Newly Updated Phosphorus and Potassium Guidelines

Originally published February 28, 2023 by Dr. Antonio Mallarino is a professor of agronomy and nutrient management research and an extension specialist at Iowa State University for Integrated Crop Management News.

Guidelines for soil-test phosphorus (P) and potassium (K) interpretations and application rates were last updated in 2013. Since then, significant field research was conducted to assure that nutrient management guidelines are kept current. The new response data, increased crop yield levels in Iowa, and better awareness of usually large spatial soil-test variability in production fields indicated a need for adjusting the boundaries of soil-test interpretation categories and suggested nutrient application rates. The updated publication PM 1688 has been posted for free download at the Extension Publications Store and the Soil Fertility Extension website.

No updates were made to the pH and lime management guidelines since recent research confirmed the suitability of the 2013 guidelines. Additionally, the soil-test zinc (Zn) interpretations for the DTPA test method were not changed; however, the updated publication now includes interpretations for Zn by the Mehlich-3 test method. This article is a brief overview of major changes made and what remains unchanged based on the available research data.

Main Reasons for the Changes

Improved crop genotypes have been introduced to production agriculture and yields of mainly corn and soybean, but also other field crops, continue increasing. Since the prior update in 2013, about 200 field response trials for P and K with both corn and soybean crops were conducted at five Iowa State research and demonstration farms — in central, northeast, north central, northwest, southeast, and southwest Iowa. These research farms have different soil types, and the weather (mainly rainfall and temperature) differs somewhat across farms. While new research was not conducted for other field crops, the guidelines were adjusted by using previous relative differences with corn and soybean and information from similar soils in neighboring states.

However, the fundamental concepts used for developing soil-test interpretation and fertilizer application guidelines remained the same. The general objective is to accomplish long-term profitability and reduced risk of yield loss while maintaining or improving the profitability and sustainability of crop production and water quality. This is attained by emphasizing crop response-based fertilizer applications for the low-testing interpretation categories targeted to maximize yield in most conditions; recommending removal-based maintenance using prevailing crop yields (not necessarily yield goal) for a soil-test category with low probability of a small yield response (the optimum category), and for the high category recommending only common starter rates for some conditions described in the publication.

What has Changed?

Two key areas have been updated in the new version of PM1688. One important change was to adjust the boundaries for the soil testing categories — mainly by moving the boundary for the optimum category up slightly to make this category wider. This is justified by the observed variation in the magnitudes of yield responses and further recognition of the intrinsic uncertainty of soil-test results mainly due to high spatial variability despite use of improved dense sampling methods. Also, despite significant improvements in laboratories quality control and many years of successful proficiency soil testing programs, the bias in test results among laboratories continue being substantial since its complete elimination is very difficult.

The left graph in Figure 1 shows the available relative grain yield responses of corn and soybean to P fertilization for a wide range of soil-test results using the Bray-1 or Mehlich-3 tests using the standard colorimetric measurement of extracted P (these tests continue to be statistically equivalent except in calcareous where the Bray-1 should not be used). For reference, the previous interpretations in the PM-1688 publication and the new categories are indicated in the figure. Even discounting a few outliers, the responses below the 95% relative yield for the low and optimum categories suggest a need for raising those boundaries by a few parts per million. These adjustments also maintain the traditional criterion for all categories – to comprise probabilities of response around 80, 60, 25, and 5% for the very low, low, optimum, and high categories. This is shown by the right bar graph in Figure 1.

Figure 1. Relationships between soil-test P and relative corn and soybean grain yield responses [the previous (top) and new (bottom) soil-test categories are shown] and probabilities of response within each new category (right). VL=very low, L=low, Opt=optimum, H=high, VH=very high.

Graphs in Figure 2 show the new relationships for soil-test using the dry and moist (or slurry) sample handing procedures and the probabilities of response for each category for the ammonium-acetate and Mehlich-3 tests (which continue to be statistically equivalent). The graphs demonstrate the need for similar changes as mentioned for P. An important finding of the recent research is that it confirmed the moist K test is much more reliable than the dry test and its use results in more accurate and profitable K fertilization – particularly in specific soils identified as problematic for the dry test. This is especially the case in productive soils that are classified with moderately poor to poor drainage, even if tile drainage is present. New study of soil properties showed that mainly poor soil drainage with alternating saturated and dry conditions over the seasons combined with slightly different clay mineralogy to other soils partially explain the bad performance of the dry K test on these soils – which are commonly found in central to northern Iowa but also in other areas.

