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Misha Manuchehri and Brian Arnall
There are many factors that influence the persistence and uptake of a herbicide that has soil activity. One of those factors is soil pH or the amount of hydrogen (H) ions present in the soil solution. Some herbicides will persist for an extended amount of time or rapidly degrade when outside the pH window of 6.0-7.0.
The triazines (atrazine, simazine, etc.) and sulfonylureas (chlorsulfuron, metsulfuron, etc.) are two herbicide chemical families that are especially affected by soil pH (Table 1). The dinitroanilines, and the active ingredient clomazone also can be affected by low and high soil pH; however, degradation by light and/or volatility are more important when it comes to the activity of these herbicides. Generally, the triazines and sulfonylureas persist longer and are more available for plant uptake in higher pH soils (>7.0) while the opposite is true for imidazolinone herbicides (imazamox, imazapic, imazethapyr, etc.). Imidazolinones persist and are more available for plant uptake in lower pH soils (<6.0). The persistence of the triazines and sulfonylureas in high pH soils is a result of a decrease in chemical and microbial breakdown, a trend that is often observed in high pH soils where neutral herbicide molecules are loosely adsorbed to the soil and are more available for plant uptake. Conversely, in low pH soils, triazine and sulfonylurea herbicides become charged and are more tightly adsorbed to the soil where they are more susceptible to breakdown.
A key management factor that must be considered when evaluating a field’s soil pH is whether or not the field is no-till and for how long it has been in no-till. Tillage will impact how deep you should take soil samples to determine soil pH. In no-till and minimum tillage fields, the traditional method of 0-6 inch or 0-8 inch soil cores may not be adequate. Instead, a 0-2 inch core depth and a 2-6 inch core depth may be needed, since application of limestone to the surface may increase surface pH more than expected or application of nitrogen fertilizer to the surface may cause a drop in pH at the surface. In many long term no-till fields with historic surface applications of N and no lime applications, soil pHs in the low 4s have been observed while the 3-6” depth will be at a 6.0. Since herbicides with a soil residual are affecting plants just below the soil surface, this is the soil zone we are the most interested in.
Oklahoma and Kansas production fields can have a wide range of soil pH from field to field and within field. In a dataset of over 300 grid sampled fields from Oklahoma (259 fields) and Kansas (47 fields), the average field pH was a nice 6.0. However, the average range in the lowest and highest soil pH within the fields was 1.9. This means the average field had a pH range from 5.0 to 7.0. It should be noted that more than 25% of the fields had a pH range of 3.0 units. This range of highs and lows has helped explain the presence of spotty herbicide issues on several fields in the past and should be taken into account when planning crop rotations.
It is extremely important to know and understand the pH of your soils and the herbicides you plan to use and how they will react. Soil testing is the only way to know your soil pH and reading your herbicide label is a great way to learn if soil pH affects the herbicide you are applying.
Table 1. Herbicide chemical families or selected herbicides that are most affected by soil pH.
Herbicide chemical family or active ingredient
Common name (trade name) examples
Importance of soil pH
Soil pH considerations
Chlorsolfuron + metsulfuron (Finesse C & F), metsulfuron (Ally XP)
pH > 7a – persist longer and are more available for plant uptake
|Atrazine (AAtrex), simazine (prince)||
pH > 7 – persist longer and are more available for plant uptake
Imazamox (Beyond), imazapic (Plateau), imazethapyr (Pursuit)
pH < 6 – persist longer and are more available for plant uptake
aAcidic Soils < 5.5, Basic Soils > 7.5
During January and February I spent a lot of time on the road giving precision ag and wheat yield / protein talks. One thing about giving the same talk multiple times and spending countless hours on the road, about 70, is the time you have to think about the little things in your talk. This time around it was the slide below. The graph is from the 502 Long Term Fertility study located in Lahoma OK. When I first put the slide together in 2016 the purpose was to show how the yield and optimum fertilizer rate is extremely varied. I went in to the 55 plus years of yield data and pulled out the past ten years and identified the nitrogen treatment, only those with full P and K fertility, that economically maximized yield each year. With the graph I was able to show how the nitrogen rate required to maximize yield changes dramatically each year and where the amount of N was not directly correlated with yield. But after showing this graph a few times I thought that added lbs of N per bushel would help me highlight the point about changing N demand. That’s the blue numbers below each year. And of course out of curiosity I averaged the numbers. The ten year average was 1.5 lbs of N, which would suggest over a ten year period you would need to apply 120% of the N removed to optimize profit.
