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With the most recent FAA UAV announcement my phone has been ringing with excited potential UAV users. Two points always comes up in the conversation. NDVI (normalized difference vegetation index) and image resolution. This blog will address the use of NDVI, resolution will come later. Before getting into the discussion, what NDVI is should be addressed. As described by Wikipedia, NDVI is a simple graphical indicator that can be used to analyze remote sensing measurements, typically but not necessarily from a space platform, and access whether the target being observed contains live green vegetation or not. NDVI is a mathematical function of the reflectance values of two wavelengths regions, near-infrared (NIR) and visable (commonly red).
The index NDVI has been tied to a great number of crop factors, the most important being biomass. Biomass being important as most things in the plant world impact biomass and biomass is related to yield. The most challenging issue with NDVI is it is highly correlated with biomass and a plants biomass is impacted by EVERYTHING!!!! Think about it, how many things can impact how a plant grows in a field.
The kicker that most do not know is that all NDVI’s values are not created equal. The source of the reflectance makes a big difference.
Measuring reflectance requires a light source, this is where the two forms of NDVI separate. Passive sensors measure reflectance using the sun (natural light) as a light source while active sensors measure the reflectance from a known light source (artificial light). The GreenSeeker is a good example of a active sensor, it emits its own light using LEDs in the sensor while satellite imagery is the classic passive sensor.
The challenge with passive remote sensing lies within the source of the light. Solar radiation and the amount of reflectance is impacted by atmospheric condition and sun angle to name a few things. That means without constant calibration, typically achieved through white plate measurements, the values are not consistent over time and space. This is the case whether the sensor is on a satellite or held held. In my research plots where I am collecting passive sensor data, so that I can measure all wavelength, I have found it necessary to collected a white plate calibration reading every 10 to 15 minutes of sensing. This is the only way I can remove the impacts of sun angle and cloud cover. When using the active sensors as long as the crop does not change the value is calibrated and repeatable.
What does this mean for those wanting to use NDVI collected from a passive sensor (satellite, plane, or UAV)? Not much if the user wants to distinguish or identify high biomass and low biomass areas. Passive NDVI is a great relative measurement for good and bad. However many who look at the measurements over time notice the values can change significantly from one day to the next. The best example I have for passive NDVI is a yield map with no legend. Even the magnitude of change between high and low is difficult to determine.
Passive NDVI in the hands of an agronomist or crop scout can be a great tool to identify zones of productivity. It becomes more complicated when decisions are made solely upon these values. One issue is this is a measure of plant biomass. It does nothing to tell us why the biomass production is different from one area to the next. That is why even with an active sensor OSU utilizes N-Rich Strips (N-Rich Strip Blog). The N-Rich Strip tells us if the difference is due to nitrogen or some other variable. We are also looking into utilizing P, K, and lime strips throughout fields. Again a good agronomist can utilize the passive NDVI data by directing sampling of the high and low biomass areas to identify the underling issues creating the differences.
OkState has been approached by many UAV companies to incorporate our nitrogen rate recommendation into their systems. This is an even greater challenge. Our sensor based nitrogen rate calculator (SBNRC blog) utilizes NDVI to predict yield based upon a model built over that last 20 years. That means to correctly work the NDVI must be calibrated and accurate to a minimum of 0.05 level (NDVI runs from 0.0 to 1.0). To date none have been able to provide a mechanism in which the NDVI could be calibrated well enough.
NDVI values collected with a passive sensor, regardless of the platform the sensor is on, has agronomic value. However its value is limited if the user is trying to make recommendations. As with any technology, to use NDVI you should have a goal in mind. It may be to identify zones or to make recommendations. Know the limitations of the technology, they all have limitations, and use the information accordingly.
This blog is a bit of a self-serving advertisement while at the same time telling an important story about a successful extension program. I am still a newbie in terms of service years as I just finished my sixth year as a state specialist. Over this time I have learned an important fact, an idea or concept does not have to be your own to be applicable. Case in point, ever since grad school I’ve had a tent card, 4”x 3.5” card folded in half to equal the size of a business card, from Potash and Phosphate Institute (PPI), now the International Plant Nutrition Institute (IPNI) pinned to my wall. The Plant Food Uptake card has the nutrient concentrations of the primary grain crops grown in the Great Plains. During my research, it was a handy piece of information. Today, it still hangs on my board today with a series of out-dated photos and thank you cards from former students.
It is from the Plant Food Uptake card that I developed the concept of the first Pete’s Sheets Do’s and Don’ts of using N-Rich Strips and N-Ramps in 2008. I personally thought this was a great idea but did not know if it would be well received by my audience. The results have been more than I could have ever imaged. Since 2009, I have developed 10 additional Pete’s Sheets with the latest out for print. I keep close tabs on how many I have ordered, and the latest order brings to total 38500 Pete’s Sheets. I currently have 8000 Pete’s Sheets on hand, which means there is approximately 30,000 cards in circulation. I doubt any fact sheet I have ever written has that level of distribution.