Figure 2. Relationships between soil-test K using the dry and moist sample handling procedures and relative corn and soybean grain yield [the previous (top) and new (bottom) soil-test categories are shown) and probabilities of response within each new category for each sampling handling procedure (bar graphs). VL=very low, L=low, Opt=optimum, H=high, VH=very high.

The other significant change was to increase the suggested P and K fertilization rates for the very low and low interpretation categories. These changes cannot be detailed here but are easy to see in tables for several crops in the publication. The reason for the increase is to maintain the traditional concept in Iowa P and K recommendations of assuring that the rates attain maximum yield in most conditions while gradually increase postharvest soil-test levels. Default yield and removal-based application rates were also increased for the optimum category. These defaults are only general guidelines for instances when laboratories do not receive yield level information. As PM 1688 as stated for decades, producers should use prevailing yield levels and provided P and K concentrations to estimate removal-based rates for the optimum category.

What has Not Changed?

The recent research confirmed that the Bray-1 soil P test is unreliable in calcareous soils and either the Olsen or Mehlich-3 P tests should be used, different soil-test interpretations are needed for the Mehlich-3 test with colorimetric or ICP measurements of extracted P, K test results by the ammonium-acetate or Mehlich-3 K tests are more reliable when using the field-moist or slurry sample handling procedure than the dried sample procedure  mainly in soils with moderately poor to very poor drainage, for all tillage systems equivalent crop responses typically occur for broadcast and planter-band P application (other than starter in some conditions), and broadcast or planter-band K applications continued showing no consistent differences.

No new deep banding research was conducted but many previous field trials before 2013 had shown that deep band P is not better than broadcast P for corn or soybean with any tillage system, but that K fertilizer should be deep-banded with ridge-tillage whereas it may be beneficial only occasionally for corn managed with no-tillage or strip-tillage mainly in dry years.

Although only the online version of the updated publication is available for free download at this time, the website will indicate when hardcopies being printed will be available for purchasing. Remember that additional useful information related to the nutrients addressed in the publication and other nutrients is included in various menus of the Soil Fertility Extension website.

Dr. Antonio Mallarino is a professor of agronomy and nutrient management research and an extension specialist at Iowa State University. His programs focus on agronomic and environmental issues of nutrient management with emphasis on phosphorus, potassium, lime, and micronutrients. Issues addressed include soil and plant-tissue sampling and testing, fertilizer and manure placement methods to increase nutrient use efficiency and crop yield, use of variable-rate technology, and phosphorous management impacts on water quality. Dr. Mallarino was a prominent member of the team that developed the Iowa Phosphorus Index and is a member of the science team of the Iowa Nutrient Export Reduction Strategy contributing about phosphorus management and water quality. He has calibrated several soil and plant tissue test methods and developed interpretations for Iowa soils.

Community-driven Responses to Water-related Climate Challenges

Please join us for the Iowa Learning Farms webinar at noon CDT, Wednesday, March 29, featuring Kara Salazar, assistant program leader for community development, Purdue Extension & Sustainable Communities, extension specialist, Illinois – Indiana Sea Grant, Purdue University Department of Forestry and Natural Resources. Salazar works with multidisciplinary teams, overseeing the development and delivery of programs to support community planning and sustainable development strategies. Her expertise and focus includes placemaking and enhancing public spaces, land use, green infrastructure, community development, facilitation and strategic planning.

In the webinar, “One Block at a Time: Community-driven planning and equitable adaptation through multi-benefit green infrastructure,” Salazar will highlight ongoing efforts that support marginalized neighborhoods in medium-sized communities through the Great Lakes Sea Grant program that take on the impacts of water-related climate challenges. Drawing on examples from paired projects in four communities, she will highlight efforts focused on improving resilience to more frequent flooding and extreme weather events. Salazar will also discuss the One Block approach to equitable and inclusive stormwater management planning and green infrastructure implementation.

“Whether the result of income insecurities or the effects of redlining, these communities experience challenges most acutely, yet have the fewest means to respond,” said Salazar. “Extension and Sea Grant programs are well-positioned to provide technical assistance and leadership in addressing these challenges. Supporting implementation through the One Block concept can provide a framework for success that is centered on equitable, community-driven engagement and planning.”     

Webinar participants are encouraged to ask questions of the presenters. People from all backgrounds and areas of interest are encouraged to join.

