the same week.
Figure 2. Visual maturity differences between nitrogen application timings. Timing of applications are listed within the figure.

Yield Response to Application Timing

Response of N was observed at all locations (Figure 3), while the delay of nitrogen varied in its effects across all locations. Grain yield from each N application was compared back to the pre-plant application to evaluate the effects of timing. All four locations responded positively to N fertilizer.  At both LCB and Lahoma grain yield was maintained with applications made as late as 42 to 63 DAP respectively before any negative trend in grain yield was observed. Perkins was the only locations to have a statistically significant increase in grain yield due to delayed N applications. At this site, which is a sandy loam, waiting until 42 DAP resulted in a 15 bushel increase over the pre-plant plot. Now Alva which was double crop showed that rainfall is key.  At this site, none of the in-season treatments made it up the level of the pre-plant. The reason for this will be discussed further below.

Figure 3. Grain sorghum yield results from the nitrogen timing studies conducted at four locations in Oklahoma.

Influence of Rainfall

            The loss in grain yield at Perkins in the pre-plant application could likely be reflective of nitrogen loss due to leaching. Pre-plant applications have been well documented in the aspect loss as a result of crop requirement and early physiological development. Long term mesonet rainfall data depicts a decline in the probability of rainfall with the progression of the growing season across all locations. In early season the probability of 0.5 inches of rainfall ranges from 8 to 10% respectively for LCB, Lahoma, and Perkins, and dramatically decline to percentages at low as 5% in mid-July during grain filling period. For Alva rainfall probability is substantially lower as its season was initiated during the drier months, which depicted a probability of 6% for 0.5 inches of rainfall, and 4.5% for 1 inch for early season rainfall crucial for pre-plant incorporation and crop establishment. These probabilities drop considerably compared to regular season as the months progress onward, mid to late August probability for 0.5 inches ranges from 0.8 to 11.5%, while for a 1 inch is 0 to 6.9%. Past weather data provided by the mesonet illustrates how later in the season rainfall and its amount is variable, suggesting that in a double crop scenario delayed application is not recommended while it is in regular season crop due to the increased chance of rainfall probability. 

Summary

            The purpose of this study was to evaluate the impacts of delayed nitrogen application in grain sorghum. In order to develop an accurate conclusion additional site years are required, although current data could suggest delaying nitrogen application for full season grain sorghum is possible without a detrimental loss in grain yield. This means producers have time to evaluate the crop and market to determine if more inputs are needed and economical, while allowing implementation of technologies such as the N-Rich Strip and SBNRC.

If you have any questions for comments please reach out.
Brian Arnall
b.arnall@okstate.edu
405.744.1722

Acknowledgement of EDC Ag Products Co LLC for support of this project.

Utilizing N fixing biologicals.

In the past couple years significant efforts have been made to produce N fixing microorganisms that can be utilized in an agriculture system. The atmosphere is 78% N2 and prokaryotic microorganisms such as the bacteria species Azotobacter, Bacillus, Clostridium, and Klebsiella take that N2 gas and turn it into plant available NH4. These organisms have been around providing nitrogen for plants, for as long as there has been plants. In agriculture we have heavily utilized their relationship with legumes however have struggled bringing them into other realms of production. Naturally they tend to be found in areas that are very low levels of nitrogen. For example, prokaryotes were found in the un-fertilized check of the 130-year-old Magruder Plots but are not found any other treatment that receives fertilizer organic or commercial.

 Nitrogen-fixing nodules on soybean roots. Image credit: Bo Ren, Purdue University

Now there are several products marketed as containing N fixing microorganisms suited for use in today’s corn, sorghum, and wheat production. While I have an active research program evaluating the use of such materials in Oklahoma, this blog will not address what works or how well. This blog will touch upon my thoughts on how to utilize a technology such as this if you pull the trigger to implement.

So there is one key to getting a ROI on products that create plant available nitrogen, and it’s a really simple key.
Under Apply Nitrogen
If you apply enough or more N than the crops needs, then there is ZERO value in a product that creates more N. For example, applying one of these products in your 250-bushel yield goal corn after you’ve already laid down 300 lbs of N preplant. Unless you lose it all to leaching, your probability of seeing a ROI on your biological investment is pretty poor. I have a hard time understanding the thought process behind paying for a N fixing product and not lowering your fertilizer rate. I can see one of two reasons. 1) You believe you historically under apply N and are losing yield because of such 2) Are in an environment which has a high potential of late season N losses, and you are unable to make recovery applications.

So what to do if using a N Fixer? I do not have the confidence yet to say, “Apply X product, it will produce Y lbs of N, so cut your rate by Y lbs”. That uncertainty is one of the greatest challenges, not knowing will I get 10 lbs or 40 lbs? If I did, then I would just subtract that off my planned rate. Side note, as someone who has been doing on farm N rate studies for a decade plus, I would have to add that most were likely over applying by that much and could cut back anyways. For me the use of the N Fixers should force your hand into utilizing in-season N applications, regardless the crop. So that you can better predict or determine impact of the product.

This is where the use of a refence strip (N-Rich or Zero N) is the golden ticket. We need a way to quickly evaluate the amount of N the crop has access to. The N-Rich method works best when preplant N is drawn way back. I would add that reduced pre-plant is a great scenario for N Fixers. The N-Rich in comparison to the rest of the field will provide you guidance towards your in-season goals. If the N-Fixers are doing a great job the N-Rich will not be showing up any time soon and you can make your N rate adjustments accordingly. If you are a Pre-plant or die kind of farmer, then I say you need to pull back the reins on the preplant rate but give the N Fixers some room to add value and add in your Zero N strips. These will again let you observe what is happening in the soil apart from your fertilizer. If it is getting on the late side of in-season N and you cannot find your zero, might be a good time to walk away and hang up the fertilizer applicator keys. I have lots of blogs and pubs on the use of reference strip so send me a note if you want to dive further into these approaches.

