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.
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.
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.
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.
If you have any questions please feel free to contact me @ email@example.com
Whoi Cho, PhD student Ag Economics advised by Dr. Wade Brorsen
Raedan Sharry, PhD Student Soil Science advised by Dr. Brian Arnall
Brian Arnall, Precision Nutrient Management Extension.
In 2014 I wrote the blog Banding P as a Band-Aid for low-pH soils. Banding phosphate to alleviate soil acidity has been a long practiced approach in the southern Great Plains. The blog that follows is a summary of a recent publication that re-evaluated this practices economic viability.
Many Oklahoma wheat fields are impacted by soil acidity and the associated aluminum (Al) toxicity that comes with the low soil pH. The increased availability of the toxic AL3+ leads to reduced grain and forage yields by impacting the ability of the plant to reach important nutrients and moisture by inhibiting root growth. Aluminum can also tie up phosphorus in the soil, further intensifying the negative effects of soil acidity. More on the causes and implication of soil acidity can be found in factsheet PSS-2239 or here (https://extension.okstate.edu/fact-sheets/cause-and-effects-of-soil-acidity.html). The acidification of many of Oklahoma’s fields has left producers with important choices on how to best manage their fields to maximize profit.
Two specific management strategies are widely utilized in Oklahoma to counter the negative impacts of soil acidification: Lime application and banding phosphorus (P) fertilizer with seed. While banding P with seed ties up Al allowing the crop to grow, this effect is only temporary, and application will be required every year. The effects of liming are longer lasting and corrects soil acidity instead of just relieving Al toxicity. Historically banding P has been a popular alternative to liming largely due to the much lower initial cost of application. However, as P fertilizers continue to increase in cost the choice between banding P and liming needed to be reconsidered.
A recent study by Cho et al.,2020 compared the profitability of liming versus banding P in a continuous wheat system considering the impacts that lime cost, wheat price and yield goal has on the comparison. This work compared the net present value (NPV) of lime and banded P. The study considered yield goal level (40 and 60 bu/ac) as well as the price of P2O5 fertilizer and Ag Lime. The price of P2O5 used in this study was $0.43 lb-1 while lime price was dictated by distance from quarry, close to quarry being approximately $43 ton-1 and far being $81 ton-1. For all intents and purposes these lime values are equivalent to total lime cost including application. Wheat prices utilized in the study were $5.10 bu-1 and $7.91 bu-1. It is important to note that baseline yield level was not considered sustainable under banded P management in this analysis. This resulted in a decrease in yield of approximately 3.2 bu ac-1 per year. This is attributable to the expected continued decline in pH when banding P is the management technique of choice.
The analysis in this work showed that lime application is cost prohibitive in the short term (1 year) when compared with banding P regardless of lime cost, yield goal level, and wheat value (within the scope of this study). This same result can be seen over a two-year span when yield is at the lower level (40 bu ac-1). While in the short-term banding P was shown to be a viable alternative to liming, as producers are able to control ground longer lime application becomes the more appealing option, especially when producers can plan for more than 3 years of future production. In fact, under no set of circumstances did banding P provide greater economic return than liming regardless of crop value, yield, or liming cost when more than 3 years of production were considered and only under one scenario did banded P provide a higher NPV in a 3-year planning horizon.
While historically banding P was a profitable alternative to lime application for many wheat producers the situation has likely drastically changed. At the time of writing this blog (09/17/2021) Diammonium Phosphate (DAP) at the Two Rivers Cooperative was priced at $0.78 lb-1. of P2O5. This is a drastic increase in P cost over the last year or so since Cho et al. was published in 2020. With P fertilizer prices remaining high it will be important for producers to continue to consider the value of liming compared to banded P. This is particularly crucial for those producers who can make plans over a longer time frame, especially those more than 3 years.
Addendum: As fertilizer prices have continued to rise a quick analysis utilizing the $0.78 lb-1 of P2O5was completed to consider the higher P fertilizer cost. Under this analysis an estimated decrease in NPV of approximately $38 an acre for P banding occurred. When considering this change in NPV, lime application becomes the more profitable option for alleviation of soil acidity symptoms even in the short term (assuming lime price values are equivalent to the previous analysis). This underlines the fact that it is imperative to consider the impact on profitability of the liming vs. banding P decision in the current economic climate for agricultural inputs.
