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Subsoil Potassium in Central Iowa Soils: Status and Future Challenges

John F. Obrycki, John L. Kovar, Douglas L. Karlen
Agrosystems, geosciences & environment 2018 v.1 no.1 pp. -
cellulose, crop production, crop residues, feeds, feedstocks, nitrogen, nutrient management, nutrient requirements, phosphorus, potassium, soil profiles, soil sampling, soil surface layers, subsoil, Iowa
A reevaluation of plant-available K within central Iowa soils is needed, especially if crop residues are harvested for animal feed or cellulosic feedstock. This study evaluated plant-available K concentrations to depths of 120 cm (n = 2433) within soil profiles at 10 experimental sites. Data from 0- to 5-, 5- to 15-, 0- to 15-, 15- to 30-, 30- to 60-, 60- to 90-, and 90- to 120-cm depths were compiled and evaluated. 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 with 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.