Nitrogen Rich Strips being applied in winter wheat. Photo credit: Zack Rendel, Rendel Farms Miami Oklahoma.

Feel free to reach out with questions or comments. B.arnall@okstate.edu

Related Blogs

Its dry and nitrogen cost a lot, what now?

The title says a lot about the primary question I am receiving right now. And the latest long range “forecast” does not make me feel any better about the current situation. But it is what it is and many great plains wheat farmers are having to make a decision.

The current situation in the wheat belt is that we are dry to depth, when the 32 inch PAW is on short supply and this comes from a combination of no rain and above average temperatures.

Average 32-inch Plant Available Water. Graph retrieved from Mesonet on January 20th, 2022.
120-Day rainfall accumulation across Oklahoma. Graph retrieved from Mesonet on January 20th, 2022. Start data of this time frame is September 22, 2021
The daily average temperature departure from the 15 year mean temperature for the Lahoma Research Station. The Mesonet long-term averages utilize 15 years of daily data (e.g. daily average, daily maximum/minimum, or daily total) for every current and past Oklahoma Mesonet station.

Fertilizer prices are holding fairly strong, at expensive, and the wheat crop currently seems to be going in reverse. So what is a wheat farmer to do? If we are looking on the bright side the lack of moisture in the surface will help reducing any potential losses through urea volatilization. It does not make the potential for loss zero though. If I am bound and determined to fertilize now, I would be very selective of the source and method of application. The biggest driver, tillage and residue amounts.

  • Conventional Till / No residue (plenty of bare soil showing) and small wheat-
    • UAN via Streamer nozzles
      • Why: With UAN (urea ammonium nitrate) you have a liquid N source that will get onto and into the soil and readily available nitrate. Streaming on will help concentrate the fertilizer and potential reduce any urea volatilization if any dews were to occur. Urea would sit until dissolved and lead to potential losses if the first moisture was heavy dew and not a incorporating rainfall.
  • No-till / high residue (no bare soil showing)-
    • Dry Urea
      • Why: If Our residue is dry when the urea is spread the wind will help push it below the residue surface providing protection until a good rain. If UAN is applied to this dry or even slightly damp residue and not washed off with a rainfall in a week or so the amount of N tied up in that residue will likely be significant.
  • The big wheat (very little bare soil, lots of wheat tissue.
    • Urea or UAN Streamer
      • Why not Flat fan. At least with the current status the wheat is not growing and bigger wheat has increasing levels of tip die back. So while UAN sprayed on actively growing wheat can be absorbed foliarly, stressed wheat can not do it as well. Plus the UAN that hits dead or damaged tissue will not make it into the plant. The UAN applied via flat fan will need incorporation via rain in a couple days.

You may have caught in the paragraph above I said, “If I was bound and determined”. If I had the option I am not pulling the trigger until after I have received some good moisture. I fully expect and have already seen rigs running before every decent chance of rain. Unfortunately many of those chances have not panned out and that will remain my concern moving forward. I want to make sure we have some water in the tank before investing in the system.

But now we increase the risk/fear by waiting and the question I get is what if we don’t get good rains or don’t get good incorporating rains. The short answer is, if we don’t get rains the N application is the least of our concerns. If we approach March 15th and we have not had the rains needed to put a little water in the tank and incorporate the N then we are not likely looking at a bumper crop which will need N. What survives in that scenario will be living off deep soil water, and where there is deep soil water there is a good chance of deep N. The shallow soils will be so stressed that nutrient demand will be very little.

Now lets talk waiting and applying N. How late before we just say we are done. To answer I am going to draw from a data set I talk about a lot, the delayed N work by Dr. Souza. This study was started in the fall of 2016 and concluded with the 2020 wheat harvest. In all, twelve trials were established and achieved maturity. This study was designed to evaluate the recovery of winter wheat grain yield and protein after the crop was N stressed. Treatments included an untreated check, pre-plant application and ten in-season treatments. The application of in-season treatments was initiated when N deficiency was confirmed and treatments were applied in progressive order every seven growing days to the point of 63 growing days after visual deficiency (DAVD). A growing degree days is any day that the average daily temperature is at or above 40⁰ F.  Ammonium nitrate (NH4NO3) was applied at a rate of 90 lbs N ac-1 for all treatments.

With this data we can answer two questions, first at what point did we lose yield compared to pre-plant and second how late could we apply and still increase yield above the check. So comparing to the pre lets us know how long could we wait with losing yield. Across the trials we lost yield three times by waiting too long, at LCB2017b that was 4/19, Lahoma18 it was around 3/30, and then Newkirk2020 we lost yield by waiting until 4/6. This data is why I am pretty comfortable waiting until mid March when and if needed. Now if we look at the check, that will tell us if things start improving late can we get still get a yield bump with added N. Newkirk 2020 was the only time and place we could increase yield above the zero after the 4/14 additions.

Table 1. Date of nitrogen application. Each month is color coded.
Table 2. Evaluation of winter wheat grain yield and protein results compared to the Pre-plant Nitrogen treatment. Red boxes means the treatment yielded statistically lower results, Yellow is no difference, Green means the treatment has increased grain yield or protein. Perkins2018, LSC2018b, and LCB2019, did not have a grain yield respond to N (no red box in Zero check) and therefore will not be discussed.

Take Home Message

My recommendation is that if you are not required to take delivery or needing to cover a lot of acres, i.e. time limited, I would not get in a hurry to apply N on this wheat crop. I think if we combine weather by market this a good time to wait and see. Once we get a rain and have some soil moisture it will be time to run the rigs. The crop currently does not need a lot of N so why spend the $. If things don’t improve by mid to late march, consider the wheat a cover and look towards a summer crop with the hopes of rains in April. If you need to take the crop to yield, then you can wait a while longer and still get a return on the N, with hopes the price could come down a bit.

Finally, While I don’t suggest running fertilizer in front of the first chance of rain, I would make sure I had an N-Rich strip on each and every single field. Strips can go out well past green up and serve a great purpose. The N-Rich strip will help you determine if the crop is able to mine any soil N or if the N tank is dry.