Webinar Access Instructions

To participate in the live webinar, shortly before noon CDT March 29:

Click this URL, or type this web address into your internet browser: https://iastate.zoom.us/j/364284172

Or go to https://iastate.zoom.us/join and enter meeting ID: 364 284 172

Or join from a dial-in phone line:

Dial: +1 312 626 6799 or +1 646 876 9923

Meeting ID: 364 284 172

The webinar will also be recorded and archived on the ILF website, so that it can be watched at any time. Archived webinars are available at https://www.iowalearningfarms.org/webinars For a list of upcoming webinars visit https://www.iowalearningfarms.org/events-1

A Certified Crop Adviser board-approved continuing education unit (CEU) has been applied for. Those who participate in the live webinar are eligible. Information about how to apply to receive the credit will be provided at the end of the live webinar.

Moving Iowa Forward – A Farmer’s Perspective

Giving hope to future generations of Iowans is Leopold Conservation Award winner and Iowa Learning Farms farmer partner Seth Watkins’ goal. Watch the webinar recording from this week to join Seth’s conservation conversation (tongue twister!) and gain some insight into the mind of a farmer.  

“When we see land as a community to which we belong, we may begin to use it with love and respect.” – Aldo Leopold. 

Seth has three ideas on how we move toward a culture of conservation: 

  1. Policy. Demand a Farm Bill that rewards the regeneration of natural resources, not production.  
  1. Embrace Equity. Be inclusive and actively work to create more opportunities to make more farmers.  
  1. Use easements to protect the land we still have.  

Seth is passionate about a variety of policy changes that can come to help farmers in Iowa while also protecting the land, water, and biodiversity. Seth also discussed the importance of diversity in farming, but with diversity must come equity. He would like to see younger farmers, female farmers, and farmers of color given space and opportunities in Iowa. Lastly, to help, Seth would like to see more conservation easements put on land. His reason why? Easements are just plain sexy. 

Tune into the most recent Iowa Learning Farms webinar Honoring Leopold’s Legacy and watch any previous webinar from the archives.  

Alena Whitaker 

Share Your Thoughts on Cover Crops in the National Cover Crop Survey!

Why do you plant cover crops…or why don’t you?

What do you want to know about cover crops? Where do you get your information about them?

Share your thoughts on cover crops in an online survey. Your insight will help guide research, communications, seed development, and more.

This National Cover Crop Survey is the seventh since 2012 conducted by the USDA-NIFA Sustainable Agriculture Research and Education (SARE) program, Conservation Technology Information Center (CTIC) and the American Seed Trade Association (ASTA), with the help of Informa/Farm Progress.

Please take a few minutes to contribute your voice here!

The survey will close on March 30, 2023. After completing the questionnaire, you may enter a drawing for one of three $100 Visa gift cards.

Where Would we be Without Leopold’s Vision?

Please join us for the Iowa Learning Farms webinar at noon CDT, Wednesday, March 22, featuring Seth Watkins, owner of Pinhook Farm in Page County Iowa. Watkins has been a leading advocate for soil and water conservation efforts across Iowa and was the recipient of the 2022 Iowa Leopold Conservation Award.

In the webinar, “Honoring Leopold’s Legacy,” Watkins will provide insights on building toward a more sustainable future gained through his experiences and observations gathered in 40  years of farming in Iowa. A staunch advocate for land stewardship, Watkins will share lessons learned from successes and failures and his efforts to encourage action among others. He will discuss conservation efforts in Iowa, agricultural practices and practical methods being employed to improve the future that all stakeholders are building together. Watkins will also address the importance of taxpayer supports for farming and the importance that farmers respond to those supports in ways that regenerate resources, restore water quality and provide healthy food.

“If we’d stop subsidizing the wrong things, we wouldn’t have to subsidize the right things… We probably wouldn’t need price supports if everyone found the most appropriate land use per acre,” said Watkins. “We must look at and listen to our land, responding to what makes it healthy and productive, and what does not.”

Webinar participants are encouraged to ask questions of the presenters. People from all backgrounds and areas of interest are encouraged to join.

Webinar Access Instructions

To participate in the live webinar, shortly before noon CDT March 22:

Click this URL, or type this web address into your internet browser: https://iastate.zoom.us/j/364284172

Or go to https://iastate.zoom.us/join and enter meeting ID: 364 284 172

Or join from a dial-in phone line:

Dial: +1 312 626 6799 or +1 646 876 9923

Meeting ID: 364 284 172

The webinar will also be recorded and archived on the ILF website, so that it can be watched at any time. Archived webinars are available at https://www.iowalearningfarms.org/webinars For a list of upcoming webinars visit https://www.iowalearningfarms.org/events-1

A Certified Crop Adviser board-approved continuing education unit (CEU) has been applied for. Those who participate in the live webinar are eligible. Information about how to apply to receive the credit will be provided at the end of the live webinar.

