Nutrient Distribution and Requirement of ‘Forelle’ Pear Trees on Two Rootstocks

Accurate water and fertilizer management are essential in modern high intensity orchard systems to enable the manipulation of both reproductive and vegetative development. Studies have been conducted to establish the annual plant fertilization requirements, of several tree crops but there exists little information regarding the requirements of pear trees on rootstocks of differing vigour. Nineyear-old ‘Forelle’ pear trees in a commercial orchard on either vigorous, BP1 or dwarfing, Quince A (QA) rootstocks were completely removed from the soil and divided into various tree parts (roots, stems, leaves, shoots and fruit), each part massed, milled and a mineral analysis conducted. Annual requirements of the nineyear-old trees for macro(N, P, K, Ca and Mg) and microelements (Mn, Fe, Cu, Zn, B) were determined by calculating losses and fixation and their requirements expressed in g or mg element/kg yield respectively. ‘Forelle’ on the more vigorous BP1 shows higher requirements than the more dwarfing QA rootstock mainly because of the higher mass of summer and winter wood removed as prunings as well as a higher mass of young shoots. From these results, and with due consideration given to various production differences, annual minimum and maximum fertilization guidelines based on the yield have been determined. INTRODUCTION Most pear orchards in SA are established on the locally bred vigorous BP3 and less vigorous BP1 rootstocks (Du Plooy and Van Huyssteen, 2000) which are more productive than seedling stocks. More dwarfing Quince (Cydonia oblonga L.) stocks, that increase precocity and fruit quality especially in the higher intensity modern orchards, are gaining importance (Jacobs et al., 2003). Fertilizer recommendations are routinely based on soil and leaf analysis but are non-specific for cultivar or rootstock (Rease, 1994). Accurate water and fertilizer management are essential in the highly intensive orchard systems to enable the manipulation of both reproductive and vegetative development, to ensure the possibility of higher quality fruit with longer storage potential and to reduce pollution and costs (Tagliavini and Marangoni, 2000). Although the important functions fulfilled by macroand microelements are well known (Clarkson and Hanson, 1980; Mengel and Kirby, 1982; Devlin and Witham, 1983; Tisdale, Nelson and Beaton, 1985; Neilsen and Neilsen, 2003), the specific elemental requirements for optimum growth, production and fruit quality per fruit kind and cultivar, especially under high planting densities, need to be determined. A relatively simple method for determining tree nutrient requirement is to base it on whole tree mineral analysis. Various studies have been conducted (Batjer et al., 1952 for apples; Conradie, 1980; 1981 for vines; Haynes and Goh, 1980 for apples; Stassen, 1987 for peach; Stassen et al., 1997a; 1997b for mango’s and Stassen et al., 1997c for avocado’s. This method takes into account mineral nutrient losses from removal of fruit and pruned wood from the orchard, part of the dropped leaf content and nutrient fixation in the permanent parts of the tree (older wood and roots) relative to tree age (Stassen, 1987 and Stassen et al., 1997a,b,c). Our objective was to study the macroand micro-element distribution and estimate