Potassium dynamics of four grassland soils contrasting in soil K management history

Potassium ( K ) management is important in grassland systems, especially in dairy farming. The interactions of K with other nutrients, such as magnesium (Mg), and enhanced K levels themselves can have impacts on metabolic health issues of dairy cattle, yields and product quality. A deeper understanding of K dynamics is necessary for sustainable management. We used four sandy soils of similar genesis but from fields contrasting in farm K management histories (>15 years) and combined these with different levels of mineral K fertilization in a 5‐month glasshouse pot trial with Italian ryegrass (four cuts). The initial plant available soil K status (calcium–acetate–lactate extraction: K CAL ) was 18, 93, 168 and 295 mg K kg −1 for low (organic), medium (organic), high (conventional) and very high (conventional) input farming systems, respectively. A mass balance approach was used to examine soil K history and treatment K fertilizer effects on soil and plant K . Potassium concentrations in shoots were high, except for the low K input soil under the 0 K fertilizer treatment, and were influenced by initial soil K and high K input. Soil K release values reflected K management history, whereby soils with higher historical K inputs released more K , and the organically managed low input soil had very limited K release and reserves. K HC l was found to be a strong ( R 2  = 0.87) and useful indicator of potential and available soil K deliveries from these sandy soils. This study has confirmed that both long‐term K management history and current K fertilizer practice have strong effects on K and Mg dynamics in the soil–plant system.