Feel free to reach out with questions or comments.
Brian Arnall Precision Nutrient Management Specialist.
b.arnall@okstate.edu

Special thanks to EDC Ag Products Co LLC for suppling NH4NO3 used in the delayed N project.

Relevant past blogs for your reading enjoyment.

The Easy Button for Nitrogen…….

Brian Arnall, 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. I outline the importance of this equation in the blog “Components of a variable rate nitrogen recommendations“.

There are nitrogen “Easy Buttons” which utilizes 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. Yield goals are generally calculated as the average of the best 3 out of 5 years, or the 5-year average times 20%. Also, the 2.0 lbs of N is more than what is in a bushel as it also adds in an efficiency factor or a 0.5 lbs per bushel cushion. This method and others like it provide an accurate N rate with slight probability of yield loss. However, the rec 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.

Illustration of accuracy versus precision.
Figure 1. Illustration of accuracy versus precision.

When it comes to nitrogen recommendations the Easy button method will use components which help ensure that the rate prescribed will maximize yield 90-95% of the time. For example, take the data presented in Figure 2. 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.

Figure 2. Grain yield (bushels per acre), economical optimal N rate (EONR), and pounds of nitrogen per bushel producer at the EONR, from 15 years of data from the long-term fertility trials located near Lahoma, Ok.

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 started to express itself. Why, well there is a lot of weather between Oct to March, a lot of environmental positive and negative impacts on that final grain yield. This is the best timing to go out with approaches, models, or techniques to estimate yield potential for N rate recs.

While I am a big fan of soil testing, pre-plant soil samples for N are just a snap shot in time. But the While I am a big fan of soil testing, pre-plant soil samples for N are just a snapshot in time, but the nitrogen cycle Figure 3, will roar on 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. While we very well what the mechanisms of the cycle are and can model the reactions quite well. Our inability to predict long term weather patterns is the greatest factor limiting our ability to predict future availability of NSoil.

Figure 3. Complete Nitrogen Cycle. http://psssoil4234.okstate.edu/lecture

This is where the reader should be asking “how can we get better site specific data” and I begin the discussion on why I have been promoting the of the Sensor Based Nitrogen Rate Calculator (SBNRC) and N-Rich strip method.

Lets talk about how the approach follows Stanford’s mechanistic approach to N management. First the Yield Potential component of the SBNRC which is related to NCrop. In effect researchers have built models over the past two decades that can correlate the 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. One reason is that we have allowed more “environmental influence” to happen. Dr. Bill Raun, a founder of the SBNRC concept kept great discussion and data sets on his NUE.OKSTATE.edu website. On the “NUE Website on YP” he provides information on how yield prediction work while on the “NUE Website YP Library” he has not listed every algorithm created, and the math behind them, but also a recipe book for how anyone can create their own algorithm. While there are a lot post sensing stresses that can bring down final grain yield, the models that have been built and continually improved, do quite a good job on predicting final grain yield in-season. Resulting a much more site specific value for NCrop. The blog”Sensing the N-Rich Strip and Using the SBNRC” goes into a further discussion of using the online SBNRC.

That now leaves NSoil, which I will argue is at least as important as NCrop. As weather so greatly influences the nitrogen cycle it would be nice to have a weather station on every field paired with a 0-4 ft soil description which could be incorporated into a model. Given those might be out of reach we have found the 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 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%. I have a previous blog which goes into the application of the reference strip. “Nitrogen Rich Strips, a Reminder

Finally we combine the two, YP and RI. By predicting the yield of the area out side the N-Rich strip we can determine environmental yield potential, YP0. Basically what can the field yield if nothing is added. We multiple YP0 by the RI to get the yield potential with added N, YPN. Then its as simple as N rate = (YPN – YP0 ) x N needed per bushel. So for example if YP0 is 40 bushel RI =2, then YPN is 80 bushel. I need to fertilize the additional 40 bushels of wheat and I can use the 2.0 N per bushel can come up with a top-dress rate of 80 lbs N per acre. We are now incorporating site specific in-season NCrop and NSoil data.

And just a reminder for those of you new to my blog, I have a lot of research documenting that it is not only OK, but often best if we wait on N application in wheat and other crops. Value of In-Season N blog.

Every step we take towards the easy button is often a step towards site specific imprecision due to the use of generalized terms or models. Depending on your goals this very well could be acceptable for your operation, but with nitrogen prices as volatile as they are, should we not be considering pushing the easy button to the side, for now. Let’s add a bit of site-specific data so that we can take advantage of the N the system may be giving us, or the yield we did not expect. Let the N-Rich Strip be that first step.

Relevant Peer Review Publications.

In-Season Prediction of Yield Potential Using Wheat Canopy Reflectance,  Agron. J. 93:131-138

Nitrogen Fertilization Optimization Algorithm Based on In-Season Estimates of Yield and Plant Nitrogen Uptake
  J. Plant Nutr. 24:885-898

Real-Time Sensing and N Fertilization with a Field Scale GreenSeeker Applicator

Identifying an In-Season Response Index and the Potential to Increase Wheat Yield with Nitrogen (pdf)

Nitrogen Response Index as a Guide to Fertilizer Management
 

Evaluation of Green, Red, and Near Infrared Bands for Predicting Winter Wheat Biomass, Nitrogen Uptake and Final Grain Yield 

Full List of NUE Publications

If you have any questions please feel free to contact me @ b.arnall@okstate.edu

Nitrogen Rich Strips, a Reminder

With the recent increase in fertilizer prices just prior to winter wheat planting season I felt it was a good opportunity to bring this older post back up and give it an update. Since the blog was originally written in 2013 there has been a lot of work done both to better understand the nitrogen fertilizer need / timing of winter wheat and efforts to updated and improve the algorithms behind the Sensor Based Nitrogen Rate Calculator.