Link to the Open Access Peer Reviewed publication “Banding of phosphorus as an alternative to lime for wheat in acid soil” https://doi.org/10.1002/agg2.20071
Bronc Finch Ph.D. student under the leadership of D.B. Arnall
Brian Arnall, Precision Nutrient Management Specialist.
A study conducted by Oklahoma State University, in cooperation with Noble Research Institute has had the opportunity to evaluate the nitrogen management of graze-out winter wheat over the past three years. This study was set up with three nitrogen management treatments of a 60 lb N pre-plant, 120 lb N pre-plant and a split application with 60 lbs N at pre-plant and 60 lbs N at spring top-dress applications. In the 2018-2019 season top-dress N was applied shortly after Feekes 6 stage (hollow-stem) due to rain and other conditions preventing timely top-dress. The 2019-2020 and 2020-2021 growing seasons received ideally timed top-dress applications in late February to early March. For this study the first cuttings were targeted for just before the appearance of first hollow stem, and second harvest was targeted at just prior to boot stage. The 2020-2021 growing season had much less growth at the time of top-dress than previous years due to a severe early spring freeze damage which resulted in only a harvest at boot.
Table 1. Rainfall totals for each year within each location. September 1 – January 1 represents the pre-vernalization grazing period, September 1 – January 1 represents the growing season total.
Crop production can be majorly impacted by the environment, and that is no different for a forage production system such as in this study. Table 1 shows the differences in rainfall totals for each growing season, and how much of that seasons total fell prior to January when hard freezing conditions typically occur. The 2018-2019 season saw at least 40% more rainfall than other years at both location. Much of this increase was in the fall and early winter which lead to a greater first harvest at both locations and greater soil moisture storage for after spring green-up. The environmental impacts continue to cause reduction in annual yields for the 2020-2021 season with a longer cool season and a late freeze (data not shown) which caused significant winter kill on what was a decent fall production.
Regardless of environmental impacts system improvements were observed with the management on N for wheat biomass production. Figure 1 shows the amount of biomass that can be attributed to the gain of a grazing animal, calculated as dry matter yield * net energy for gain. Gain-yield production reported substantial increases in total system production with the increase in N rate in comparison to a lower 60 lb per acre rate. When forage dry matter yields were near or above 5 tons, such as in 2018-2019 the split application of 60 lbs pre-plant and 60 lbs top dress significantly increased the yield and gain-yield, while lower yielding years only saw increases with increased rate. Overall system production improved with the increase of application and slight improvements were reported when application of N was closer to plant utilization.
Total N uptake of harvested biomass is documented in Figure 2 and follows a similar trend as total biomass yield. Total N uptake can be directly related to protein, as protein is total N * 6.25. As observed with the total biomass production, N uptake was reduced annually due to the reduction in available soil moisture and plant growth. The increase of rate increased overall system N uptake as well as increased annual N uptake. Split applications of N resulted in an increased N uptake in comparison to the same total rate applied as pre-plant for the entire system and in most individual years. Increasing the uptake of N not only leads to the increase in biomass and protein concentration but also allows for the more efficient usage of fertilizers. By utilizing as much fertilizer as possible allowing for increased root growth to mine N from the soil. These increases in yield production and nitrogen uptake improve the gain potential of the forage crop which leads to increases in return on investment for the producer.
Net returns can be calculated by multiplying gain by cattle price. Table 2 shows the 3-year Gain, Return, and Profit of the 120 pre and 60-60 split treatment. The dollar value of gain was set at $1.12 per pound as reported by Dr. Derrell Peel in the Cow-Calf Corner August 16, 2021 newsletter. Interestingly, the delay of the additional 60 lbs N until a top-dress application results in an average increase of $458 per acre over the 120-pre-plant rate. This increase in profit does take into account the extra application cost of top-dress. The rate used was $7.82 ac-1 per year: OSU Fact sheet “CR-230. This results in an increased cost of $23.46 over a three year time frame.
