Soil bulk density and its influence upon tree root growth

Bulk density is an indicator of soil compaction. It is calculated as the dry weight of soil divided by its volume. This volume includes the volume of soil particles and the volume of pores among soil particles. Bulk density is typically expressed in g/cm³.

Soil bulk density is, to some degree and in combination with other soil characteristics, reflective of the mechanical resistance roots meet in the soil. At higher bulk densities, root growth can be restricted as the forces exerted by the roots as they push through the soil cannot ‘overcome’ the resistant forces of the compacted soil. Further to this, increased bulk density reduces soil porosity and therefore offers less viable space for roots to grow within (given roots grow most often within aerated pockets of the soil structure). By-and-large, root growth is optimal at 1.2g/cm³ and limited seriously when bulk densities exceed 1.6 g/cm³ (Bassett et al., 2005). Road foundations in Denmark are typically compacted to bulk densities exceeding 2 g/cm³ however (Buhler et al., 2007), which indicates the extent of the problem for urban trees in particular.

Studies with tree seedlings in containers have shown that soil compaction reduces vertical root penetration, thereby meaning areas of high bulk density might increase the likelihood of shallow rooting. In larger, mature trees, higher bulk density can decrease the fine root density profile quite significantly – by up to 60% (Watson & Kelsey, 2006). This can impact upon water and nutrient uptake. Therefore, if a tree develops and matures in a soil with high bulk density, not only may it have shallow roots but also have a reduction of fine root mass. Research has however shown that trees have a remarkable ability to reach down to points of lower soil bulk density, where surface compaction is an issue (Nambiar & Sands, 1992) – assuming deeper root penetration is actually possible. Interestingly, a certain amount of compaction, in turn increasing bulk density, can aid with root growth – given the increase in root-soil contact (Alameda & Villar, 2009). This is particularly the case for roots within very loose, sandy soils (such as sand dunes).

Additionally, with the loss of macro-pore space as soil density increases, water infiltration and gas diffusion is reduced, soil oxygen concentration is decreased, and carbon dioxide concentration can increase – possibly to toxic levels (Watson & Kelsey, 2006). This deterioration in quality of the soil environment thereby renders the soil less favourable for root growth and nutrient uptake (respiration for active transport is less feasible, for instance) (Batey, 2009; Day et al., 2010; Kozlowski, 1999), as well as mycorrhizal fungi establishment (Shigo, 1986).

The overall impact of soil bulk density upon root morphology and function does ultimately vary between species however (Bassett et al., 2005). Species will preferentially reside at different places upon a larger continuum of bulk densities, with tolerance ranges also differing – certain species may tolerate a wider range of soil densities than other species.

Sources:

Alameda, D. & Villar, R. (2009) Moderate soil compaction: implications on growth and architecture in seedlings of 17 woody plant species. Soil and Tillage Research. 103 (2). p325-331.

Bassett, I., Simcock, R., & Mitchell, N. (2005) Consequences of soil compaction for seedling establishment: Implications for natural regeneration and restoration. Austral Ecology. 30 (8). p827-833.

Batey, T. (2009) Soil compaction and soil management–a review. Soil Use and Management. 25 (4). p335-345.

Buhler, O., Kristoffersen, P., & Larsen, S. (2007) Growth of street trees in Copenhagen with emphasis on the effect of different establishment concepts. Arboriculture & Urban Forestry. 33 (5). p330-337.

Day, S., Wiseman, P., Dickinson, S., & Harris, J. (2010) Tree root ecology in the urban environment and implications for a sustainable rhizosphere. Journal of Arboriculture. 36 (5). p193-205.

Kozlowski, T. (1999) Soil compaction and growth of woody plants. Scandinavian Journal of Forest Research. 14 (6). p596-619.

Nambiar, E. & Sands, R. (1992) Effects of compaction and simulated root channels in the subsoil on root development, water uptake and growth of radiata pine. Tree Physiology. 10 (3). p297-306.

Shigo, A. (1986) A New Tree Biology. USA: Shigo and Trees Associates.

Watson, G. & Kelsey, P. (2006) The impact of soil compaction on soil aeration and fine root density of Quercus palustris. Urban Forestry & Urban Greening. 4 (2). p69-74.

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