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ARS Home » Midwest Area » Ames, Iowa » National Laboratory for Agriculture and The Environment » Soil, Water & Air Resources Research » Research » Publications at this Location » Publication #354277

Research Project: Managing Carbon and Nutrients in Midwestern U.S. Agroecosystems for Enhanced Soil Health and Environmental Quality

Location: Soil, Water & Air Resources Research

Title: Subsoil potassium in central Iowa soils: Status and future challenges

item OBRYCKI, JOHN - Orise Fellow
item Kovar, John
item Karlen, Douglas

Submitted to: Agrosystems, Geosciences & Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/6/2018
Publication Date: 8/22/2018
Publication URL:
Citation: Obrycki, J.F., Kovar, J.L., Karlen, D.L. 2018. Subsoil potassium in central Iowa soils: Status and future challenges. Agrosystems, Geosciences & Environment.

Interpretive Summary: Potassium (K) has received less attention than nitrogen (N) or phosphorus (P) because it has no major unintended environmental consequences. Unfortunately this appears to have depleted soil profile K concentrations. An analysis of over 2400 Iowa soil samples to a depth of 120 cm (4 feet) showed average surface (0 to 15-cm) and subsurface (15 to 120-cm) concentrations of 150 and 105 ppm K. Based on Iowa soil test interpretations, those values are considered low and very low, respectively. This information will be useful to producers, crop consultants, fertilizer industry leaders, as well as soil fertility/plant nutrition research and extension personnel as they strive to meet increased K demands associated with higher plant populations and potential grain yields.

Technical Abstract: Earlier planting, increased seeding rates, and higher corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] yields, coupled with decreased alfalfa (Medicago sativa L.) and forage production, warrant a reevaluation of plant available potassium (K) within central Iowa soils. Quantifying K relationships is especially important if crop residues are harvested for animal feed or cellulosic feedstock since vegetation contains more K than grain. This study focused on datasets from ten experimental sites that provided soil profile distributions of plant-available K to depths of 120 cm (n=2,433). Depth increments used included: 0 to 5-, 5 to 15-, 0 to 15-, 15 to 30-, 30 to 60-, 60 to 90-, and 90 to 120- cm. Based on over 400 samples collected from each depth increment below 15-cm, a reference value of 100 mg kg-1 appears common for central Iowa subsoils. Sixty-four percent of all samples had soil test K levels below the 120 mg kg-1 threshold for very low nutrient status for crop production in Iowa. Only 28% of surface soil samples (<15 cm depth) were considered to have at least optimum nutrient status with concentrations greater than 160 mg kg-1. As K is not an environmentally sensitive nutrient, its management in central Iowa soils has generally been neglected in recent years, particularly when compared to N and P management initiatives. Building awareness of soil K profile concentrations, rather than focusing only on surface concentrations, will be necessary to meet nutrient requirements for 21st century agricultural production.