The Influence of Mineral Nutrition on the Root Development of Trees

Abstract

Sitka spruce seedlings were grown with divided root systems in sand culture and the applica tion of N, P, and K was varied while maintaining a complete supply of other nutrients. There was a localized stimulation of the growth of roots to which nutrients were applied and N pro duced the greatest stimulation ; P was also important but K did not enhance growth. Measurements of Ca and Mg concentrations in the roots showed that differential root growth, which is influenced by the concentration of N and P in the bathing medium, could not be ade quately explained by the effect of N or P on the uptake of the immobile cations. The two parts of the divided root system appeared to compete for assimilates ; the enhanced growth of one root was accompanied by reduced growth in the other, and vice versa. INTRODUCTION A localized stimulation of the growth of Sitka spruce roots occurs following the application of a solution containing N, P, and K to a portion of the root system (Coutts and Philipson, 1976), and similar responses have been reported in cereals (Hackett, 1972; Duncan and Ohlrogge, 1958). By withholding specific nutrients Drew (1975) has demonstrated that both N, either as nitrate or ammonium, and phosphate will individually produce a localized stimulation of growth in barley roots but that K had no such effect. The localized stimulation of growth does not appear to be associated solely with the internal concentrations of these major elements since they are mobile and have been shown to occur in the roots which receive no external nutrient supply (McClure, 1972 ; Coutts and Philipson, 1976) : the mineral nutrients stimulate the growth of only those roots which are bathed in the nutrient medium. In this paper we examine the relative importance of N, P, and K individually in stimulating tree root growth and investigate McClure's hypothesis (1972) that a localized stimulation of root growth by phosphate may result from an increased up take of immobile ions such as calcium and magnesium. MATERIALS AND METHODS