Root growth of forests on peatlands with a high pH subsoil or marl layer

Inhibited growth is observed in many forest stands planted on marl and other calcareous soils. Currently in Ireland, state grant aid for afforestation is refused on sites which contain marl or other material which audibly or visibly reacts when dilute Hydrochloric acid (Hcl) is applied (reactive layer), within 70 cm of the soil surface. This known as acid testing or fizz testing and the reaction occurs to due carbon dioxide evolution when dilture Hcl contacts carbonate soil material. The aims of this study were to investigate siites where forests have been established on marl or other calacareous soils, investigate if these subsoils are root limiting environments comparing both soil physical and soil checmical properties with root numbers and to investigate the validity of the field ‘Acid Test’ as utilised in Irish Forest Policy. Initial analysis was completed on 50 forest stands of which 19 were selected for further investigation. Tree species included; alder, sycamore, Sitka spruce, Norway spruce, Scots pine and birch. These sites were located in the midlands of Ireland. An investigation was conducted into the presence of reactive layers composed of marl or other high pH material in the soil, the soil properties, and the penetration of the tree roots into the calcareous layer. Tree growth above ground was assessed in a separate study. A soil-coring survey sampled the boundary between the calcareous layer and the overlying soil to distinguish between root penetrating and root-restricting environments. Split-tube undisturbed root cores of 40 cm length and 7 cm diameter were collected, three methods of root analysis were utilised; the core break method, X-Ray Computed Tomography (CT) and WinRHIZOTM 2D scanning and software. Analysis of root scanning images showed roots penetrating all reactive layers. Soil samples were collected at each site and pH, acidneutralising capacity (ANC) andsoil organic matter content were analysed. Soil properties were then compared to root numbers. Soil physical properties: porosity and penetration resistance (PR) were seen to have a statistically significant relationship with root count whilst soil chemical properties; pH and Effervescence Class (EC) did not. The use of the three methods of root analysis was also analysed and the combination of method utilised was beneficial to provide a variety of both quantitative and qualitative data. These results indicate that the use of the ‘acid test’ may not be the most efficient method to indicate a soil’s suitability for forestry and a pocket penetrometer may be a better in field test for a root-restricting environment.