The 1.5 lbs per bushel over time was an important number. Not long before I had reached out to half of dozen producers that I have spent at least 5 years with working on their N management. My question to them, what was your average yield and average N rate over all your fields and years. Turns out that most of these producers who were using N-Rich strips and making 2 or 3 trips over the field were averaging 1.5-1.6 lbs N per bushel of wheat produced across a farm. Of course when they told me this I was excited, that’s such an improvement over 2.0 lbs of N per bushel.
The real thought came with me combining these two independent tid bits. Can we provide a Nitrogen Management Report Card if we look at several years of yield history? Let me preface what is presented below is not a scientifically tested or proved concept, yet. The more I think about it the more I am beginning to think that YES we can do a beneficial postmortem analysis. This is not a 1 year analysis, in fact based on the long term data I have been looking at there needs to be 5 years of data per field evaluated. I also strongly contend that this is a by field process. This will provide the opportunity to look at management over a broad spectrum of soil types and weather.
The calculation for lbs of N per bushel is not tough. In a continuous grain only winter wheat system you would add up the amount of nitrogen applied per acre over the period you are evaluating. Sum up the annual average grain yield and multiple that value by 1.3. Divide the total N applied by the total N removed per acre. This will be a decimal value, to compare with the tables below multiple by 100 to get a percent. Based on the long-term trials there needs to be at minimum five years of data. But the more the better.
I would like to reiterate the grades provided below were not developed from any given data set. The report comments are of my own opinion. I do hope in the near future to utilize the Oklahoma State University long-term fertility studies to refine these tables.
For a field with a crop rotation the way to calculate is the same you will just need to go into each harvest and multiple yield by the N in the crop, then sum up those values.
For a field with a crop rotation with legume (or cover crop), I have adjusted the grade scale with the assumption less total N will be needed due to the addition of N fixed by the legumes.
Hopefully with concept will give you a different way to evaluate your N management strategies. This will not and cannot tell you what you need to apply next year. I mean just look at the data from Lahoma, from 2011 to 2015 optimum N rate ranged from 0 to 100 lbs N pre acre and N per bushel grown ranged from 0 to 2.2. Also as you look at the charts, understand that if you follow the old rule of thumbs 2.0 lbs N per bushel winter wheat and 1.2 lbs N per bushel for corn and sorghum, you are likely in the RED. These values are not that wrong for yield goal, 100% preplant application nitrogen management strategy. It is just with today technology, equipment, and agronomic practices we can do a lot better.
My final recommendations/comments would be:
1) If you are in the greens and yellows you are overall doing well. However there is always room for improvement. Are you currently accounting for the temporal variability in N demand, how about the spatial variability?
2) If you are in the orange and reds on the low side, are you there because you are underestimating yield or you are applying less because of grain prices?
There is likely money to be made by increasing yields with a little more nitrogen in these fields.
3) If you are in the orange and reds on the high side, are you there because you are consistently overestimating yield? Perhaps your yield estimation is not off but your lbs of N per bushel value is too high? Are you applying all of your N pre-plant. This practice is the most inefficient way, in terms of N use efficiency, to fertilize.
Questions or comments?
Please feel free to reach out to me via email or phone.