The Nitrogen Rich Strip, or N-Rich Strip, is a technique/tool/process that I spend a great deal of time working with and talking about.  It is one of the most simplistic forms of precision agriculture a producer can adopt.  The concept of the N-Rich strip is to have an area in the field that has more nitrogen (N) than the rest.  In recent years we have been utilizing Zero-N strips in corn. The approach to some may be new but at one point most producers have had N-Rich Strips in their fields, albeit accidentally.  Before the days of auto-steer it was not uncommon, and honestly still is not, to see a area in the field that the fertilizer applicator either doubled up on or skipped.  In our pastures and dual purpose/graze out wheat every spring we can see the tell-tale signs of livestock deposits.  When over laps or “Cow Pox” become visible we can assume the rest of the field is behind in nitrogen.  The goal of an N-Rich Strip is to let the field tell you when it needs more N. Research has shown wheat can be yellow and recover completely and it may even be a benefit. See the link for the Value of In-season Nitrogen at the end of this blog.

Cow Pox, Image courtesy Kaitlyn Nelson
Cow Pox, Image courtesy Kaitlyn Nelson

What I like most about the N-Rich Strip approach is its Simplicity.  The N-Rich Strip is applied and; Scenario 1. The N-Rich Strip becomes visible (Greener) you APPLY NITROGEN, Scenario 2.  The strip is not visible you Option A. DON’T APPLY NITROGEN Option B. Apply Nitrogen Anyways.  The conclusion to apply N or not is based on the reasoning that the only difference between the N-Rich Strip and the area 10 ft from it is nitrogen, so if the strip is greener the rest of the field needs nitrogen.  If there is no difference N is not limiting and our research shows N does not have to be applied.  However producers who decide to be risk adverse (in terms of yield) can apply N but it would be advised to do so at a reduce the rate.  Now is a good time to note that the N-Rich Strip alone provides a Yes or No, not rate recommendation.  At OSU we use the GreenSeeker optical sensor and Sensor Based Nitrogen Rate Calculator (SBNRC) to determine the rate, but that discussion will come later.  I equate the change from using yield goal N rate recs to the N-Rich Strip as to going from foam markers to light bars on a sprayer.   Not 100% accurate but a great improvement.

N-Rich Strip in no-till wheat near Hobart OK.
N-Rich Strip in no-till wheat near Hobart OK.
N-Rich Strips showing up on google earth image. You can see how the strip on the left is darker than the right suggested a greater need for nitrogen.

Now that we have covered the WHY, lets get down to the nuts and bolts HOW, WHEN, WHERE.

How the strip is applied has more to do with convenience and availability than anything else but there are a few criteria I suggest be met. The strip should be at least 10 ft wide and 300 ft long. The rate should be 40 to 50 lbs N (above the rest of the field) for grain only wheat and canola, 80 lbs N for dual purpose wheat. The normal recommendation is that when applying pre-plant either have a second, higher rate programmed into the applicator or make a second pass over an area already fertilized. Many will choose to rent a pull type spreader with urea for a day, hitting each field.


Also popular are applicators made or adapted for this specific use. ATV sprayers are the most common as they can be multi-purpose. In most cases a 20-25 gallon tank with a 1 gpm pump is placed on the ATV with an 8-10ft breakover boom. The third applicator is a ride away sprayer with a boom running along the rear of the trailer. In all cases when liquid is the source I recommend some form of streamer nozzle.
If this all sounds like to much then the easiest application method might just be a push spreader. No need for trailer or even a truck. In most cases I recommend whichever N source is the easiest, cheapest, and most convenient to apply.

Vincent N-Rich Strip Applicator, Ponca City OK
Vincent N-Rich Strip Applicator, Ponca City OK
Gard N-Rich Strip Applicator, Fairview Ok
Gard N-Rich Strip Applicator, Fairview Ok
Push spreader used by Oklahoma State Cooperative Extension service. Check with your local office. If they don’t have one, we can send one.

When the strip is applied in winter crops proper timing is regionally dependent. For the Central Great Plains, ideally the fertilizer should be applied pre-plant or soon after.  However,  in most cases as long as the fertilizer is down by December or even January everything works. Timing is more about how much the wheat is growing. If it is slow growing fall, timing can be delayed. When the N-Rich Strip approach is used on the Eastern Shore in Virginia and Maryland the strips have to be applied at green up. We have been trying this in Oklahoma and Kansas with good success.  It is always important to make the tools fit your specific regional needs and practices and not the other way around.

Where is actually the biggest unknown.  The basic answer is to place the N-Rich Strip in the area that best represents the field.  Many people question this as it doesn’t account for spatial variability in the field, and they are correct.  But my response is that in this case spatial variability is not the goal, temporal variability is.  Keeping in mind the goal is to take a field which has been receiving a flat yield goal recommendation for the last 30+ years and make a better flat rate recommendation.  My typically request is that on a field with significant variability either apply a strip long enough to cross the zones or apply smaller strips in each significant area.  This allows for in-season decisions.  I have seen some make the choice to ignore the variability in the field, made evident by the strip, and apply one rate and others choose the address the variability by applying two or more rates.  One key to the placement of N-Rich Strips is record keeping.  Either via notes or GPS, record the location of every strip.  This allows for the strips to be easily located at non-response sites.  It is also recommended to move the strip each year to avoid overloading the area with N.  

For more information on N-Rich Strips

Factsheets

https://extension.okstate.edu/fact-sheets/applying-nitrogen-rich-strips.html

https://extension.okstate.edu/fact-sheets/using-the-greenseeker-handheld-sensor-and-sensor-based-nitrogen-rate-calculator.html

https://extension.okstate.edu/fact-sheets/impact-of-sensor-based-nitrogen-management-on-yield-and-soil-quality.html

Related Blogs

YouTube Videos  

Can Grain Sorghum Wait on Nitrogen?

