Variability in Soil Test Potassium and Crop Yield

Information from this project led to several revisions of the state guidelines for potassium management, published in the Cooperative Extension Publication PM1688. Revisions were made in 2002, 2011, and 2013.

Some key findings have been:

• Historical interpretation of soil tests significantly overestimated the fertility of Iowa soils. Soils were actually much more deficient in potassium than previously thought. Soil tests once classified as "optimum" are now classified as "very low."
• As a result, potassium fertilizer consumption in Iowa increased substantially to meet the newly discovered soil fertility needs. At the start of the project in 1999, consumption of K2O was 414,000 short tons and was part of a long period of decline that had started in mid 1980s. After the first published revision in potassium recommendations in 2002, consumption began growing again and reached an historical high of 669,000 short tons in 2011.
• IPNI soil test summaries from 2001 to 2015 have confirmed that the percentages of soil samples with inadequate potassium have been decreasing over time in Iowa - consistent with the increase in fertilizer use.

The changes made to soil test interpretations have occurred over many years as data were collected and analyzed from this project.

In 2002, the following changes were made:

Right Rate
• The first major change was made to interpretations of Very Low, Low, Optimum, High, and Very High potassium fertility categories.
• Soil test interpretations were added for the Mehlich III extractant. Previously, interpretations existed only for the ammonium acetate extractant. Many commercial laboratories at that time were moving to the Mehlich III extractant because it could be used to extract other nutrients.
• Probabilities of crop response to K additions were added to each soil test category: 80% for Very Low, 65% for Low, 25% for Optimum, 5% for High, and less than 1% for Very High.
• New guidance was provided on potassium rates for the second and third seasons after soil sampling. Resampling is suggested every 2 to 4 years, and questions about how to fertilize in those years between sampling were addressed.
• A "partial crop removal rate" was added for the High category between sampling years to limit rapid soil test declines.
Right Time
• Annual applications of K were recommended for silage and forage crops.
• N-P-K starter (applied at planting) was recommended for the High soil test category under specific conditions: limited soil drainage, cool soil, residue-covered soil, or late planting with full-season hybrids.
Right Place
• Deep band placement of K was added for corn and soybean grown on ridge-till and for corn grown under no-till.

In 2011, the following changes were made:

Right Rate
• Average margins of error were added to laboratory analyses for K: 10%

In 2013, the following changes were made:

Right Rate
• The second major change was made to the interpretations of Very Low, Low, Optimum, High, and Very High potassium fertility categories.
• New interpretations were added for field-moist and slurry samples. Previous interpretations were for air-dried soil samples only.
• A cautionary statement was added that laboratories devising new methods needed "field-response based interpretations" and not rely solely on "in-house adjustment factors."
• Potassium removal estimates for harvested crop portions were adjusted downward for several crops. These estimates are used to calculate the "maintenance" fertilizer rate - the rate needed to maintain soil fertility in the Optimum category.
• Potassium removal estimates were added for corn stover, soybean residue, and several forages
• Moisture percentages were added to potassium removal estimates, eliminating confusion and allowing dry matter-based calculations to be conducted
• Equations were provided for rate calculations, to facilitate their use in site-specific potassium applications using variable rate application technologies
• The range of potassium rates required to raise a soil test level 1 ppm was quantified for the first time: 6-20 lb K2O/acre, and advice was given not to exceed the recommended rates in the Very Low and Low categories because of this uncertainty unless economics were very favorable.
• Greater emphasis was given to more frequent soil sampling, every two years, as opposed to the more traditional four years.
Right Time
• Annual applications of potassium were recommended for the highest applications rates for soils testing in the Very Low category.
• N-P-K starter applications (at planting) were clarified to have low chances of producing a yield increase on soils testing in the Very High category. Additionally, it was stated as unlikely that crops will respond to a N-P-K starter when two seasons' worth of potassium is applied before the corn season in a corn-soybean rotation.
Right Place
• The depth of deep band placement was quantified: 5 to 6 inches deep

This project has also produced many scientists proficient in potassium science. Past students involved in this project and their current positions are listed below:

Dr. Pedro Barbagelata, Scientist, INTA, Argentina
https://inta.gob.ar/personas/barbagelata.pedro

Mr. Agustin Bianchini, General Manager, Okandu, USA
https://www.linkedin.com/in/agustinbianchini/

Dr. Matthew Clover, Agronomy Research Manager, DuPont Pioneer, USA
https://www.linkedin.com/in/matthew-clover-b5983170/

Dr. Daniel Kaiser, Associate Professor, Department of Soil, Water, and Climate, University of Minnesota, USA
https://www.swac.umn.edu/directory/faculty/daniel-kaiser

Dr. Jorge Sawchik, Principal Investigator, Instituto Nacional de Investigación Agropecuaria, Uruguay
http://www.inia.uy/Personas/Paginas/jorge-sawchik-pintos.aspx

Dr. David Wittry, AMVC Management Services, USA
http://www.amvcms.com

Dr. Antonio Mallarino, the Principal Investigator for the project, has produced a number of peer-reviewed publications from this project, drawing international interest in his work. He has been to several countries and spoken at numerous international conferences. The peer-reviewed publications from this project are listed below (in chronological order):

• Oltmans, R.R. and Mallarino, A.P. (2015). Potassium uptake by corn and soybean, recycling to soil, and impact on soil test potassium. Soil Science Society of America Journal, 79, 314-327. http://dx.doi.org/10.2136/sssaj2014.07.0272. (672 downloads: verified 11 Jan. 2018).
• Clover, M.W. and Mallarino, A.P. (2013). Corn and soybean tissue potassium content responses to potassium fertilization and relationships with grain yield. Soil Science Society of America Journal, 77, 630-642. http://dx.doi.org/10.2136/sssaj2012.0223. (707 downloads: verified 11 Jan. 2018)
• Barbagelata, P.A. and Mallarino. A.P. (2012). Field correlation of potassium soil test methods based on dried and field-moist soil samples for corn and soybean. Soil Science Society of America Journal, 77, 318-327. http://dx.doi.org/10.2136/sssaj2012.0253. (970 downloads, verified 11 Jan. 2018)
• Mallarino, A.P., Bergmann, N., and Kaiser, D.E. (2011). Corn response to in-furrow phosphorus and potassium starter fertilizer applications. Agronomy Journal, 103, 685-694. http://dx.doi.org/10.2134/agronj2010.0377. (793 downloads, verified 11 Jan. 2018)
• Sawchick, J. and Mallarino, A.P. (2007). Evaluation of zone soil sampling approaches for phosphorus and potassium based on corn and soybean response to fertilization. Agronomy Journal, 99, 1564-1578. http://dx.doi.org/10.2134/agronj2006.0151. (445 downloads, verified 11 Jan. 2018)
• Mallarino, A.P. and Borges, R. (2006). Phosphorus and potassium distribution in soil following long-term deep-band fertilization in different tillage systems. Soil Science Society of America Proceedings, 70, 702-707. http://dx.doi.org/10.2136/sssaj2005.0129. (710 downloads, verified 11 Jan. 2018)
• Borges, R. and Mallarino, A.P. (2003). Broadcast and deep-band placement of phosphorus and potassium for soybean managed with ridge tillage. Soil Science Society of America Journal, 67, 1920-1927. http://dx.doi.org/10.2136/sssaj2003.1920. (430 downloads, verified 11 Jan. 2018)