Comparison of Effects Between Foliar and Soil N Applications on Soil N and Growth of Young Gala/M9 Apple Trees

The increasing concern about environmental contamination by nitrate leaching from agricultural land has stimulated a search for more efficient ways to apply N fertilizers. An experiment was carried out to compare the effectiveness of foliar and soil N applications on apple tree growth and soil nitrogen status in order to evaluate N use efficiency. One-year-old, uniform Gala/M9 apple trees were selected and transplanted into 4-litre pots with a typical loamy sand soil. The plants grew in the greenhouse and were randomly divided into five groups of 17 trees. The experiment included a control (no N) and 4 N treatments where the same amount of N as urea was applied once every two weeks started when shoot growth exceeded 5cm. N was applied by 1) foliar only, 2) soil only, 3) soil early and late, foliar midsummer (combination I), or 4) foliar early and late, soil midsummer (combination II). New shoot number was recorded, and shoot length, plant and soil N nutrients were measured throughout the growing season. Plants were moved from the greenhouse in late October to over-winter outside. Five trees were sampled for total N analysis at the dormant stage. The remainder was put in the screen house for re-growth evaluation in the following season. Shoot number was unaffected by treatments in the first season, but soil N application alone and combination I significantly increased shoot length. All N treatments significantly increased leaf N status and leaf color (as indicated by SPAD-chlorophyll readings) compared to the unfertilized control, but there were no significant differences among different N treatments. Foliar application increased reserve N content at the dormant stage and promoted re-growth, but all N treatments resulted in similar root/shoot ratios, which were lower than that of the control treatment. Both soil NO3 and NH4 were significantly increased by soil N application compared with either control or foliar application, an indication that soil N application created a higher potential for leaching loss. INTRODUCTION Nitrogen (N) is an essential element for agricultural production, and N fertilizers are the most widely used fertilizers in the world (Mengel and Kirkby 1978). Although the application of N fertilizers has dramatically improved plant production, it has also resulted in a world-wide problem of environmental contamination. Increasing public concern about excessive nitrate leaching from agricultural lands has encouraged a search for a more efficient way to apply N fertilizers (Dinnes et al, 2002). Soil application is the traditional method of supplying N to sustain plant production in agriculture, but it is often associated with high soil N leaching and low efficiency of use. Foliar N application to fruit trees can have higher use efficiency (Weinbaum, 1988), but it may adversely affect plant vigor and reduce the economic yield if relied upon solely. Combinations of soil and foliar N application might achieve optimum N use and plant production, but there have been no consistent results reported in literature (Johnson et al, 2001). The objective of this study is to compare effects of foliar, Proc. XXVI IHC – Sustainability of Horticultural Systems Eds. L. Bertschinger and J.D. Anderson Acta Hort. 638, ISHS 2004 Publication supported by Can. Int. Dev. Agency (CIDA) 268 soil or their combined N application on the growth of young apple trees. MATERIALS AND METHODS One-year-old uniform Gala/M9 apple trees were selected and transplanted into 4litre pots with loamy sand soils. The plants were grown in the greenhouse under standard growth conditions and randomly divided into five groups with 17 trees in each group. The experiment included a control (no N application, CK) and 4 N treatments where the same amount of N as urea was applied once every two weeks from mid May (when shoots were 5cm-long) to early mid September and about 4g N in total was applied to each tree. N was applied by 1) foliar only (Foliar), 2) soil only (Soil), 3) soil early and late, foliar midsummer (combination I, Com.I), or 4) foliar early and late, soil midsummer (combination II, Com.II). New shoot number was counted, and shoot length, plant and soil N nutrients were measured periodically during the growing season. Leaf greenness readings, measured by a SPAD meter (Minolta Corporation), were also made. Plants were moved from the greenhouse in late October and allowed to over-winter outside. Five trees were sampled for total N analysis in the dormant season. The rest of trees were placed in a screen house for an evaluation of regrowth with no N application in the following season. Plants were portioned into leaves, stems and roots, dried and weighed. Total N in leaves, stems and roots was determined with a LECO FP-528 N analyzer (LECO CORPORATION, 3000 Lakeview Avenue, St. Joseph, MI 49085-2396, USA). Soil NH4 was analyzed by the Nessler method, and NO3 by the nitrophenoldisulfonic-yellow method (Jackson, 1958). All data were subjected an ANOVA analysis with NCSS’97 Statistical System software (NCSS Statistical Analysis Software, Kaysville, UT, USA).