Michaela Smith, Masters student under advisement of B. Arnall
Brian Arnall, Precision Nutrient Management Specialist

            Grain sorghum producers in Oklahoma are challenged greatly by their environment and sporadic rainfall patterns, which diminish as the season progresses. These uncontrollable variables influence timing of nitrogen (N) application and nitrogen use efficiency. Using rainfall events as an incorporation method forces producers to apply before the event regardless of its intensity or delay application until field conditions are acceptable while anxiously waiting for another rainfall event. When deciding to delay N application it’s important to know the effects on physiological development and grain yield.

Figure 1. Field trial at Perkins, showing visual heading differences among nitrogen application timings. Timing from left to right were made 49 DAP, CHECK, 63 DAP.

Trial structure and breakdown

            This study was conducted over the 2020 growing season consisting four locations, including one double cropping system following wheat. Ten in-season applications were made using ammonium nitrate (AN) as the N source at a rate of 90 lbs. ac. Using AN as the N source reduced the risk of nitrogen loss through the process of volatilization as the goal of the research was to test the plant not the fertilizer. A pre-plant treatment served as the standard check, while in-season applications were initiated at 21 Days After Planting (DAP) and applications made sequentially at 7-day intervals. A non-fertilized check was included to the study to confirm locations were responsive to N fertilized applications Hybrid, plant date, and seeding rate can be found in Table 1.

Table 1. Planting information or the delayed nitrogen sorghum trials.

Physiological Response to Application Timing

            Two of the four locations demonstrated an effect to physiological development and maturity with the delay of nitrogen application. A delay in heading by a one to two-week period was observed at Perkins and Lahoma for applications made after May 21st (Table 2.). This delay in heading contributed to similar delay in maturity and potential harvest date. At Perkins decreased plant height was observed in the pre-plant plot and was associated with the onset of late season nitrogen deficiency (Figure 2). While this response was unexpected, the impact of nitrogen deficiency experienced early in the crop growth on the root and shoot growth has been well documented in many species. As a plant experiences nitrogen limitations growth changes from above ground to the below ground parts (roots) in an attempt to alleviate nitrogen stress. This increase in root growth could contribute to a more efficient uptake of nitrogen and decrease loss. In contrast to Figure 2, pre-plant application is shorter than compared to later season applications, this could be a result of inadequate N uptake thus leading to N loss by leaching, whereas later applications had increased root growth for efficient N interception and uptake.

Table 2. Delay in Heading for the Perkins (gray) and Lahoma (green). Letters indicate the start and finish of heading. S represent the start of heading while F indicants the finish of heading, SF denote treatments the started heading and finished within the same week.
Figure 2. Visual maturity differences between nitrogen application timings. Timing of applications are listed within the figure.

Yield Response to Application Timing

Response of N was observed at all locations (Figure 3), while the delay of nitrogen varied in its effects across all locations. Grain yield from each N application was compared back to the pre-plant application to evaluate the effects of timing. All four locations responded positively to N fertilizer.  At both LCB and Lahoma grain yield was maintained with applications made as late as 42 to 63 DAP respectively before any negative trend in grain yield was observed. Perkins was the only locations to have a statistically significant increase in grain yield due to delayed N applications. At this site, which is a sandy loam, waiting until 42 DAP resulted in a 15 bushel increase over the pre-plant plot. Now Alva which was double crop showed that rainfall is key.  At this site, none of the in-season treatments made it up the level of the pre-plant. The reason for this will be discussed further below.

Figure 3. Grain sorghum yield results from the nitrogen timing studies conducted at four locations in Oklahoma.

Influence of Rainfall

            The loss in grain yield at Perkins in the pre-plant application could likely be reflective of nitrogen loss due to leaching. Pre-plant applications have been well documented in the aspect loss as a result of crop requirement and early physiological development. Long term mesonet rainfall data depicts a decline in the probability of rainfall with the progression of the growing season across all locations. In early season the probability of 0.5 inches of rainfall ranges from 8 to 10% respectively for LCB, Lahoma, and Perkins, and dramatically decline to percentages at low as 5% in mid-July during grain filling period. For Alva rainfall probability is substantially lower as its season was initiated during the drier months, which depicted a probability of 6% for 0.5 inches of rainfall, and 4.5% for 1 inch for early season rainfall crucial for pre-plant incorporation and crop establishment. These probabilities drop considerably compared to regular season as the months progress onward, mid to late August probability for 0.5 inches ranges from 0.8 to 11.5%, while for a 1 inch is 0 to 6.9%. Past weather data provided by the mesonet illustrates how later in the season rainfall and its amount is variable, suggesting that in a double crop scenario delayed application is not recommended while it is in regular season crop due to the increased chance of rainfall probability. 

Summary

            The purpose of this study was to evaluate the impacts of delayed nitrogen application in grain sorghum. In order to develop an accurate conclusion additional site years are required, although current data could suggest delaying nitrogen application for full season grain sorghum is possible without a detrimental loss in grain yield. This means producers have time to evaluate the crop and market to determine if more inputs are needed and economical, while allowing implementation of technologies such as the N-Rich Strip and SBNRC.

If you have any questions for comments please reach out.
Brian Arnall
b.arnall@okstate.edu
405.744.1722

Acknowledgement of EDC Ag Products Co LLC for support of this project.

Yellow Wheat the 2020 Edition.

I have been trying to write this blog addressing the yellow wheat for about two weeks now. But with finally finding a dry”ish” day or two and a lot of calls and emails about yellow wheat, I am just now getting to it.
So the short story is there is a lot of wheat out there in the state that is show signs of chlorosis, or yellowing. I wish I could say I have all the answers for you in this article, but I will have to lay heavily upon the agronomist best answer, “Well it Depends.”.

Cow pox showing up in a wheat field in Kay County.