Table 2. Evaluation of Gain (dry matter yield * net energy for gain) in tons ac-1, Return (Gain * $1.12) in $ ac-1, Profit (Return minus fertilizer and application cost $241.03 for 120 and $264.49 for 60-60 split) and the Difference between the profit of 120 lbs of N applied preplant and the 60-60 split application.
As the winter wheat planting begins and decisions are being made, the management of nitrogen can be a very challenging one. From this data set, yield increased with the application of N above 60 lbs, as expected. While the decision to pre-plant or split apply can add an element of difficulty to the decision, the split application has been shown to be equal to or better than 100% pre-plant application. This study indicates the application of a split of N can be more profitable, than utilizing a pre-plant application of the same rate. Resulting in improved livestock gain-yield production, and system profitability. While the split application did not have a big payoff every year, taking advantage of it in the good years resulted in such a significant increase that split application resulted in a $458 per acre increase in profit across the two sites.
For questions or comments please feel free to reach out.
Acknowledgement of the support by Noble Research Institute for this project.
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.
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.
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.
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
Raedan Sharry, Ph.D. Student, Precision Nutrient Management
Brian Arnall, Extension Specialist, Precision Nutrient Management
Note, this blog is focused on grain only winter wheat production.
Crop producers looking to increase profits often consider how to reduce costs without sacrificing yield and/or quality. This applies to essentially all production functions including nitrogen application. Winter wheat growers in the southern Great Plains have a wide number of options available to them when considering nitrogen source and application technique. At the time of writing (08/27/2021) fertilizer prices obtained from the Two Rivers Farmers Cooperative are as follows ($/unit): UAN (28-0-0) $0.62, NH3 (82-0-0) $0.45, and Urea (46-0-0) $0.62. These price levels equate to approximately a 57% increase in urea cost, 65% increase in UAN28, and a 65% increase in NH
Winter wheat producers in the southern plains have historically applied nitrogen (N) fertilizer prior to planting, often utilizing anhydrous ammonia for application due to its generally lower price point per unit of N relative to other sources. However, research at Oklahoma State shows that if the total N application is delayed until approximately feekes 5 to feekes 7 stages (jointing) yields were increased 23% of the time while grain protein was increased 68% of the time. By delaying N application to later in the growing season N is more likely to be available when the crop requires by avoiding conditions conducive to losses. Further reading on delaying nitrogen application can be found here (https://osunpk.com/2020/09/10/value-of-in-season-application-for-grain-only-wheat-production/)
A study located a Perkins, OK observing yield and protein response provides an example of an expected response to delayed N. In this study 3 N fertilizer rates (180, 90 and 45/45 split) across 5 different timings (Pre, 30, 60, 90, and 120 days after planting) where investigated. Grain yield was maximized by the 180 lb. rate applied 60 days after planting, while protein was maximized at the 120 days after planting timing. This same trend continues across all N rate levels as the later N applications whether at 60 or 90 increased yield relative to the pre while the 120 days after planting application maximized protein level regardless of rate level. However, maturity of the 120 day application treatment was severely delayed. This experiment shows the ability to sustain yield while decreasing N rate if N application is pushed to later in the season to avoid conditions that lead to N losses as displayed by the 90 lbs. at 90 days after planting treatment compared to the 180 lb. pre-plant rate.
Application costs are directly related to choice of source utilized. For instance; anhydrous ammonia application is predicated on the use of a pulled implement such as a low disturbance applicator for in-season application or a tillage implement for pre-season application. This is compared to other sources such as urea or ammonium nitrate which may be broadcast, or UAN that can be applied using a sprayer. The relationship between source and cost of application is inherently related to the application efficiency of the equipment used. Table 2 below provides a rough idea of cost associated with different application methods. (Information Retrieved from Iowa State). Fuel cost assumed at $2.60/gal. Labor cost assumed to be $15.00/hr.
|Implement||Operating Efficiency||Fuel cost/ac||Labor Cost/ac||Operating cost/ac|
|90’ SP Sprayer||~78 ac/hr||$0.34||$0.19||$0.53|
|60’ Dry Spreader||~30 ac/hr||$0.39||$0.50||$0.89|
|35’ Sweep Plow||~21 ac/hr||$1.43||$0.71||$2.14|
In many operations across the southern plains efficiency has become a key factor in decisions such as input selection and equipment purchases. This has come in response to the need to cover more acres with less labor. With that in mind and looking back to table 2 it is easy to see that a self-propelled sprayer is likely able to cover more acres than other equipment options. This most likely should be considered when considering options for N management in the wheat crop.
