The impact of forestry on soil formation and health; tales of the podzol.

Soil formation is a gradual process resulting from the combined effects of parent material, topography, climate and time. Land management can disrupt natural soil formation and certain forms of land management can exaggerate soil forming or degrading processes. Surveying soil profiles can reveal how land has been managed over the last decades to centuries and can guide the future management of land to maintain or enhance soil health.

Figure 1: Area of woodland with significant ground cover.

Woodlands and forestry greatly influence soil formation by creating their own microclimate of shaded and cool soil surface conditions vastly different to adjacent open glades and fields. Plantation conifer forestry illustrates this clearly and to its greatest extent, where tightly spaced trees are concentrated into blocks interspersed by rides, creating light and heat limited conditions at the soil surface (Figure 1). Woodlands and forestry also add considerably to organic matter in the form of leaf and needle litter in the surface horizon of soils. The decomposition of this surface litter in cool and damp shaded conditions generate organic acids which percolate the soil profile displacing suspended Fe and Mn by a process generally referred to as podzolisation. Podzol soils therefore display clear horizons of leached or ‘eluviated’ strata below the surface deposited litter, which are evident by bleached sand grains. Lower in the soil profile the deposition of Fe and Mn are evident by an orange coloured ‘illuviated’ horizon (Figure 2).

Figure 2: Soil profile impacted by podzolisation.

Soil classification systems for forestry clearly recognize the importance of podzolisation in the nutrient regimes of forestry soils and account for podzolic soils in their own right, as well as podzolisation processes occurring within other soil types. Amongst other factors, the degree of podzolisation of soils results in nutrient impoverished horizons in soils which can limit forest productivity. Over time, and with repeat restocking of cleared stands for example, podzolisation can increase and soils may be rendered nutrient limited.

With increasing interest in expanding woodland and forestry cover, whether for amenity, biodiversity or carbon capture and accounting, it is vital to understand the health of soil assets, and key field measures by which this can be assessed and monitored. Forest soils, or land newly designated for woodland and forestry, should be surveyed or re-surveyed to account for the degree of podzolistion and its extent, so that management plans can best serve the maintenance and enhancement of soil health in the long term.