How Much Sulfur is Needed?

Why are there more sulfur products on the market? How much sulfur do I need to put on my corn, soybeans, and alfalfa? All of this and more are answered in the most recent Iowa Learning Farms webinar with Daniel Kaiser from the University of Minnesota.

The form of sulfur that crops like corn, soybeans, and alfalfa take up is sulfate. Readily available sulfate is ammonium sulfate, gypsum (calcium sulfate), and potassium sulfate. Elemental sulfur is different and has to be oxidized to allow plant uptake. “That means you might need two to four times as much elemental sulfur compared to sulfate,” said Dr. Kaiser.

Dr. Kaiser usually recommends 5-10 lbs/acre/year of sulfur for continuous corn. Alfalfa response to sulfur is usually best observed when 20 lbs/acre/year is applied. With these recommendations though, Dr. Kaiser empathized it depends a lot on your location. Soil types, erodibility, and water all have big effects on how useful the sulfur application will be.

Tune into last week’s webinar here and find any previous Iowa Learning Farms webinars here. Check out our website for upcoming webinar topics and other events happening.

Alena Whitaker

Lawns and Golf Courses: A Leading Cause of Nitrate-N Loading?

As the calendar turns to spring, my mind turns to golf. Golf has also been on my mind because of what I heard at some recent farmer/landowner meetings we hosted throughout the state. When we asked the participants what the leading causes of water quality issues are in Iowa, participants listed urban such as golf courses and lawns as one of the top three causes.

As an avid golfer, I started wondering if we could better quantify the amount of nutrients that might be coming from those sources. While it is critical to look at nutrient reduction no matter our land use whether urban, rural, or agricultural, we also need to recognize the magnitude of the various contributions.

I am a data guy, so I immediately sought out data on land uses in Iowa from the USDA NRCS Natural Resources Inventory (2017). According to this, there were about 24.9 million acres of cultivated cropland and slightly over 1 million acres of urban and built-up land* in Iowa. So, there is 24 times more cultivated land in Iowa than urban and built-up land. This isn’t really news to anyone who lives here. We are an agricultural state, and the majority of the land is in agricultural lands.

What is less clear is whether the urban and built-up land sends more nitrate into our waterways per acre than the agricultural lands? How big of a problem is the nutrient loss from golf courses and lawns?  

Well, there has been some work done on this in Iowa, and other parts of the country. From a meta-analysis done by Bock and Easton (2020) of studies across the country, they summarized that the median nitrate-N loss via leaching and runoff was 8.9 lb/acre from turfgrass plot studies. More locally, Schilling and Streeter (2018) monitored a number of golf courses in Iowa and estimated the mass of nitrate-N recharged to groundwater in golf courses to be about 3 lb/acre. For comparison, Lawlor et al. (2007) reported an average annual nitrate-N loss of 27 lb/acre from fifteen years of a corn-soybean cropping system at 150-160 lb-N/acre fertilization rate which would be a common fertilization rate. 

While not all the urban and built-up land is in lawns or golf courses and not all these acres would lose the same amount of nitrate-N, if we multiply the 8.9 lb/acre loss by 1 million acres we get 8.9 million lbs of nitrate-N loss from urban and built upland. In comparison, if we have the 24.9 million acres and each acre lost 27 lb/acre, which again I grant is an average and approximate value, there would be 672 million lbs of nitrate-N loss from cultivated cropland.

While those of us with lawns should work to decrease our nitrate-N loss through appropriate nutrient management, it is important to recognize that these areas likely only contribute a couple percent to downstream nitrate load. If you are an avid golfer like me, you can continue to love/hate the sport knowing that the course you are playing on is not a leading contributor to water quality issues in the state.

Matt Helmers

Bock, E. M., & Easton, Z. M. (2020). Export of nitrogen and phosphorus from golf courses: A review. Journal of Environmental Management, 255. doi:10.1016/j.jenvman.2019.109817

Schilling, K. E., & Streeter, M. T. (2018). Groundwater Nutrient Concentrations and Mass Loading Rates at Iowa Golf Courses. Journal of the American Water Resources Association, 54(1), 211-224. doi:10.1111/1752-1688.12604

*This urban and built-up land includes residential, industrial, commercial, and institutional land; construction sites; public administrative sites; railroad yards; cemeteries; airports; golf courses; sanitary landfills; sewage treatment plants; water control structures and spillways; and highways, railroads, and other transportation facilities if they are surrounded by urban areas.

