Assessment of Plant-Available Potassium in Soils.

Characteristics such as a soil’s landscape position, moisture content, texture and mineralogy can influence the amount and location of potassium (K) in a profile, and can appreciably influence the variability of soil test K (STK) levels. Plant-available non-exchangeable K (PANK) can be a significant source of K in soils. Commonly used soil tests such as 1 M NH₄OAc and Mehlich 3 that do not access this K pool may affect the accuracy of K fertilizer recommendations. An incubation study was performed on the Ap and Bt horizons of soils representing two drainage sequences common to Indiana. The Nappannee-Hoytville sequence consists of lacustrine soils having illitic mineralogies. The Cincinnati-Avonburg-Cobbsfork sequence represents loess derived soils with mixed mineralogies. Soils were incubated at 25°C under two moisture contents (Saturation and Field Capacity) and K was extracted with 1M NH₄OAc, Mehlich 3, and NaBPh₄ after periods of 1, 4, 7, 10, and 14 days. Each soil was then allowed to air-dry in a greenhouse before again being analyzed for K with the extracting solutions. Significant differences (P ≤ 0.05) in extractable K levels were found within incubated moist soils, and between incubated moist soils and their respective air-dry counterparts. Winter wheat was grown in the greenhouse in the Ap horizon of these 5 soils for three 28 day growth cycles.  Mehlich 3 and NaBPh₄ extraction of soil K after each harvest was conducted.  The ability of these soil tests to predicted changes in soil K due to plant removal will be discussed.