Arboriculture & Urban Forestry 33(2): March 2007 145 creased to 0.07 in the uncompacted samples and 0.06 in the compacted samples. From unamended sandy loam to 75% added amendment (food waste compost or peat), macropo- rosity of the compacted samples increased 200%. Macropo- rosity of the uncompacted samples increased 50% with 75% added food waste compost and macroporosity increased 75% with 75% added peat. Clay Loam For the clay loam samples, there were no significant increases in the macroporosity of the uncompacted samples when amended with any rate of food waste compost. However, the compacted clay loam samples showed an increase in macro- porosity—from 0.01 to 0.02—when 33% food waste compost was added, an increase to 0.03 when 50% food waste com- post was added, and macroporosity held steady at 0.03 with the addition of 75% food waste compost. The addition of peat to the clay loam increased macropo- rosity steadily in both uncompacted and compacted soil samples. In uncompacted soil, macroporosity increased from 0.03 at 10% added peat, to 0.04 at 33% and 50% added peat, and to 0.05 at 75% added peat. In compacted soil, macropo- rosity increased from 0.02 at 10%, to 0.03 at 33%, and to 0.04 at 50% and 75% added peat. The overall increase in macro- porosity from unamended clay loam to clay loam with 75% added peat was 67% in uncompacted samples and 100% in compacted samples. DISCUSSION This investigation showed that the addition of organic amend- ment (at least 33% for sandy loam and 50% for clay loam) to a compacted soil reduced bulk density to below root restrict- ing thresholds and increased macroporosity significantly— more than 100% in some cases. Sphagnum peat was margin- ally more effective at lowering bulk density and increasing macroporosity than food waste compost, probably because of the deleterious “particle nesting” effects of the added sand in the food waste compost. For both bulk density and macroporosity, linear regres- sions showed that, in most cases, the volume of amendment at which significant change occurred might be slightly lower than the actual volumes tested. The results suggest that amending a clay loam might not produce as many physical changes as amending a sandy loam. To amend a clay loam, at least 50% by volume sphag- num peat should be added to reduce bulk density to below root restricting thresholds; macroporosity may still be low. Fifty percent by volume is a large quantity of amendment and may create other problems, including severe shrinkage. Be- cause this shrinkage could allow water to pool, if amending clay loam with 50% organic matter, one should select plants that tolerate poor drainage. The results of this study confirm some of the existing literature while challenging some studies that question the value of amending tree-planting sites. Pellet (1971) stated that there were no significant improvements to plant growth in amended soils. Our research shows a distinct difference between amended and unamended soil samples. Although this was a purely soil-focused investigation, one could hy- pothesize that with increased soil macroporosity and de- creased bulk density, tree roots would establish more readily, enhancing plant growth. Hodge (1995) found a significant improvement in plant growth when a clay soil was amended with organic matter. He found inconsistencies with amending clay soil that paral- lel our inconsistencies with clay samples amended with food waste compost. In both cases, the sand content of the compost may have decreased macroporosity. However, bulk density decreased, and bulk density may play a more crucial role in root growth than macroporosity. If this is the case, then the clay loam was effectively remediated through the addition of organic amendment. Our study aligns with the work done by Day and Bassuk (1994) that showed that callery pear (Pyrus calleryana ‘Red- spire’) in compacted soil grew best with amendment as long as drainage was adequate. Previous amendment studies neglected to measure initial soil-limiting characteristics. In most cases, researchers merely assumed that the soils to be amended were disturbed and had root-restricting properties. By contrast, in this study, we used soil samples that had known limiting properties, including bulk density above root-restricting thresholds and low percolation rates. This investigation showed that the bulk density and mac- roporosity of two disturbed soil types, sandy loam and clay loam, were positively affected through amendation with or- ganic matter. Bulk density decreased below root-restricting thresholds and macroporosity increased significantly. Modi- fication of the soils with food waste compost and peat could be expected to reduce root impedance and increase soil aera- tion and drainage. The addition of organic amendment to a compacted soil increases the potential for better root growth even after soil is recompacted. LITERATURE CITED Aubertin, G.M., and L.T. Kardos. 1965. Root growth through porous media under controlled conditions. Soil Science of America Proceedings 29:290–293. Brady, N.C. 1990. The Nature and Properties of Soils. Mac- millan Publishing Co., New York, NY. 639 pp. Corley, W.L. 1984. Soil amendments at planting. Journal of Environmental Horticulture 2:27–30. Craul, P.J. 1985. A description of urban soils and their de- sired characteristics. Journal of Arboriculture 11: 330–339. ©2007 International Society of Arboriculture
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