This the recent rains across the dry wheat belt the N-Rich Strips are going to start showing up. Because I am re-posting ans older blog that walks users through the sensing process and inputting data in to SBNRC. But since post we have also release a iOS version of the Online Calculator. iOS N-Rate Calc
With the significant swing in temperature over the last few weeks many are chomping at the bit to get outside. The wheat is starting to respond to the good weather and N-Rich Strips are showing up around the state. Over the past week I have had several calls concerning the impact of the cold weather on the N-Rich Strips. Many of the fields either are still small due to limited days of warm weather and growth or may have a good deal of damage to the foliage. If the field of concern has only a little or no damage and the strip is visible, the time to go is NOW, but if you cannot see the strip and your field has tissue damage or is small, similar to the first two images, then you will need to wait a week or two for sensor based recommendations. Another situation fits with the third image, the field has freeze damage but the N-Rich Strip is also visible. In this case the predicted yield level would be reduced do to the dead tissue making the N rate recommendation a little off. I still however recommend using the sensor and online SBNRC (http://www.soiltesting.okstate.edu/SBNRC/SBNRC.php) to make or base top-dress N rate. Even if the recommendation is a little off it will still be much more accurate than just guessing. However you must look at the SBNRC and ensure that it makes agronomic sense, if it does not consult your county educator or myself. This is discussed in more detail in my earlier blog about freeze damage. Keep in mind no matter what, if you can see the N-Rich Strip, everything outside of the strip is suffering from nitrogen deficiency. Decisions and fertilizer applications need to be made soon, to maximize yield.
Regardless of whether or not the strip is visible you should be planning to sense with the GreenSeeker Handheld very soon. Remember the sensor has the ability to detect differences before your eyes can. To sense the N-Rich Strip and Farmer Practice the user should carry the sensor approximately 30 to 40 inches above the crop canopy while holding the sensor level over the crop. While you are walking the two area the trigger should be held the entire time. I recommend walking at minimum 100 paces for each. The average NDVI value seen on the screen will only stay on the screen for a few seconds. Therefore it is critical you have a method of recording the number for later use. The sensor has limited memory so it will time out is the trigger is held for an extended period of time. If you wish to collect more NDVI readings just do it in multiple trigger pulls recording each. Once you have the average NDVI for the N-Rich Strip and Farmer Practice you can go to the SBNRC site mentioned above to retrieve the N rate recommendation. Once in the calculator, for those in Oklahoma, choose the “within Oklahoma” option in the bottom left hand corner of the screen. This will allow the calculator to access the Oklahoma Mesonet to determine growing degree days. After the location is picked from the options you will need to enter Planting Date and Date Prior to Sensing. Additional information requested is the expected grain and fertilizer prices. While these inputs will provide some economic evaluations they will not impact recommended N rate.
Below is a YouTube video in which I describe how to use the GreenSeeker to collect NDVI readings, describe the data needed to complete the online calculator, and how to interrupt the calculators output.
Normally the alarm for beginning wheat topdressing gets sounded right away in early January. However, it might be understatement to say this year has been dry so far – “drier than a popcorn fart” may be a better description. At the time of writing this blog, a significant portion of the Oklahoma wheat belt has now gone 90+ days with less than 0.25” of rain. The great folks at Mesonet reminded us on January 18 that the long term forecast is not providing us much hope either.
Because it has been dry and no significant rain is in the current forecast, the question is what do we do now about topdressing? This is a tough question to answer as there is not a really “good” option at the moment to be honest. Here are some thoughts to consider:
- In the parts of the state where it is dry and dry deeper than the majority of the rooting zone (> 6”), we should not worry about filling up the nitrogen tank as long as the water tank is empty. As it stands currently, the best option is to hold off for now and wait to apply topdress N right in front of a real chance of rain. The good news is we still have some time yet to get N applied and not limit yield potential if we do get that rain. Ideally, we need the N down in the rooting zone just prior to jointing. Several things, including the number of potential grain sites, are determined just prior to jointing, and it is imperative that the plant has the fuel it needs to complete these tasks. Jointing occurs around the end of February in southern OK and around the second week of March in northern OK. Jointing also marks the beginning of rapid nitrogen uptake by the plant which is used to build new leaves, stem, and the developing grain head. The nitrogen stored in these plant parts will be used to fill the grain later in the season, and the plant is dependent on this stored nitrogen to complete grain fill. And while it does seem like it right now, we still have the potential to make a decent crop if we can get rain before we break winter dormancy. If we do not get the rain though soon as it is appearing, we will not have spent as much money on this crop by holding off on topdress N, and the likelihood of getting the return on our N investment goes down as our yield potential goes down.
