Assessing soil characteristics, and their likely effect on tree growth, typically requires specialist laboratory tests. But the cost of planting trees in unsuitable soils can also be high. Two recent academic papers assess measures that urban tree managers can themselves employ to gauge soil quality.
Writing in the Arboricultural Journal, Myerscough College senior lecturers Duncan Slater and Irene Weir, and arboricultural consultant Kit Hardy, a recent Myerscough graduate, state: "For the urban forester, development of an effective test or tool that could determine the relative health of urban soils would be beneficial as it would inform tree planting and soil management choices."
They argue that to this end, biological activity is a key indicator of soil quality "because it represents the living component of soil responsible for a multitude of processes vital to soil function".
Simply measuring the soil’s organic content "is arguably not an ideal measure of soil quality, as many other factors affect the extent of biological activity that occurs in urban soils and that is potentially a more important measure", for which measuring CO2 respiration in the soil "is a robust test for assessing that activity".
However, they note: "For urban foresters, there are currently only a limited number of tools at their disposal to assess the biological component of a soil in order to gauge the soil’s potential to support tree growth or its need for amendments prior to tree planting."
Colorimetric soil test
A gel-based colorimetric soil test developed in the USA for farmers, the Solvita Gel System indicates levels of biological activity by measuring CO2 respiration rates in soil samples. The Myerscough team compared this with other analytical tests using soil samples from two contrasting urban sites — a wooded area and a grassed area with semi-mature trees — and concluded that there were "significant correlations between basal respiration results and soil organic matter content".
By conventional measures the wooded site appeared the healthier, with 156% higher levels of NPK, 240% more soil organic matter and 252% more easily oxidisable soil organic carbon than the grassed site. Correspondingly, the Solvita Colour Reader value and basal respiration was nearly twice as high for the wooded site. Soil bulk density at the woodland site was also found to be significantly lower, at 0.97g/cu cm compared to 1.45g/cu cm for the grassed site.
They conclude that the Solvita system "could prove to be a useful indicator of initial soil quality for urban foresters, and also provide a repeatable means of assessing how soils change due to afforestation". They go on to suggest that "by quantifying these soil characteristics, practitioners can efficiently employ soil amelioration methods to alleviate inadequate site conditions for urban trees and other greening projects".
Meanwhile, a paper by US researchers in Urban Forestry & Urban Greening found a wide disparity in the field performance of commercial soil pH and moisture sensors that urban tree managers might use for assessing site soils.
The team, from the University of Wisconsin, the Morton Arboretum and Bartlett Tree Research Laboratories, assessed the accuracy of 21 different "low-cost" soil pH and moisture sensors from 10 US and one Chinese suppliers across a range of soil types and moisture levels.
They found that two soil pH sensor models from Oakton Instruments, employing a glass-electrode in a 1:2 solution of soil to de-ionized water, did indeed "accurately and precisely" measure soil pH compared with a standard laboratory test. By contrast, pH sensors using metal electrodes inserted into the soil "failed to significantly and accurately measure soil pH across all soil textures and moisture contents".
As for sensors for assessing volumetric soil moisture content, those measuring soil dielectric permittivity, using time domain reflectometry and frequency domain reflectometry methods, "can be calibrated to produce accurate readings in most soils" and so "performed best", though these typically cost more than $500.
Those based on measuring electrical conductivity "were highly variable in cost, accuracy and precision", they found, though they noted that General and Extech models "performed well while also being relatively affordable" at under $300.