146 Etemadi and Nezhad: Production Method and Humic Acid Application in Arid Climate growth responses of hardened-off container-grown trees, with those of freshly dug field-grown trees (B&B or BR), and 3) evaluate the possible effect of HA as a growth biostimulant, in an arid climate. MATERIALS AND METHODS In the present study, two field experiments using oriental thuja trees were conducted at Isfahan municipality research station located at Mahmoud- Abad (Isfahan, Iran, latitude 32°47’49”N, lon- gitude 51°3512”E, altitude 1,584.6 m), from March 2010 to October 2012. All trees were obtained from a field nursery for ornamental trees inside the Mahmoud-Abad research station. According to 20 years of climatic informa- tion obtained from Iran Meteorological Organi- zation, the study area was classified as cold-arid, with mean monthly temperature ranging from 3.2°C in winter to 30.5°C in summer. Mean daily temperature frequently exceeded 40°C during summer. Throughout the experimental period, mean annual evaporation and rainfall were 1,923 mm and 120 mm, respectively. Rainfall was not seasonally well distributed and mostly concentrated in the November to May period. First Experiment (hardening-off) For the first experiment, 48 thuja trees were se- lected for uniformity (trunk diameter 10 cm above the soil line, approx. 4.5 cm; height, approx. 200 cm) and dug up using a backhoe with root ball width about 10 times the trunk diameter in late March 2010. Three to four days before harvesting, trees were irrigated in order to facilitate the dig- ging operation. Half of the trees were bare rooted; their roots were sprayed with a stream of water and then kept in shade to maintain moisture. The root balls of 24 remaining trees were wrapped in burlap and laced with wire to prevent them from disintegrating. All harvested trees were shipped to the nearby field and potted in bottomless, aboveground woody containers (upper cross sec- tion = 100 cm × 100 cm, lower cross section = Depth (cm) 0–30 0–60 Clay Soil particles (%) Clay 53 Silt 26 48 27 70 cm × 70 cm, height = 120 cm) with horizontal slits embedded in the walls to stimulate lateral air pruning and prevent root spiraling. This ap- proach to digging up trees and holding them in the containers prior to transplanting to the main field was considered a hardening-off process. Containers were placed on the ground at a dis- tance of 1 m apart. Before planting, roots or root balls of half BR or B&B trees were successively sprayed with 300 mg/L of humic acid solution prepared from leonardite [containing carbon (C), 61.2%; nitrogen (N), 3.13 g/kg dry matter; and phosphorus (P), 2.89 g/kg dry matter] until com- plete drench of roots and surrounding soil. Con- tainers were refilled with the same soil type as the trees were produced. The physical and chemical characteristics of the field soil are shown in Table 1. Long wooden stakes were driven into containers soil just outside of the periphery of the root ball in order to support installed trees against prevailing wind at the experimental site. Newly potted trees were irrigated daily, for four weeks, as long as water percolated into soil and drained from the bottom of containers; then irrigation was reduced to three to four times weekly, depending on the weather, to maintain field-capacity moist conditions. This amount of irrigation water was greater than that used for trees in Isfahan landscape, but it was applied until tree establishment. Throughout the study, weeds were controlled by hand and no supple- mental fertilization was applied to the three blocks arranged in a trees. Four single-tree replications per treatment combinations (two harvesting method × two humic acid concentrations) were used in each of random- ized complete block design (4 × 2 × 2 × 3 = 48). Data collection for all trees was performed in late June and again in late October 2010, when the growth rate was greatly diminished. Per- cent survival at each measuring date was cal- culated as the total number of survived trees in each year divided by the initial number of trees. Table 1. The physical and chemical characteristics of the experimental site’s soil. Soil texture pH Sand 21 25 7.7 7.9 EC (dS/m) 2.5 2.8 K (ppm) 21.8 18.7 ©2017 International Society of Arboriculture
July 2017
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