With wheat sowing quickly approaching for many and field preparation nearing completion it is important to consider your nitrogen management options. Delayed N application allows for flexibility in management plan and depending on source utilized may increase application efficiency over pre-plant applications requiring a tillage implement. As fertilizer prices continue to remain high it is also important to consider the likely increase in N use efficiency due to applying N closer to when N requirement is peaking. Controlling cost while continuing to maximize output is imperative to sustainable profitability in crop production.
Any Question or Comments please feel free to reach out me.
Brian Arnall firstname.lastname@example.org
Raedan Sharry, Precision Nutrient Management Ph.D. student
Brian Arnall, Precision Nutrient Management Extension Specialist
Cotton production in Oklahoma has expanded the past decade into areas which other production systems such as continuous wheat may have traditionally dominated. Fields that have been managed for continuous wheat production may have become acidic in response to management practices, such as the use of ammoniacal nitrogen fertilizers. In response to the acidification of these soils it may be important to recognize and understand the potential impacts of soil acidification on cotton production.
To better document the impact of soil pH on cotton lint yield and quality a study was conducted over the 2019 and 2020 growing seasons at two locations: Stillwater, OK (EFAW farm) and Perkins, OK. Soil pH in these experiments were adjusted to a depth of approximately 6 inches using aluminum sulfate (acidifying) and hydrated lime (alkalinizing). All three locations were planted to two varieties; NexGen 3930 and Deltapine 1612 at a rate of approximately 35,000 seeds per acre, into plots with soils ranging in pH from 4.0 to 8.0.
In season measurements taken included stand count, plant height, node count and NDVI (normalized difference vegetative index. All four in season measurements demonstrated a significant critical threshold in which soil pH negatively impacted crop performance. Stand was significantly decreased at a soil pH of 5.3 or lower. This trend is displayed in Figure 1. Plant height and node count depicted in figures 2 and 3 respectively were both significantly decreased when soil pH dropped below 5.3 and 4.9, while NDVI began to deteriorate around a pH of 5.1 (Figure 4). Response to soil pH level was also visually observable as shown by Figure 5 from the Perkins 2019 location.
Yield levels across this experiment ranged from 0 to 1284 lbs. of lint per acre. In this work yield is reported as relative yield. To calculate relative yield the yield of each plot is normalized to the average of the three highest yields for that site. This method of reporting yield response allows this work to be applied across a range of yield environments.
When all sites and cultivars were combined relative yield reached a plateau at approximately 73% of yield with a critical threshold observed at a pH of 5.4. Below this critical threshold yield decreased at a rate of 37% per point of pH decline. This equates to 15% yield loss at a pH of 5.0, 33% yield loss at a pH of 4.5, and 52% yield loss at a pH of 4.5. This relationship is depicted in Figure 6. It is important to note that yield was 0 when pH was 4.3 in two plots. This represents the possibility for total crop failure when planting into very acidic conditions.
Differences in relative yield between cultivars were insignificant. However, further investigation may produce a significant difference. Critical threshold for the DP 1612 and NG 3930 cultivars were 6.1 and 5.2 respectively. This suggests that there may be value in further examining the influence of genetics on cotton response to soil acidity.
The influence of soil pH level on cotton productivity is confirmed by this study. All three sites evaluated provided a strong correlation between soil pH and lint yield, as well as the in-season growth parameters measured. While this study is likely to be expanded to another location to provide a more robust evaluation of the potential impact of soil acidity on cotton, the current dataset provides ample evidence to conclude that soil acidity is likely to be detrimental to cotton production in the southern plains. Soil pH levels below 5.5 appear to provide the greatest opportunity for yield loss as depicted above. Lint quality measurements taken in the study (micronaire, length, uniformity, and strength) showed no consistent trends in the relationship between quality parameters and soil pH suggesting that soil acidity had a limited influence on lint quality characteristics. Cotton response to soil pH is likely to be influenced by the environment of a specific location and growing season. This underlines the importance of understanding the soil properties that negatively impact productivity, such as the presence of toxic forms of aluminum or manganese. It is also important to highlight the possibility of acidic conditions significantly affecting the ability of the crop to access important nutrients such as phosphorus. Under acidic conditions cotton productivity is likely to be significantly decreased unless soils are neutralized using a soil amendment such as lime.
