Arboriculture & Urban Forestry 35(5): September 2009 Arboriculture & Urban Forestry 2009. 35(5): 271–278 271 Plant Available Moisture in Stone-soil Media for Use Under Pavement While Allowing Urban Tree Root Growth Jason Grabosky, Edward Haffner, and Nina Bassuk Abstract. Three avenues of experimental observation detail aspects of plant available water holding capacity in compacted stone-soil media designed for urban tree establishment in paved situations. The various compacted media provided an estimated plant available moisture content of 7%–11% by volume, comparable to a loamy sand. Changes in aggregate and of soil influenced initial field capacity moisture content, but high matric potential moisture content was consistent, presumably as a reflection of the aggregate content of the designed system. A large portion of plant available moisture was weakly held in large voids, consistent with related infiltration and permeability data, and could be an influence in water storage and irrigation planning to use layers of designed soils in a layered pavement section for urban vegetation. Key Words. Stalite; Structural Soil; Plant Available Moisture. Urban tree establishment, particularly when a tree is planted close to pavement, can become problematic for the tree and/or the pavement. High levels of soil and base course compaction to support pavement typically impedes or excludes root growth, limiting access to the water and mineral resources they contain. Several designed soil systems have been advanced to integrate pavement section design and tree root growth in spaces where trees are close to pavement with limited soil resources (Costello and Jones 2003). Gap-graded, large aggregate-soil systems—at times referred to as structural, skeletal or bimodal soils—rely on an aggregate matrix to meet pavement design needs by hav- ing aggregates form a load-bearing matrix, or skeleton, while suspending soil as a rooting medium within the interconnected voids within the aggregate matrix. While there are no published studies explicitly testing plant available moisture properties of these designed soils, there are several assumptions and opin- ions which vary widely regarding plant available moisture hold- ing capacity and irrigation management. There is also a need to quantify water storage and drainage behavior such as system porosity, infiltration capacity and hydraulic conductivity prior to developing soil volumes designed to integrate storm water management and urban canopy development into a paved site. The following three studies were conducted to define compact- ed porosity, infiltration, and plant available moisture in varied CU Structural Soil™ Stalite Structural Soils™ mixtures, with the third study also using Carolina (henceforth termed CU and CS, respec- tively). Stalite is a heat-expanded slate which has been reasonably assumed to provide a greater plant available moisture content over a traditional aggregate given its rough surface and porous nature. The testing of different soil and aggregate components in the fol- lowing studies reflect the time and location differences in the test- ing effort, but also reflect the component variability which might be encountered by regional availability of soils and aggregates. Developing empirical data for cultural management of plant ma- terials growing in compacted systems can assist in assigning root zone soil volumes of the stone-soil mixtures with regard to ir- rigation scheduling and stormwater management consideration. MATERIALS AND METHODS Initial Studies 1996–1997 Three tests were conducted from a CU mix design, representative of a compacted field installation. The tests were designed to as- sess consistency in sample preparation and porosity for laborato- ry testing, define plant available moisture in compacted samples using a standard desorption plate method (Topp et al. 1993), and define hydraulic conductivity (see Table 1, for protocol testing methods used). These tests were in prepared compacted samples. In the first test, six samples were tested to check for system and sample consistency. In the second test, three samples were used in plate desorption testing, but repeated plate failures on other rep- licates limited data collection. A final two-point sample test for constant head hydraulic conductivity in compaction permeame- ters was used to describe the behavior in compacted samples. A CU was developed from a gravimetric ratio of 83% blocky crushed limestone NYSDOT #2 [2.5–1.25 cm (1–0.5 in) range in size] aggregate (NYSDOT _702-02, 1990), 17% shredded clay loam and an agricultural hydrogel (0.025%) used as a sys- tem stabilizer during mixing and placement (Gelscape hydro- gel, Amereq, New City, NY). For consistency of data between horticulture and engineering protocol and to minimize potential sidewall effects in sample behavior given the size of the aggre- gate, the samples were prepared and compacted into 15.24 cm (6 in) i.d. California Bearing Ratio testing molds of 2317 cm3 (141.4 in3 ) with a standard effort of 592.7 kJ m-3 (12,375 ft lbf ft-3) (protocol references Table 1, data Table 2). Six samples were ©2009 International Society of Arboriculture
September 2009
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