Sulfur Source Selection Makes a Yield Impact

Please join us for the Iowa Learning Farms webinar at noon CDT, Wednesday, March 15, featuring Daniel Kaiser, associate professor at the University of Minnesota. Kaiser focuses on the evaluation of fertilizer guidelines for major agronomic crops grown in Minnesota. One key area has been the development of sulfur guidelines including rate, timing and source of application to facilitate crop productivity.

In the webinar, “Choosing the Right Source of Sulfur,” Kaiser will discuss sulfur sources and how selection can affect crop yields. He will share insights on the importance of sulfur application and availability of this important nutrient to plants in corn, soybean and alfalfa crops. Drawing on his research which compares the effects of different sources of sulfur on crop yields, Kaiser will discuss data showing if and when elemental sulfur oxidizes to plant available forms necessary for utilization by crops. He will also provide insights into the need for better widespread understanding of sulfur forms and application practices.

“I don’t think farmers always know what source of sulfur is being applied to their field, and therefore may not be aware whether the sulfur being applied will be available at the time a crop needs it,” said Kaiser. “It is critical that farmers choose the correct source of sulfur to ensure that it will have a significant impact on crop yields.”

Webinar participants are encouraged to ask questions of the presenters. People from all backgrounds and areas of interest are encouraged to join.

Webinar Access Instructions

To participate in the live webinar, shortly before noon CDT March 15:

Click this URL, or type this web address into your internet browser: https://iastate.zoom.us/j/364284172

Or go to https://iastate.zoom.us/join and enter meeting ID: 364 284 172

Or join from a dial-in phone line:

Dial: +1 312 626 6799 or +1 646 876 9923

Meeting ID: 364 284 172

The webinar will also be recorded and archived on the ILF website, so that it can be watched at any time. Archived webinars are available at https://www.iowalearningfarms.org/webinars For a list of upcoming webinars visit https://www.iowalearningfarms.org/events-1

A Certified Crop Adviser board-approved continuing education unit (CEU) has been applied for. Those who participate in the live webinar are eligible. Information about how to apply to receive the credit will be provided at the end of the live webinar.

Iowa’s Wetlands from a Drone’s Perspective

On last weeks webinar Iowa Learning Farms welcomed Adam Janke, extension wildlife specialist and associate professor, Iowa State University, and Kay Stefanik, assistant director of the Iowa Nutrient Research Center housed at Iowa State University. Janke is a trained wildlife biologist who is active in regional and national efforts for wildlife conservation and education. Stefanik’s focus and research is in water quality, nutrient cycling and aquatic ecosystems.

In this weeks webinar, “Iowa’s Wetlands from the Sky: Educational videos on wetland wildlife and water quality with drones,” Janke and Stefiank discussed their program designed to fill the knowledge gap in wetland ecology and diversity by leading an educational video program to explore wetland ecology and function through using drones and aerial photography throughout the state of Iowa. Throughout the presentation they discuss the win-win relationship between wildlife habitat, water quality and people.

Want to see more of their drone footage? Seven videos of their footage are live now on their website, will more to come in April! If you have any questions about the project or the website, feel free to contact Adam Janke ajanke@iastate.edu or Kay Stefanik kcstefan@iastate.edu.

Watch the webinar recording today!

-Hannah Preston

Accelerating the Construction of Edge of Field Practices

What do the Iowa Department of Agriculture and Land Stewardship, Story County and the City of Ames have in common?

They are partnering on the state’s fourth batch and build project that aims to accelerate the construction of edge-of-field practices like saturated buffers and bioreactors. Megan Volkens, IDALS, and Sara Carmichael, Story Coutny, are leading the charge in reaching out to landowners in the county and conducting surveys to determine if sites will work to install these practices. Dustin Albrecht, with City of Ames, is helping facilitate the city’s fiscal agent role in the project.

This batch and build concept has often been referred to as the ‘easy button’ because it allows for efficiencies in time and financial resources for the landowners, engineers, and contractors and has resulted in approximately 150 saturated buffers and bioreactors across the state. Building on the success of the current project, Iowa Secretary of Agriculture Mike Naig announced at the workshop that there are 12 additional projects in the planning process across the state.

Iowa Sectary of Agriculture Mike Naig highlighing the importance of partnership to reach the Iowa Nutrient Reduction Strategy goals March 2, 2023 in Ames.

During the workshop, attendees had a chance to explore a saturated buffer through a video explaining how they reduce nitrate loss through denitrification, view our Conservation Station On The Edge trailer and visit a site currently under construction near Roland.

To learn more about each of these practices, check out our archived virtual field days and webinars below!

Liz Ripley