Project Supported by the
Oklahoma Cotton Council
Oklahoma Fertilizer Checkoff Program
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.
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.
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.
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.
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.
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.
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Acknowledgement of LSB Industries for support of this project.
Bronc Finch Ph.D. student under the leadership of D.B. Arnall
Brian Arnall, Precision Nutrient Management Specialist.
The recent weather conditions have caused a delay in the ability to top-dress winter wheat in some parts of Oklahoma. Despite this delay, conditions have still been good for growth, which means a steady increase towards the hollow stem and jointing stages. As these stages approach, or have passed, many concerns have been raised about the decision to apply nitrogen to increase spring forage production of winter wheat. A study conducted over the past two years at Oklahoma State University, in cooperation with Noble Research Institute has had the opportunity to evaluate how a nitrogen application at or just after hollow stem impacts the forage production of winter wheat. This study was set up with three fertilizer treatments of a 60 lb N pre-plant only, 120 lb N pre-plant only, and a 60 lb pre-plant and 60 lb top-dress applications. In the first season of the trial, 2018-2019, the fertilizer application was applied shortly after the wheat achieved the Feekes 6 stage (hollow-stem) due to rain and other conditions preventing a timely top-dress. In the 2019-2020 season of the trial the treatments were applied at a more ideal time, near the end of February and beginning of March before hollow stem. For this study the first cutting was targeted for just prior to hollow stem and the second cutting conducted at early boot stage.
In 2018-2019 the additional 60 lb N applied in the 120 lb N pre-plant increased the dry biomass production in the second harvest by 0.4 and 0.9 tons per acre above the 60 lbs treatment at Chickasha and Lake Carl Blackwell, respectively (Figure 1). The delay of the additional 60 lbs of N increased the yield by an additional 0.7 and 1.7 tons per acre, respectively. The 2019-2020 season showed similar results at the Lake Carl Blackwell location in the second harvest, where the additional 60 lbs N at pre-plant increased biomass yield by 0.6 tons, with the delaying of the additional 60 lbs increasing biomass yield by 0.6 more as compared to the same rate when applied at pre-plant. But that additional yield gained with the split application came at cost as the 120 pre-plant resulted in 0.6 tons more in the first harvest. These results suggest that more N was needed in the pre-plant and top-dress application. The 2019-2020 Chickasha trial showed little difference in rate, more than likely 60 lbs N was enough maximize forage yield.
Total biomass production for 2018-19 winter wheat forage (Figure 2) showed to have a greater increase in total biomass production when the N was split applied with the second application being made shortly after hollow stem. The split application increased total biomass production by as much as 1.3 tons per acre more than the same rate applied as all pre-plant. The 2019-2020 year total biomass production shows to be about the same whether the N was applied all pre or split and applied in February.
Figure 3 documents N uptake of winter wheat biomass for both years continues the same trend as total biomass. Nitrogen uptake can be directly related to protein as the calculation for protein is %N * 6.25. In all cases uptake was greater than applied. In 2018-2019 split application increased over all nitrogen uptake. Much like the yield of 2019-2020 the N uptake was not significantly impacted by the timing of the N application.
Although the assessment of N application made at or after hollow stem in a winter wheat forage system was not an objective of this study, the circumstances have given a unique opportunity to evaluate the outcome. In the 2018-2019 trials when top-dress application was applied at or just following the hollow stem, yield and nitrogen uptake were both increased over the equivalent pre-plant application. For the 2019-2020 season where N was applied at the planned time yields and N uptake were equivalent to the pre-plant. While this data is not conclusive it does indicate the producers can apply N fertilizer to winter wheat forage at or after hollow stem and successfully increase both forage yield and nitrogen uptake.
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Acknowledgement of LSB Industries for support of these projects.