The influence of soil potassium status on the productivity of docks (Rumex obtusifolius L.) in intensively managed temperate grassland

Docks are a widespread problem in grassland, necessitating novel management solutions as pesticides become increasingly regulated. The objective was to investigate the role of soil test Potassium (K) concentrations (Morgan's solution; Na acetate + acetic acid, pH 4.8; STK) on competitiveness of docks in grassland. The experimental site (52°21 N, 7°18 W) was reseeded with perennial ryegrass in October 2009. A plot experiment was laid down in a randomised complete block design with seven rates of fertiliser K (0, 50, 100, 150, 200, 250 and 300 kg ha⁻¹) and eight replicates. Dock numbers were measured over 6 years (2010–2015) and herbage production over 5 years (2012–2016). There was poor alignment between fertiliser K and STK in the early years of the study, however this alignment improved over time. While fertiliser K had no effect on dock numbers, there were positive correlations between STK early in the study and dock numbers m⁻² throughout the study. Dock numbers m⁻² were relatively static between 2010 and 2012 and increased substantially between 2013 and 2015. Between 2013 and 2016 there were strong inverse relationships (r > −.765; p < .001) between dock and grass herbage production. Towards the end of the study the lower annual fertiliser K inputs (0 and 50 kg ha⁻¹) resulted in lower (p < .01) dock root and herbage production with an associated increase (p < .05) in grass herbage production. It is evident there is a threshold STK for permanent grassland infested with docks at which grass is more competitive for plant-available soil K than the docks to the detriment of the dock component of the sward.