First we will start with the things I know least about and then move on to things that are more in my wheelhouse. In the last two weeks I have been on multiple email strings trying to chase down the cause of chlorosis in fields all over the state.  One of these included Dr. Bob Hunger and the Plant Disease & Insect Diag Lab (PDIDL) and in one field his final thought was “So, my best guess is cold and wet soils along with fungi colonizing the older leaves that are starting to senesce.” At the same time I am finding regular occurrence of Tan Spot and Leaf Rust increase. All these pathogen cause some level of chlorosis and if you do not get down and pull some samples you will never know the cause.

Originally thought to be leaf rust, but corrected by Dr. Hunger who suggested it is early stages of striped rust, found in Stillwater Oklahoma 3.27.2020.

 

A new for me this year is what I am calling the herbicide ding. I was able to get over a lot of my wheat that first week of March with a shot of herbicide, everything was almost to hollowstem. The wheat really got dinged. Very visual yellowing and stunting of the plants. Talking with Dr Manucheri, she had seen the same thing in her plots in Tipton. I have also visited several farmer fields with the same symptoms. Dr. Manucheri shared with me the Finesse label. Directly from the label “Temporary discolorations and/or crop injury may occur if herbicide is applied when the crop is stressed by severe weather conditions (such as heavy rainfall, prolonged cold weather, or wide fluctuations in day/night temps), disease or insect damage, low fertility, applications to course soils, or when applied in combination with surfactant and high rates of liquid fertilizer solutions.” This can be found on page 5, http://www.cdms.net/ldat/ldFSL002.pdf . You can just about mark off every weather and application condition mentions, on the same field.

Image collected 3.25.20. The right side was treated with Powerflex on 3.5.20. The left side was not treated so that sorghum could be planted in April.

Now to the yellow wheat I can comfortably talk about. There is nitrogen deficiencies out there. That should not come as a shock with the amount of rain we have received over the last couple months. I also believe that a fair amount of the wheat crop out there is a bit lacking on roots department.

 

The overarching wet cools soils that we have more than likely have led to reduced root exploration in some areas. And if you combine short roots with a nitrate leaching then the probability of N being out of the reach of the crop is high. Then the question is “Is there still time to do anything?”. The trip I look over the weekend (3/28, 3/29) that encompassed a great deal of the North Central Ok wheat belt showed me that the majority of the wheat had really progressed physiologically in the last two weeks. At this point, a positive return on N investment hinges on the stage the wheat is at.

Our delayed N work over the past several years show that we have maintained the yield on our trials even when fertilizer was delayed into the first week of April. https://osunpk.com/2019/08/14/how-long-can-wheat-wait-for-nitrogen-one-more-year-of-data/

Each graph is from a location where the delayed N study was preformed. The objective of our study was to determine the impact of prolonged nitrogen deficiency on winter wheat grain yield and protein. Eight studies were conducted with 11 N application timings in no-till dryland conditions. A pre-plant treatment of 90 lbs ac-1 of N was broadcast applied as ammonium nitrate (AN). We used AN as our source because we wanted to measure the crops ability to recover and eliminate the impact of source efficiencies. When visual symptom differentiation (VSD) was documented between the pre-plant and the non-fertilized check, i.e the N-Rich Strip showed up, top-dress applications were performed every seven growth days (GDD> 0) (https://www.mesonet.org/index.php) until 63 growth days after VSD at all sites. The only N the treatments received where applied according to treatment structure. No pre-plant N was applied on the trials other than the Pre-plant treatment.

This table shows the application dates of the 10 site years of the delayed nitrogen study. The first column is the location, to the right of the location is two rows the top is grain yield and the bottom is grain protein. Each of the following columns corresponds to an application date. Applications began at each study when the The colors are related to whether that application was statistically (Alpha=0.05) worse than, equal too, or better than applying nitrogen at the first sign of deficiency (0DAVD). For this comparison it is important to know that at no location did preplant have significantly greater yield than 0DAVD.In the majority of those years that first week of April corresponded with the growth stage  Feekes 8, last leaf just visible. As the crop moves beyond that point, catching up did not happen. Currently there is wheat out there in the state that has not hit hollow stem (Feekes 6) and there is wheat at Flag leaf (Feekes 9).

 

The Feekes Scale focused in on the stem extension growth phase. The period extends from hollow-stem (Feekes 6) to boot (Feekes 10).

The high rainfall totals we have could have also led to another deficiency sulfur. In the past S deficiency is fairly hard to find in Oklahoma. Our long history of low S using winter wheat and high sub-soil S levels have kept the response to Sulfur low, but not uncommon. Sulfur is a mobile nutrient and will also be lost via leaching especially in sandy soils in the northern part of the state. Sulfur deficient is different from N in that it shows in the newer growth as a general yellowing of crop. Kansas State has a lot of great resources on sulfur management in wheat.  https://webapp.agron.ksu.edu/agr_social/m_eu_article.throck?article_id=2132

https://bookstore.ksre.ksu.edu/pubs/MF2264.pdf

Sulfur deficiency in wheat. Photos by Dorivar Ruiz Diaz, K-State Research and Extension

If your wheat is yellow and before you call the fertilizer applicator, first confirm it is nitrogen and or sulfur and not something else. A key point to nitrogen deficiency is that the cholorsis will be worst on the oldest leafs while new growth is green. If N deficiency is confirmed then figure out how far along your wheat is. If the crop is around hollow stem to Feekes 8,  if you can get the N on soon there is a good chance to get your money back plus. Keep in mind with air temps above 60 degrees UAN will burn the tissue so it is best to use streamer nozzles, which will still burn but the tissue damage is lessened. If you do not have access to streamers you can dilute the UAN with water and use flat fan nozzles. Cutting the UAN with water reduce the impact of leaf burn, I typically recommend at least 2 part UAN to 1 part water, but a 1 to 1 is the safest.

Image of wheat with forage burn from UAN applied with streamer nozzles. Application was made two days prior with air temps where above 80 degrees.

If you have any questions or concerns please feel free to email any questions you may have.

Brian Arnall
b.arnall@okstate.edu

 

DAP in short supply? Revisiting, DAP vs MAP, Source may matter!