- What we can and should do right now is apply N-rich strips. An N-rich strip can help put your mind more at ease by taking the guesswork out of knowing if nitrogen needs to be applied and how much should be applied. The N-rich strip can be as simple as using a small lawn fertilizer spreader with a bag of urea. You local county extension educator can also provide more information on N-rich strips and even has access to lending small fertilizer spreaders!
For those producers who have too much ground and cannot cover all of it just prior to a rain or for those who want to apply now as they are worried about nitrogen being limited after it does start raining, here are few more considerations:
- For conventional-tilled fields that have limited to no residue, applying UAN through streamer nozzles is an okay option. Why? With UAN, there is a very high percentage of soil-fertilizer contact. This immediately improves the efficiency compared to urea. In fields with crop residue, flat fan nozzles are not recommended right now as the likelihood up of N tie-up is too high.
- For no-till fields, the two big concerns are ammonia (NH3) volatilization with dry urea and tie-up on the residue with liquid UAN. Picking the best option in this scenario is a much tougher decision with not a real good conclusion. So, here it goes. If there is tall standing stubble with dry soil below, the dry urea gets the edge. Why? If the stubble is not in a mat, the urea prill can work its way down towards the soil surface. If it can get there, it is out of the high winds, and it will remain there until we get a rain, heavy dew, or increase in humidity. Is there still a chance for loss due to volatilization? Absolutely. Again, it goes back to whether there is any chance that you can wait to apply?
- There have been some questions about using urease inhibitors with broadcasting urea. That is a good question, but it is hard to make an argument for their use until we get a good chance of rain in the 10-day forecast. Typically, these products do not have the life span to hold off urease (i.e., the enzyme that breaks urea into NH3) for more than 10-12 days.
The latter points also apply to those who use the local co-op or ag retailer for application. Some of these groups require 30 days or more to cover all of the acres they service.
Since it is dry and we still have some time yet to apply N, this may turn out to be the perfect year to topdress urea with a grain drill. For those interested in this method, you can find research results from last year on this topic, as well as a calibration guide, by clicking here . More information about nitrogen applications that are “thinking outside the box” can be found by clicking here.
For more information contact Brian Arnall or David Marburger.
A result of the 2016-17 winter wheat crop was a significant amount of discussion focused on protein levels. For two years running now, the protein levels have been low across the board. Low protein brings in a challenge to sell, could impact local basis, and even more concerning is that low protein is an indicator that nitrogen was limiting during grain fill. Therefore, the field maximum yield potential was not achieved. In this blog, we talk about what protein is, what can be done to maintain a good protein level, and what can be done to increase protein if desired.
First, the definition of protein is any of a class of nitrogenous organic compounds that consist of large molecules composed of one or more long chains of amino acids and are an essential part of all living organisms, especially as structural components of body tissues such as muscle, hair, collagen, etc., and as enzymes and antibodies. Protein is also one of the many attributes that determines end-use quality and marketability of winter wheat. Sunup TV met with Dr. Carver in the baking and milling lab to create a great video discussing wheat quality impact on baking and milling.
We determine protein by measuring the percent of nitrogen in the grain and multiplying by a factor of 5.7. So if the grain has N % of 2.5, the protein content is 14.25. The amount of N in the grain is affected by many variables such as weather during grain fill, yield level, and N availability during grain fill. If weather is conducive to good grain fill and test weight is high, we will often see protein values dip. On the other hand when grain fill conditions are hot and dry and we have light test weight, wheat protein will be higher. Research has shown (Figures 1 and 2) that generally as yields increase protein levels decrease. Of course if N is limited during grain fill, the N available for the grain is reduced, and the plant is forced to get all grain N from re-immobilizing N in the leaf tissue.
Maintaining Protein, and yield.
