Refining potassium fertilizer rates for Midsouth soybeans
Potassium [K] uptake by actively-growing soybeans is high. This essential plant nutrient is supplied to plants from that already in soil plus that supplied by added K fertilizers. Soil testing is conducted to determine just how much plant-available K is in soil so any additional required amount can be added via fertilization. Click here for MSU Extension Information Sheet 894 titled “Potassium in Mississippi Soils” [by Oldham and Jones] that provides details about K forms in the soil, factors affecting plant uptake of K from soils of varying textures/CEC’s, and management of soil K and K fertilization rates for various crops. Click here for the latest MSU recommendation [publication 3434 by Jones] for K fertilization of soybeans in Mississippi.
A Nov. 2020 article titled “Profit-maximizing potassium fertilizer recommendations for soybean” by Popp, Slaton, and Roberts was published in Agronomy Journal [2020;112:5081-5095, https://doi.org/10.1002/agj220424], and reports results from K application rate studies conducted in Arkansas from 2004-2019 [86 site-years]. Pertinent details about and main results from that research follow.
• K fertilizer resources are a finite supply. This, plus current high and increasing prices for fertilizers, should encourage soybean producers to use K fertilizers efficiently.
• These facts resulted in the current study with the objective of developing profit-maximizing fertilizer-K application rates for soybean that are based not only on expected crop response and soil test K [STK] values, but also on soybean yield potential, soybean commodity price, and fertilizer cost. Also, the authors compared applying K fertilizer using variable rate technology [VRT] vs. applying a uniform, profit-maximizing rate.
• Soil series at the research sites were predominately silt loam, and conventional, Roundup Ready, Liberty Link, and Xtend soybean varieties were used over the multi-year period.
• From their research, the authors developed profit-maximizing fertilizer-K rates using data from irrigated K fertilizer rate trials that were conducted in Arkansas for more than 15 years.
• This research determined the following. 1) Reaching near-maximum soybean yield [95% relative yield] without added K fertilizer can be achieved when STK is 143 ppm [mg/kg]. However, the current soil-test-based recommendation for K fertilizer addition suggests applying K fertilizer until STK reaches 175 ppm. Thus, the current uniform rate recommendation to apply K fertilizer to soybean grown on soils with an optimum STK level will not likely increase yield, and is made only to replace a portion of the K removed with the harvested soybean seed and to help maintain the existing soil-test K level. 2) Results from this research suggest limited profitability improvement and essentially no yield change when VRT vs. uniform rate application was used to apply K fertilizer to soybean, especially at lower STK levels. 3) These results suggest lowering the K fertilizer rate recommendations for soybean, and encourage soybean producers to place increased weight on soybean price and fertilizer cost when making K-fertilizer rate decisions. 4) The results from this research highlight the need to use decision support software to assist with selecting profit-maximizing K fertilizer rates that account for yield response, fertilizer cost, and commodity price vs. making the decision based mainly on expected yield response alone. 5) Adding K fertilizer in an amount that is above the profit-maximizing rate will lead to potentially greater K leaching, less profitability, and paying for fertilizer earlier than it is needed. 6) Adding K fertilizer above the profit-maximizing rate will provide insufficient yield benefit to offset the cost of the fertilizer and its application. 7) And finally, these results show that K fertilizer use can be profitably curtailed in Midsouth soybean production fields.
The decision support software [Potash Rate Calculator (PRC)] developed from results reported in this article can be accessed here. A PRC tutorial can be accessed here. Midsouth soybean producers are encouraged to access and use this calculator to determine if K fertilizer cost can be reduced in their operation so as to increase profitability from soybean production. Producers are also encouraged to access the article titled “Deep soil testing for determining complete soil nutrient status” on this website to determine if deep soil testing for STK level below the usual sampling depth could also lead to reduced K fertilizer addition to soybean production sites.
Composed by Larry G. Heatherly, July 2023, larryh91746@gmail.com