328 Scharenbroch and Watson: Wood Chips and Compost Improve Soil Quality and Increase Growth 10% PAN would release 1 kg of N yr-1 post could be applied over 100 m2 , so this com- clause for N application rate. Assuming a compost density of 500 kg m-3 , this would be 1 m3 to meet the ANSI of compost. In addition to the improvements in overall soil quality and tree growth observed in this research with organic materials, there are many environ- mental reasons to favor organic materials to inor- ganic fertilizers (Follett et al. 1981; Finck 1982). Nutrients in organic amendments are less likely to leach, run off, or volatilize compared to nutrients in inorganic fertilizers. Because organic materials tend to release nutrients more slowly, less consid- eration is required for matching application to plant demands. Organic amendments also provide additional nutrients (e.g., manganese, zinc, sulfur) that are oſten not included in inorganic fertilizers. Many organic amendments are created from resid- uals that would potentially enter landfills. Fossil fuel consumption is required to make and trans- port inorganic fertilizers—albeit, transportation of organic materials also requires fossil fuels (Jenssen and Kongshaug 2003). Long-term fertilization with inorganic N has been found to decrease soil C stor- age, whereas, organic materials tend to increase soil organic matter and C storage (Khan et al. 2007). Soil and tree properties were not significantly impacted by ACT or CBP compared to NULL con- trols. Nutrients added per year by ACT and CBP were <80 g N 100 m-2 K 100 m-2 , <2 g P 100 m-2 , and <0.02 g and K added with COMP and WC were 0.4 and 0.5 kg N 100 m-2 (Table 1). Conversely, amounts of N, P, y-1 0.03 to 0.04 kg K 100 m-2 , 0.07 and 0.06 kg P 100 m-2 y-1 y-1 , and , respectively. Nutrients in COMP and WC were magnitudes greater than added in ACT and CBP. Total microbial biomass added per year with CBP and ACT was 0.427 and 0.576 kg 100 m-2 for CBP and ACT, respectively. On average, the total soil microbial biomass levels for these soils are 33.5 kg 100 m-2 , which is 79 times and 58 times greater than the microbial biomass added per year in CBP and ACT. To match the existing soil microbial biomass levels, concentrated ACT would have to be applied at 12,213 L 100 m-2 y-1 In comparison, the total microbial biomass added per year in COMP treatment was 12.9 kg 100 m-2 . , or 40% of the existing soil microbial biomass level. The annual cost of these treatments on a per tree basis was computed (Table 6). Labor costs com- prised the largest portion of the budget for all treat- ments. Total costs were greatest for ACT followed by WC, FERT, COMP, CBP, and lastly NULL. The amount of biomass gained per tree per year was greatest for WC and lowest for the NULL treat- ment. The most efficient ($ g-1 ) treatments were WC, COMP, and NULL. As a result of the large initial investment in a compost tea brewer, large amount of labor, and relatively low growth response, ACT was highly inefficient compared to the other treatments. CONCLUSION Compost topdressings and wood chip mulches should be used as soil management techniques for trees growing in compacted urban soils. These results confirm the hypothesis that greater im- provements in soil quality and tree growth would be observed with solid organic materials (COMP and WC) compared to liquid-based treatments (ACT and CBP). The research demonstrates that COMP and WC are effective and cost-efficient alternatives to inorganic fertilizer for improving soil quality and increasing tree growth in compacted urban soil. It is reasonable to expect that combin- ing wood chips and composts may have even greater benefit for improving soil quality for urban trees. Future research should examine the effectiveness of combining these and other organic materials in attempts to mimic the organic and A-horizons Table 6. Annual costs for materials, equipment, and labor, and growth and efficiency per tree for water control (NULL), commercial biological product (CBP), aerated compost tea (ACT), NPK fertilizer (FERT), compost (COMP), and wood-chip mulch (WC). Treatment Materials ($ tree-1 Equipment ($ tree-1 Labor ($ tree-1 ) ) ) Total cost ($ tree-1 Growth (g tree-1 Efficiency ($ g-1 ) ) ) NULL 0.0 0.5 7.5 8.0 22.8 0.35 CBP 3.5 3.5 11.3 18.3 23.9 0.77 ACT 7.5 28.5 30.0 66.0 24.1 2.74 FERT 6.5 3.5 11.3 21.3 38.7 0.55 COMP 3.0 1.0 15.0 19.0 41.5 0.46 WC 10.0 1.0 15.0 26.0 61.3 0.42 ©2014 International Society of Arboriculture
November 2014
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