Sensitivity of plants to acidic soils exemplified by the forest grass Bromus benekenii

The aim of this paper is to study the role of Al- and H-ions, base cation deficiency and N-nutrition as solely NH₄ in disturbing plant growth on acid soils. Effects of these factors on growth of the forest grass Bromus benekenii were studied experimentally and in the field and compared to other deciduous forest plants. Bromus benekenii is most frequent at pH >4.5 (soil solution) and absent at pH 3.8. In a pot experiment using an acid soil of pH 3.3, raised by steps to pH 5.2 by carbonate addition, growth of Bromus benekenii and nine other species (out of 17) was limited at pH <4.1. Toxicity of H-ions to Bromus benekenii was confirmed in a solution experiment at pH ≤4.2. Al-toxicity occurred at low Al-concentrations (20 μM) in Bromus benekenii and nine other tested species. Base cation (Ca+Mg) mitigation of Al-toxicity was found in a factorial solution experiment. Our experiments also indicate that H- and Al-ions interact in a way that makes combined treatments equally toxic as concentrations of H-or Al-ions alone. Incubation experiments showed that soil nitrification is retarded in acid soils. This may affect Bromus benekenii as it was most demanding on NO₃-availability out of 15 species in a solution experiment. Only 50% growth was attained in a NH₄- as compared to a NH₄+NO₃-treatment. Comparing experimental results with field data, we conclude that H- and Al-toxicity and unfavourable N-nutrition (as solely NH₄) to a similar extent may explain the acidifuge behaviour of field layer plants.