Why plants grow poorly on very acid soils: are ecologists missing the obvious?

Factors associated with soil acidity are considered to be limiting for plants in many parts of the world. This work was undertaken to investigate the role of the toxicity of hydrogen (H+) which seems to have been underconsidered by ecologists as an explanation of the reduced plant growth observed in very acid soils. Racial differences are reported in plant growth response to increasing acidity in the grass Holcus lanatus L. (Yorkshire-fog) and the tree Betula pendula Roth (Silver Birch). Soils and seeds were collected from four Scottish sites which covered a range of soils from acid (organic and mineral) to more base-rich. The sites and their pH (1 : 2.5 fresh soil : 0.01 M CaCl2) were: Flanders Moss (FM), pH 3.2 +/- 0.03; Kippenrait Glen (KP), pH 4.8 +/- 0.05; Kinloch Rannoch (KR), pH 6.1 +/- 0.16; and Sheriffmuir (SMM), pH 4.3 +/- 0.11. The growth rates of two races of H. lanatus, FM and KP, and three races of B. pendula (SMM, KP and KR) were measured in nutrient solution cultures at pH 2.0 (H. lanatus only), 3.0, 4.0, 5.0, and 5.6. Results showed races from acid organic soils (FM) were H+-tolerant while those from acid mineral soils (SMM) were Al3+- tolerant but not necessarily H+-tolerant. These results confirmed that populations were separately adapted to H+ or Al3+ toxicity and this was dependent upon the soil characteristics at their site of collection. The fact of plant adaptation to H+ toxicity supports the view that this is an important factor in very acid soils.