I am able to travel to meetings and speaking engagements with relevant Pete’s Sheets easily in tow, and I also provide bulk quantities for free via request. Another enjoyable aspect of the Pete’s Sheets is I am able to place them in a tray on my office door. It is always a good day when I have the need to refill card holders, and this happens surprisingly often.
The spin-off from the PS has also been quite impressive. One of the best complaints I have ever received was from a producer who quipped he could no longer carry all of the Pete’s Sheets in his wallet. I took that as a major complement and went to work at compiling the cards immediately. The outcome was the 5″x 8″ spiral bound Nutrient Management Field Guide which has been equally successful. Not only did I “borrow” the tent card concept from PPI/IPNI, but I have also “borrowed” the idea of providing a customized logo just as Purdue has with its fields guides. The most famous is probably the Purdue Corn and Soybean Field Guide. While the custom logo has not been widely utilized, the times it has the revenue has been used to purchase more standard cards.
My take home from this experience is in extension old ideas can easily be incorporated into new educational tools. Often, it takes just a little time and imagination to be successful. Pete’s Sheets now greatly aid both my extension and teaching programs.
Now to the self-serving portion of the blog an announcement for the newest card titled Nitrogen Cycle. Within the Pete’s Sheet is a visual representation of the N-cycle, which every student enrolled in my Soil Nutrient Management course must learn, and the back of has a list of important N-cycle terms with definitions. I will mail Pete’s Sheets to anyone requesting a set, with a maximum of 100 per style. I continuously look for new ideas and topics and welcome any suggestions. View pdfs of the cards and booklets or download the custom order form on my NPK website at http://npk.okstate.edu/petesheets. To receive sets of the standard Pete’s Sheets, simply email me your request and provide a mailing address.
Great write up by Dr. Edwards.
There are few crop inputs that deliver as much return on investment as nitrogen fertilizer. It takes approximately two pounds of nitrogen, costing approximately $1.00, to produce one bushel of grain worth about $6.00. Of course, nitrogen is not the only yield determining factor in a wheat crop. Also, the law of diminishing marginal returns eventually kicks in, but nitrogen fertilizer is still one of the safest bets in the house.
Top dress nitrogen fertilizer is especially important because it is applied and utilized at a time when the plant is transitioning from vegetative to reproductive growth. 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 also marks the beginning of rapid nitrogen uptake by the plant which is used to build new leaves, stem, and the…
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Since my Ag App post in July I have presented on the topic an additional five times and have two more on the books for 2014. A good thing about doing talks is that you have to update the information to remain current. Which in all honesty, when it comes to technology of any kind this is quite challenging and even more so for Smart Phone Apps. In July when I first blogged on the subject I had 76 apps on my iPad. Today (1.3.14) I have 111 apps on my iPad, for both the iPhone and iPad, that I deem to be Ag related. Since the summer I have found new favorites, changed some, and added categories but for the most part I still maintain my 2 minute rule stated in the first blog. I have allowed a bit more leniency in that I now say “If I cannot figure it out in 3 minutes it’s GONE. An app should be intuitive, easy to use and have a purpose. They only exception to the 3 minute rule is the Scouting and Mapping Apps. Because of their complexity I allow them 5 minutes, and then I am done. Any app with GIS in its name gets much more time” I guess I am just getting soft.
Again I must make the obligatory statement; I am not a developer, designer, or expert. I am just a user who has had a chance to look at a few apps. Almost all of the apps I have are free and I am sure I have missed a few. Please share those with me. I am also not discussing Mobi’s, this is another large group of quality decision aid tools. I am also not discussing none apples apps. This is not because they are not relevant or important, it is because I do not have that technology.
I now have nine Ag folders on my iPad:
Ag News/Weather/Markets, Scouting/Mapping, Record Keeping, ID Tools, Crop Tools, Calculators, Sprayer/Chemicals, Fertilizer, Seed Select.
Apps are nice because the majority are stand alone and do not need internet or cell connection. This means they can be used when you are in the middle of nowhere, which is a great deal of Oklahoma, and have no service. This will exclude many of the Ag News/Weather/Markets, Scouting/Mapping, and Record Keeping apps that need positioning or location information.
Now let’s discuss some of the new and old apps.
Not much change in this group however I have added one or two.
