The health of a tree is generally a function of its ability to find nutrients and water and hence is influenced largely by the condition of its rootzone. Rootzones comprise 20% of the tree and are often overlooked with focus solely on canopy health and structure. When care is distributed holistically there is often significant improvement in general canopy health and vigour.
Urban trees are often ‘captives', dependent on inherited soil fertility and are often victims to disrupted natural processes. In a natural environment, topsoil fertility is continually enhanced by a process called bioaccumulation. Bioaccumulation results from litter-fall and the associated cycling of nutrients and organic matter. In urban settings, litter-fall is usually removed for visual aesthetics, neatness, or turf care resulting in decline of soil organic matter and depletion of surface nutrients. This creates a knock-on effect of decreased soil organism activity, and consequently a decline in soil void space and porosity. Combined with compaction from pedestrian foot traffic, results in increased density, reduced oxygen and nutrient levels, decreased aeration and drainage quality. Combined with the high proportion of urban impervious surfaces, accompanied by surface runoff, compacted and degraded rootzone soils can quickly become waterlogged.
Simon Leake, SESL Australia’s principal soil scientist and founder testifies for this-
“I have seen large mature figs killed within a few weeks by the installation of a bund wall on the downhill side and the diversion of runoff into the bund. The cause was water-logging or a reduction in soil aeration.”
Soil atmosphere comprises 21% oxygen. A minimum of 2.5% is required for root survival, 5% for root growth, 12% for root initiation and 15% for nutrient absorption. Root growth and function is compromised when soil oxygen level falls below 21% resulting from compaction combined with high rain fall events and urban run-off.
Sands and sandy loams have a naturally high porosity and resistance to compaction; however, most topsoils are clay or silt dominant and can easily be compacted to densities exceeding 1.5 g/cc in an urban environment. In a major Sydney Park densities in silty loam soil regularly exceeded 1.7 g/cc. A common factor in urban tree decline, aside from degradation of the rootzone soil is simply a reduced rootzone for the tree to explore. Services, buildings, roads, pathways and driveways all reduce the available rootzone surface area, causing a stunted root volume therefore reduced access to nutrients. Low fertility will slow growth rates, leading to reduced canopy cover in urban environments and a decline in
mature tree populations.
Table 1. Some possible predicted symptoms that will present in the tree canopy due to various soil conditions.
Improving Rootzone Quality.
Given reduced overall rooting volumes, it is important to maximise the functioning of what soil volume remains. A tree species growing naturally in poor soils may not thrive in the same soil if root volume is reduced and it should not be concluded that it does not need feeding in an urban situation.
Soils should be optimised chemically to improve tree health. Analysis for basic chemical properties allows recommendations for lime, dolomite, soil acidifying agents, clay breakers and fertilisers. Generally, in urban environments, trace element deficiencies are rare. The commonly feared soil contaminants such as lead, and cadmium do not typically cause too much harm to plants. It is toxicities such as zinc or copper toxicity that are becoming increasingly common and are more likely to cause long-term problems to the tree and rootzone health.
Mulching of tree rootzones provide very significant physical improvements to canopy health sometimes even allowing for complete tree recovery. Within the soil rootzone, mulch can improve water infiltration, reduce evaporation rates and increase soil porosity due to greater soil organism activity. Mulching can also deter pedestrian foot traffic. Mulch greatly improves chemically infertile soils by increasing cation exchange capacity and improving nitrogen cycling and the potential stimulatory factors provided by the decaying organic matter can do wonders for tree health.
Use composted mixed green waste mulch from widely available city collections, Very woody mulches can induce, nitrogen draw-down however can be overcome with 20-30g/sqm of urea applied to the mulch surface. It may need two additions over 6 months.
Mulch should be no deeper than 50mm in humid zones and may be as deep as 75mm in dry areas. There is generally no benefit from using deeper mulch and actively decaying mulch can severely limit soil oxygen levels if placed too deep. We have seen trees killed by 200mm deep of fresh pruning mulch. Mulch should be applied under the canopy of decurrent canopy forms and at least out to half a canopy height away from the trunk in excurrent forms.
Reducing Soil Density
In cases of severe compaction, mulching alone may not be sufficient to encourage rapid improvement. This is often the case in hard compacted clays and the heavier loam textures. There are several degrees of intervention that can be used. From least to most interventionist are:
Soil coring: Spiking is not recommended as the penetration of spikes actually increases soil density providing very temporary relief. Coring machines to remove cores and backfill with sand is most effective.
Sand Slitting: Usually accompanied by soil coring a sand slit is a narrow trench which is backfilled with sand. It provides better penetration of air and water and some drainage of excess water. Avoid major structural roots by operating in a radial fashion away from the trunk and not going within the critical rootzone.
Soil Removal and Replacement: Compacted silty soils can be extremely severe and justify soil removal. Replacement of soil in radiating trenches avoiding major roots and backfilled with a sandy medium is a successful approach. Alternatively, removal and replacement of the entire surface 100-200mm of soil either manually or by using air or water knives. The more soil removed and replaced generally the more effective the treatment will be. Steer clear of the soil within the critical rootzone as this provides more structural support than active root feeding. Replacement soil must be a gap graded loamy sand similar to a sports turf rootzone mix. Some organic matter (10%) and fertiliser are beneficial.
Drainage: The most effective drain is an intercept drain, simply a surface dish drain, that removes any run-on water before it impacts on the rootzone and conducts it away. Subsoil drains dug in to around 300mm and backfilled with sand can accompany other compaction relief work. Avoid cutting major roots of course.
Aeration Ag Pipes: New plantings, especially of large stock planted into clay soil pits often show suffocation shock. This is exacerbated by adding organic matter to the planting hole. Short of draining the hole, the installation of vertical pipes of slotted Ag drain can admit oxygen to the rootzone.
When examining thin, sparse canopies, prone to drought or water-logging, think always of improvements to the rootzone rather than top work to improve tree health. The tree may need top work as a result of structural decline but it is often a treatment of symptoms rather than treating the root cause. You cannot have a healthy canopy without a healthy rootzone.