From mid August through October, fertilizer applicators and grain drills are running across the southern great plains wheat ground. Di-ammonium Phosphate (DAP 18-46-0) is maybe the most popular form of phosphorous (P) utilized in wheat production today. DAP delivers a high content of nitrogen (N) while efficiently binding the toxic aluminum (Al) ions low pH soils. However due to the flooding that occurred throughout the spring the Ports have been closed and DAP could be in short supply. I have heard that many companies and Co-ops have already sourced Mono-ammonium phosphate (MAP 11-52-0) to supplement the lack of DAP for our early planted wheat crop. With this happening, I wanted to share some points about the two sources. In a broadcast incorporated scenario I do not have much preference for one over the other. It is when producers are applying the fertilizer in-furrow or applying to alleviate Al toxicity that source can matter. As described in the post below, in a soil with a neutral pH DAP and MAP perform equally well. So in this scenario I give MAP a slight edge over DAP if the price is the same. I say this as you can run less material per acre with MAP and refill a little less often. Many worry about the drop in N delivered with MAP versus DAP but in my work I see that is the P in the starter that gives us a good response and not the N, which can be delivered with pre-plant or top-dress. However, DAP wins out in soils with a pH below 5.5. The original blog below shows the results when DAP and MAP are banded in acidic soils using the same rate of P per acre.  The reason we see this happen is that when MAP dissolves it forms a slightly acidic solution (approx 4.0) while DAP will dissolve to form a slightly basic solution (approx 7.5). In our work BOTH DAP and MAP increased yield above the check in acidic soils, which goes to show MAP is an affect tool for short term remediation of aluminium toxicity (Band P for Al toxicity Blog). However it might require more MAP per acre to reach the equivalent results. Because of what we saw when comparing the two sources at equivalent rates of P, I would recommend increasing the rate of P2O5 from 30 lbs per acre to 35 or 40. This would be going from 65 lbs of DAP to 67-77 lbs of MAP per acre.

So the take home would be as this wheat season takes off and you find that DAP is hard to locate and you A) Have overall neutral (5.5+) pH levels do not hesitate using MAP. Run the normal amount of material getting a few extra lbs of P or apply less material to get the same amount of phosphate. B) Have a acidic situation and are banding to alleviate aluminum toxicity use the same amount of material or a little bit more.  Keep in mind in acidic soils with a low soil test P level you have to apply enough phosphate to take care of the Al and enough to take care of the P deficiency. Note the results of the NPKS wheat response strip (NPKS BLOG)

Original Post Published July 18, 2016
DAP vs MAP, Source may matter!

Historically the two primary sources of phosphorus have had different homes in Oklahoma. In general terms MAP (11-52-0) sales was focused in Panhandle and  south west, while DAP (18-46-0) dominated the central plains.  Now I see the availability of MAP is increasing in central Oklahoma. For many this is great, with MAP more P can be applied with less material. which can over all reduce the cost per acre. There is a significant amount of good research that documents that source of phosphorus seldom matters. However this said, there is a fairly large subset of the area that needs to watch what they buy and where they apply it.

If you are operating under optimum soil conditions the research shows time and time again source does not matter especially for a starter.  In a recent study just completed by OSU multiple sources (dry, liquid, ortho, poly ect ect) of P were evaluated.  Regardless of source there was no significant difference in yield.  With the exception of the low pH site. The reason DAP was so predominate in central Ok, soil acidity.  See an older blog on Banding P in acidic soils.

Picture1

Figure 1. The cover of an extension brochure distributed in Oklahoma during the 1980s.

When DAP is applied, the soil solution pH surrounding the granule will be alkaline with a pH of 7.8-8.2. This is a two fold win on soil acidity aka aluminum (Al) toxicity.  The increase in pH around the prill reduces Al content and extends the life of P, and as the pH comes back down the P ties up Al and allows the plant to keep going. However, the initial pH around the MAP granule ranges from an acid pH of 3.5-4.2.  There is short term  pH change in the opposite direction of DAP, however the the Al right around the prill becomes more available and in theory ties up P even faster.

Below is a table showing the yield, relative to untreated check, of in-furrow DAP and MAP treatments in winter wheat.  The N401 location had a ph 6.1  while Perk (green) has a pH of 4.8.  At Perkins in the low pH, both forms of P significantly increased yield, almost 20 bushel on the average.  DAP however was 5 bushel per acre better than MAP. At the N40 site the yield difference between the two sources was 1 bushel.

MAPvDAP2

Relative yield winter wheat grain yield MAP and DAP both applied at equal rates of P (32 lbs P2O5 ac) when compared to a untreated check.

In general it can be said that in acid soils DAP will out preform MAP while in calcareous high pH soils MAP can out preform DAP. So regarding the earlier statement about the traditional sales area of MAP or DAP if you look at the soil pH of samples went into the Oklahoma State University Soil, Water, and Forage Analytical lab the distribution makes since.

State pH

Average soil pH of samples sent into OSU soil water forage analytical lab by county.

In the end game price point and accessibility drives the system.  In soils with adequate soil pH levels, from about 5.7 to around 7.0, get the source which is cheapest per lbs of nutrient delivered and easiest to work with. But if you are banding phosphorus in row with your wheat crop because you have soil acidity, DAP should be your primary source.

Double Crop Response to Additional N, P, K and S.

Vaughn Reed, PhD. Student Precision Nutrient Management
Brain Arnall Precision Nutrient Management Extension Specialist.

Data presented below are the results of Mr. Reeds Masters research project.

