Establishing grading indices of available soil potassium on paddy soils in Hubei province, China

Abstract

Abstract Soil testing is an important diagnostic tool for assessing crop-available soil potassium (K) and hence making appropriate fertilizer recommendation. This study was aimed at correlating grain yield response data to soil-test K extracted with ammonium acetate (NH 4 OAc), cold nitric acid (HNO 3 ), sodium tetraphenylboron (NaTPB) and boiling HNO 3 solution, based on 54 field trials conducted during 2011 to 2015 across 15 counties in Hubei province, China. The specific objectives were to establish abundance and deficiency indices of available soil-K (ASK) for rice ( Oryza sativa L.) and make accurate K fertilizer recommendations. Potassium extracted with NaTPB and boiling HNO 3 was 1.47 times and 3.61 times higher respectively than that extracted with cold HNO 3 , while K extracted with cold HNO 3 was 1.32 times higher than that extracted with NH 4 OAc. There were significant logarithmic relationships between crop response and soil-test K. The R 2 values for cold HNO 3 -K and NaTPB-K methods were much higher than for NH 4 OAc-K method. In order to calibrate the application, the abundance and deficiency indices of ASK categorized by cold HNO 3 -K in low, medium, high and very high ranges were <50 mg kg −1 , 50 to 100 mg kg −1 , 100 to 200 mg kg −1 and >200 mg kg −1 respectively, and that defined by NaTPB-K were <60 mg kg −1 , 60 to 150 mg kg −1 , 150 to 330 mg kg −1 and >330 mg kg −1 , respectively. These values could be used to evaluate soil K supplying capacity and make appropriate K fertilizer recommendations for rice.