Journal of Arboriculture 31(4): July 2005 185 Table 3. Total chlorophyll content (µg cm–2 ) 90 and 130 days after budbreak (DABB) as affected by different planting hole backfill soils in the second year and third year of research on Quercus robur trees. 2nd Year Chlorophyll Chlorophyll Treatment 90 DABB 130 DABB Compost 14.90 b* Fertilization 17.13 a Leonardite 14.92 b Control 12.93 c 13.08 b 16.92 a 12.61 c 12.27 c 15.26 b 18.09 a 14.54 b 14.89 b 3rd year Chlorophyll Chlorophyll 90 DABB 130 DABB 13.79 bc 17.21 a 14.16 b 12.81 c *Values differ significantly when followed by different letters at P ≤ 0.05 (LSD test); ns = not significant. Plant Growth and Leaf Gas Exchange, Third Year Shoot extension was much greater in the fertilized trees (Table 1). This finding is different from other studies that indicate that if there is a benefit from fertilization after planting, it is not very relevant (Struve 2002). Other investi- gations suggest that it is of secondary importance after irrigation (Herms 1998). However, some evidence exists demonstrating how the response to fertilizers is variable depending on species, tree size, type of planting material (container or B&B, plant size, etc.), and soil quality (Funk 2003). The few studies carried out in this field have, in fact, shown that whenever trees are regularly watered, they benefit notably from a balanced supply of nutrients (Gilman 1994). It is, therefore, probable that in the first season following planting, an absence of rainfall accompanied by high tem- peratures and the stress quite common after transplanting may have a limiting effect on the mineral elements added to the planting hole. This hypothesis is consistent with findings that showed that though there was no response to fertiliza- tion in the first year after transplanting, fertilized trees grew more in the second, third, and fourth years after transplanting (Wyman 1936; Gilman and Yeager 2003). Considering the data collected during the entire season, photosynthesis was greater in fertilized plants, while no significant effect was produced with regard to transpiration and water use efficiency (Table 2). Fertilized trees showed significantly higher values of chlorophyll at both sampling dates, while the control plants had clearly lower values (Table 3). Soil Physical Characteristics Measured bulk density values are given in Table 4. Statistical analysis did not show significant differences for treatments and sampling depth, even though slightly lower bulk density values were observed in the soil amended with leonardite. However, bulk densities able to restrict root growth (rb > 1.47 for soil with more than 45% clay) (NRCS Soil Quality Institute 1999) were never observed in the soil surrounding the planting hole. Cone resistance decreased in soil amended with compost and leonardite with respect to control and fertilization treatments (Figure 1). This finding indicate a noteworthy improvement in soil physical conditions, considering that penetrometer resis- tance has been found to correlate with some quantitative and qualitative parameters, such as crustability, compressibility, workability, erodibility, bulk density, root growth, seedling emergence, and plant yield (Bradford 1986; Dexter 1987). Compost addition exhibited significantly lower penetrometric resistance respect to control and fertilization treatments along the whole soil profile (0 to 60 cm [0 to 24 in.]). Moreover, backfill soil added with compost and leonardite never showed cone resistance values (>2000 kPa) (Schafer et al. 1992) that may seriously restrict root elongation. Total macroporosity and porosity values regarding different pores type at two sampling depth are reported in Table 5. Both treatments failed to show differences in total porosity at 10 to 20 and 30 to 40 cm (4 to 8 and 12 to 16 in.) depths, indicating a rather homogeneous distribution throughout the soil profile. Regular and irregular pores were not affected by various treatments. The elongated pores significantly increased in the leonardite- and compost-amended soil compared to control and fertilization treatments. Because it has been widely demonstrated that the increase of elongated pores improves the soil structure quality (Pellegrini et al. 2000; Pagliai and Vignozzi 2002), these results indicate the presence of a better soil structure after the addition of leonardite and compost. For a thorough characterization of soil macropores, the main aspects to be considered are not only pore shape but also pore size distribution, especially of elongated continu- ous pores, because many of these pores directly affect plant growth by easing root penetration and storage and transmis- sion of water and gases. For example, pores of equivalent pore diameter ranging from 0.5 to 50 mm are the storage pores, which function as a water reservoir for plants and Table 4. Effect of treatments on soil bulk density (g cm–3 at 10 to 20 and 30 to 40 cm (4 to 8 and 12 to 16 in.) depths. Sampling depth Treatment Compost Leonardite Fertilization Control 10–20 cm 1.30 ns* 1.17 1.29 1.25 30–40 cm 1.30 ns 1.16 1.29 1.33 *Values differ significantly when followed by different letters at P ≤ 0.05 (LSD test); ns = not significant. ©2005 International Society of Arboriculture )
July 2005
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