This category has changed the most. Record keeping apps have been removed and several new apps added. The only free apps which can create boundaries are still Scout and Sirrus. To date Scout remains to be my favorite app for in field scouting notes. Pictures tagged with Lat Long and a note is very useful. My knock on is app is its boundary creation. It is a challenge every time as it is hard to remember the steps and not make a mistake. That is where Sirrus comes to play, by far the best boundary creation app. Sirrus has easy to use tools for both point and pivot boundaries. I like the edit vertex zoom in tool that resembles a rifle scope. I was able to add 12 fields in a matter of 20 minutes. Being able to create grid soil sampling scheme and record samples is also a very nice tool. My favorite part of the app, the UNDO button, and all apps should include this. The drawback to Sirrus is that it has no ability to take notes such as Scout. An additional nice scouting tool is South Dakota States NPIPM (North Plains IPM) app. This app provides not only a pest id tool with morphological drop down, I will discuss this in the ID Tools cat, but also management recommendation for the identified insect.
The majority of the apps in this category are “Pay to Play”, which makes since as they deal with data management and storage. Many would also fit the Scouting/Mapping category. As I do not pay for many apps I do not have experience with any of these. However this is the category that I would recommend any group to look at as they should be the all-inclusive app. However, PeRK by the University of Nebraska is a free app designed for field records of pesticide applicators.
I have added a few apps to this category but my favorites have not changed. I regularly use Plant Images, ID Weeds, and the Pestbook as references. I will add more discuss to app ID tools. The importance of being able to ID weeds and Pest via morphological drop down menus (ID Weeds and NPIPM) is extremely important. Many of the ID tools just have pictures and names. Well is I am using an ID Tool I likely do not know what I am looking at or what it is called.
Crop Tools includes my second “Paid in Full” app. And this one hurt a bit more. Not because it cost money but because I have multiple versions of the hard copy. However Field Guide by Purdue is one of my most recommended apps. Field Guide is the electronic version of the Purdue Corn and Soybean Field Guide, which the majority of consultants in the Corn Belt likely have this sitting in their truck. The Stoller apps also have nice very nice image bank of plant developmental phases. FieldGuide and CornAdvisor, another good app, are great examples of what I expect to be coming out of the majority of the Land Grant Universities very soon. Cooperative Extension has hundreds if not thousands of quality hard copy publications just waiting to be turned in to handy dandy apps. To be honest I am working on turning my Nutrient Management Field Guide into an app right now.
Only two apps has been added to this category. I am still using Fert.Removal, HarvestLoss and Growing Degrees on a regular basis.
Many apps have been added to this group but none of them have been good enough to kick TankMixCalc and SpraySelect of my favorites list.
Similar to the Sprayer/Chemicals category several apps have been added to this group, including several from Ok State. For me the Fert Cost Calc is still very useful. I do not get to use the Manure Calc I am very impressed by its layout and user friendliness. This app allows for applicator calibration, nutrient recs and manure value estimator.
It is no surprise the apps in this category are company created. I will say for the central Great Plains Pioneer’s Canola Calc is very useful tool for selecting canola planting rate providing input for row spacing live plants, seed weight, Germ percent, and survival percent.
To wrap up this blog I want to share with you may new Favorite none ag app. Bump is a huge time saver for anyone who takes pics with your iPhone or iPad. Bump allows easy transfer between mobile devices but more importantly between your mobile device and desktop by a simple tap of the space bar. This file share will go both directions. This means no more emailing pictures from your phone so that you can have them on your desktop. Bump is a iPhone app that can work on the iPad.
When searching with an IPad, remember to switch the search to include IPhone apps, there are some good ones out there that are IPhone only. Check out www.npk.osktate.edu/presentations to see screen shots from many of my favorite apps.
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. Due to our fertilizer applicators this is typically a strip. The approach maybe somewhat 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. I like to tell producers that the goal of the N-Rich strip is to make a really big cow pie.
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 no less than 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.
Becoming more popular are applicators made or adapted for 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. In most cases there is not a great deal of thought put into what source. I recommend whichever 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 the first of November everything works. This does not say a strip applied after this time doesn’t work but it leaves more room for error. There is a chance the crop could already be stressed or the nitrogen tied up and not release in time. However 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. The soils in that region are very deep sands and nitrogen applied in the fall may not make it to the spring. Also most wheat producers in the area make three or more applications of nitrogen unlike the two (pre and top) of the Great Plains. 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 spacial 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.
I hear a great deal of talk about how it would take to much time to put out the N-Rich Strip. However the majority of producers that do it once on one field, end up doing it every year on every field. There is very likely someone in your area who is using the N-Rich Strips. As top-dress grows closer keep an eye out for a blog “Using the GreenSeeker Sensor and Sensor Based Nitrogen Rate Calculator”.
For more information on N-Rich Strips check out the YouTube video below, visit http://www.npk.okstate.edu or contact me directly at firstname.lastname@example.org. I have lots of material I am happy to share and distribute.
See the YouTube Video http://www.youtube.com/watch?v=kJ3DSwWYgE8