Managing nitrogen to maintaining protein and maximizing yield comes down to making sure that N is available at critical growth periods. With wheat, the critical uptake stage is typically the time frame between hollow stem and soft dough. The two graphs below show nitrogen uptake in wheat and barley. If the same graph was made for dual purpose wheat, the upward swing would start sooner but would follow the same general trend.
When it comes to making sure N is available during this time of peak need, the only way we can do that is apply just before it is needed. This means split application. While putting all the nitrogen out pre-plant as anhydrous ammonia is the cheapest method, it is also the method that provides the lowest nitrogen use efficiency and is most likely to show deficiencies late in the season. One of the challenges with 100% preplant N application is that years with good yield potential coincide with years with good/high high rainfall, which means more nitrogen loss. Some interesting results from studies implemented in the 2016-17 cropping season showed the importance of nitrogen application timing. The study is determining how long nitrogen application can be delayed after the N-Rich strip becomes visible (https://osunpk.com/2013/09/19/nitrogen-rich-strips/). For the study, 90 lbs of N was applied on one of the treatments at planting When that plot became visibly greener or bigger than the rest, N application was triggered. After the 0 DAVD (Days after visual difference where the day had growing degree days >0), another treatment was applied every 7 growing days for 63 growing days. Each plot, excluding the zero N check, received 90 lbs as NH4NO3 (we use this to take the variable of volatilization out of the data). In all cases, 90 lbs applied in late January to early February was better than 90 lbs pre-plant. Keep in mind there was 0 N applied at planting for each DAVD application timing; yet, we still hit 50-80 bushel wheat with nothing but in-season N. This is the result of supplying the N when the plant needs it. I should add this is just one year of data, and every year is different. The study is being replicated again this year and will be highlighted at the Lahoma field day.
For dual-purpose wheat, the total amount of N expected for the forage production needs to be applied pre-plant. Oklahoma State recommends 30 lbs N for every 1,000 lbs of forage expected For grain-only wheat, there needs to be only 20 to 40 lbs of N available to the crop when planted (this includes residual N). The remaining N should be applied at green up or early spring. The only way to ensure that N is applied when the crop needs it is to utilize the N-Rich Strip method. Having a N-Rich strip in your field lets you know when the wheat needs more nitrogen and when it does not.
Two years testing the N-Rich Strip and Sensor based nitrogen rate calculator (SBNRC) from the Texas boarder to the Kansas boarder showed that the SBNRC on average reduced N but maintained yield and protein when compared to standard farmer practice (Table 1).
Some producers may plan to market high protein for a premium if available. Fortunately, there are opportunities to increase protein via management. While most of the strategies for increasing protein happen later in the growing season, some of the early decisions can be a significant contributing factor. Variety selection and keeping the plant healthy and free of competition (i.e., pest management) throughout the growing season are going to increase the opportunity to produce high protein wheat. After that, the equation goes back to Figures 3 and 4 and making sure the crop has access to nitrogen during peak periods, including grain fill. If you will note, the bottom two graphs of Figure 5 both show significant increases in protein on the later applications. For both locations, this was when N (90 lbs N ac-1) was applied after full flag leaf emergence. There has been a significant amount of work at OSU looking at late application of N stretching back into the 1990s http://nue.okstate.edu/Index_Publications/Foliar_N_Curt.pdf. The focus has been looking at timing, source, and rate. The take home of decades of work can be summarized as such. Yes, protein can be increased with late season application, but not always. Applying N at or after flowering has a significantly greater probability of increasing protein than a application at flag-leaf. Source of N has had little impact if managed properly (UAN, 28-0-0, has to be watered down so that it does not burn the plant). The rate of N does matter quite a bit. Most of the work suggests that for every pound of N applied, the percent grain protein could increase by .05%. So to increase protein from a 12.5% to 13.5%, it would require approximately 20 lbs of N per acre. My work has shown the same trend that a 20 lbs application at post-flowering had more consistent increases in protein than lower rates at the same time or similar rates applied at flag leaf.
This wheat season we are looking to improve our knowledge of management on protein content through multiple studies by continuing the evaluation of varieties and management practices.
If you have any questions for comments please feel free to contact me.