A Composite Sampling Technique to Assess Urban Soils Under Roadside Trees

Soils underlying roadside sugar maple {Acer saccharum Marsh.) trees in an urban setting were studied to determine the relative advantages of a ten-point composite sampling scheme over the more common practice of averaging analytical results from a few individual samples. The soil samples near the crown periphery were systematically extracted using a hammer-driven tube sampler and composited using equal volumes from each point. Mineral soils were partitioned into 0-15 and 15-30 cm depth components. Duplicate composite samples were obtained for each of 12 trees. Four individual soil samples also were extracted with a larger tube sampler from the same region and depths. Coefficients of variation and numbers of samples needed to estimate the mean within ten percent at the .05 probability level were computed for various chemical and physical measurements. The ten-point composite method consistently displayed a pronounced reduction in variation of analytical results and provides a considerable savings of laboratory time. Specifications for sampler design and utilization are described. Trees situated in an urban environment are subjected to stress factors not encountered by trees in a forested setting (Tattar 1 978). The area encompassed by urban and industrial developments continues to expand at a high rate, making it increasingly necessary to deal with problems specific to metropolitan soils. Frequently, these problems are associated physical or chemical properties (Mader and Thompson 1969). Proper diagnosis of soil conditions is dependent on extracting representative soil samples from the site. Attempts to assess soil parameters are complicated by the disturbed and heterogeneous nature of most urban soils. Urban soils should be sampled by depth increments rather than by soil horizons due to the lack of a distinct vertical profile. Information is needed on the variability of urban soils in order to implement sampling designs that will allow discrimination of appropriate soil properties. This is crucial if valid prescriptions are to be generated (Mader 1974). This study is part of a larger study investigating the decline of roadside sugar maple (Acer saccharum) trees. The intent of this study is to demonstrate whether a composite sampling scheme can appreciably lower the variation of analytical results when compared with the more common practice of averaging results from several individual point samples. The description and implementation of a sampling device for urban soils are detailed. Materials and Methods Twelve roadside sugar maple trees ranging in size from 50-106 cm (20-42 in) dbh were chosen from a study group of 40 trees. All trees had soil under 60-100% of their crowns. Several assumptions were made concerning feeder root distribution: 1) Roots will not occupy the area under a paved road due to low O2 status (Van Camp 1961). 2) If a driveway is not paved, roots will occur at a depth where soil compaction no longer impedes root growth (Goss et al 1975, Wiersum 1957). 3) When roots encounter the edge of a paved road they tend to branch out and parallel the road (Bernatzky 1978, Horsley and Wilson 1971). 4) The greatest concentration of feeder roots will be in the top 30 cm of mineral soil (Pritchett 1979). 5) Feeder roots of sugar maple will predominate at the crown periphery (Tubbs 1977). Figure 1 illustrates the typical situation for a tree in this study. The asterisks designate where the ten composite points were taken. If the dripline extended over an unpaved drive, a sample point was located in the drive. Along the road the samples were taken 20 cm (8 in) from the road's 1 Paper No. 2077, Massachusetts Experiment Station, University of Massachusetts, Amherst, MA 01003. Research supported in part by U.S. Forest Service Northeast Forest Experiment Station through the Consortium for Environmental Studies. Graduate student in Forest Soils, Professor of Forest Soils, and Associate Professor of Plant Pathology, respectively. Journal of Arboriculture 8(4): April 1 982 97 edge or curb. The four stars indicate where individual point samples were drawn. Two sets of ten-point composite samples were taken for each tree. The second set was taken at the crown periphery from points lying halfway between the first set's points. Analytical results from averaging two composite sets from the 0-15 cm (0-6 in) and two composites from the 15-30 cm (6-12 in) depths were compared to the results obtained from taking four individual point samples from the starred areas and averaging the separate lab results for these samples by depth. The sampler developed for the ten-point composite scheme is shown in Figure 2. It is essentially a hammer driven, open faced auger that is designed to probe mineral soil to a depth of 30 cm. Organic layers are not analyzed and should be dismissed. Each sample point will yield 1 25 cc (4 oz) of soil or 62.5 cc (2 oz) for each of the two depth components. It is best to discard a portion of the soil by leveling the cutout trough with a blunt knife. This insures a constant volume per sample point and eliminates contamination of the sample engendered by smearing as the sampler is retrieved from the ground. The net yield is closer to 50 cc (1.5 oz) for each depth. This results in a composited sample of 500 cc (16 oz) per depth component. If a bulk density of 1.2 g/cc is assumed, a 600 g (1.3 Ib) sample will be available for lab analysis. Even if the coarse fraction is as high as 50%, a sample of sufficient size remains. 12