On farm research trials are important, because they give us the ability to see responses over a larger geographic area, and even more importantly, evaluate our recommendations on fields that are managed by producers, not researchers.  They also allow us to look at current production practices and see if there are any missed opportunities. Several years ago, we looked at whether producers were leaving yield on the table by not applying enough nitrogen (N), phosphorus (P), potassium(K), and sulfur (S) to winter wheat. We did this by applying strips of N, P, K, and S fertilizer on farmers’ fields with the instructions to not change their fertilizer management strategies. If one or more of the strips resulted in higher yields then it could be assumed that either the nutrient was under-applied by the producer, or in the case of N, lost.  That study concluded that at 75% of the locations, yield was maximized by the producer with [their respective] NPKS management system, however the greatest responses came from the addition of P and that Oklahoma State University’s soil testing and analysis was adequate for nutrient recommendations. That studies results were published in 2017 and is open access, so available for anyone to read. https://dl.sciencesocieties.org/publications/cftm/abstracts/3/1/cftm2017.02.0014

Locations of double crop fertility response strips applied in the summers of 2016 and 2017.

There are many producers around the state that follow winter wheat with double crops (DC). Often, this practice is done with limited inputs to reduce economic risk.  Oklahoma State does not make different recommendations for DC or full season crops, with the exception that yield potentials can differ.  In 2016 and 2017 we duplicated the Wheat NPKS study across 3 double crops (soybean, grain sorghum, sunflower) following winter wheat and canola. With a recent climb in DC yields we wanted to investigate if producers were applying enough nutrients to maximize grain yield. Additionally it would allow us evaluate the accuracy of OSU’s soil test based fertilizer recommendations in a double crop. Over the two years, 61 on-farm sites ranging from central to NE Oklahoma had 200 lb/ac of product per nutrient applied in strips 6ft wide by 150 ft long.  Urea (46-0-0), triple super phosphate (0-46-0), muriate of potash (0-0-60), and gypsum (0-0-0-19) were used for sources N, P, K, and S, respectively (92 lbs N, 92 lbs P, 120 lbs K, 38 lbs S). In most cases the fertilizer was applied post planting and post-emergence to ensure strips were applied an areas with good stand.

NPKS Strip Applicator. This ground driven 3pt rig uses Gandy boxes to deliver fertilizer into tubes which is then blown, by a PTO driven fan, out into strips 6 feet wide, per box. This applicator was putting out 200 lbs of Urea, 0-46-0, potash, and gypsum out per acre.

Much like with the wheat-NPKS study 75% of the locations did not respond to additional fertilizer. Twenty treatment comparisons of the 244 made across all 61 locations (50 soybean, 7 grain sorghum, 4 sunflower) yielded a statistically significant change in yield due to the addition of N-P-K-or S. For this report, a comparison was the yield of each nutrient versus the non-treated check, therefore there were four comparisons made per location. Seventeen of the twenty positive responses were found in soybean, three with grain sorghum, and no responses were found in sunflower plots.  Lack of response from grain sorghum and sunflower locations is contributed to small amount of grain sorghum and sunflower fields in the study.

Double crop soybeans in Ottawa County with strips of nitrogen, phosphorus, potassium, and sulfur applied post plant.

Nitrogen rates, for non-legumous crops, are yield driven, meaning the higher yielding a crop, the higher amount of N required.  Both grain sorghum and sunflower crops, due to neither being legumes, were expected to see N response, especially to those locations that applied little to no N to begin with.  A yield response from the addition of N was found in one grain sorghum location, where the producer application was not enough to maximize yield, and the additional N pushed the yields.  As expected, there were no soybean locations that responded to the addition of N.

Phosphorus and potassium are both sufficiency based, not yield driven.  This means that if the soil is at 100% sufficiency, the crop will produce at its highest rate achievable, based on that nutrient.  100% sufficiency for P and K are approximately 65 STP and 250 STK, respectively.  Phosphorus and potassium strips yielded the most results, especially in soybean locations. Of 20 responses, five responses were due to P, ten due to K, and four due to S. Locations that responded to the addition to P were locations that either had low levels of STP (approx. 80% sufficiency or less), or had low pH, which leads to less availability of P (pH>5.0).

Potassium yielded the most positive results, with ten responsive locations, as well as the most interesting results, with only three sites falling below 100% sufficiency.  The other responses were attributed to having low Cl levels (Cl, as in Chloride, which while responses are rare, is a necessary nutrient, and sometimes can lead to losses in yield, especially in sandy environments), as well as drought stress conditions.  Potassium has been shown to have a vital role in nutrient uptake and water retention, as it is found to be critical for root growth, and these are displayed highest in crops found in drought like conditions. One hypothesis for the K response is related to root growth. The later planted DC will spend less resources in root development before going reproductive. Soybean is a heavy user of K, combine smaller roots, typically hot drier soils, and high K demand it is not surprising to find this occurrence.

Sulfur, while not wide-spread reported in Oklahoma, has recommendations by OSU built on a yield driven scale.  There were four responsive locations found in this project. While one location had low soil test S values there were located areas that received high rainfall events during the growing season, and therefore the response was attributed to leaching of S.

So, after all that, what is the bottom line?  Here is our observations:

  • Producers maximized yield 75% of the time, with 25% of locations responding to any additional nutrient.
  • The 20 responses to additional nutrients occurred across 15 locations, four locations had responses to more than one nutrient
  • By nutrient: Note for P and K, due to site variability it was not expected to observe statistic yield increase due to P or K unless soil test was below 70% sufficiency, of which no location had soil test P or K below 70%.
    • 38 locations were below 100% sufficiency of phosphorus, with five observed responses
    • Seven locations were below 100% sufficiency of potassium, two observed responses. An additional eight locations responded that were not predicted by soil test
    • Based on pre-plant soil test there were no sites expected to respond to the addition of Sulfur, 4 locations did respond.
  • Soil test results were adequate in correctly identifying locations that would not respond to the addition of nutrients (93.5% accurate), while not as accurate at predicting sites that would respond.
  • For K, soil testing was less accurate, as eight of the ten responsive locations had soil test values above 250 soil test K (125 ppm or 100% sufficiency). For this reason, we are currently doing work evaluating K recommendations for soybeans.

This work confirms that of the fields we evaluated, the majority was not yield limited by N, P, K or S. However, as with anything, we have more work to do in order to further refine our recommendations, and always looking to learn more about how to aid producers.