The Response of Young Valencia Orange Trees to Differential Boron Supply in Sand Culture.

Twelve young Valencia orange trees which were budded on Rough lemon stock, were planted into 50-gallon containers on April 28, 1947. The containers were perforated in the bottom and filled with a very fine, white quartz sand. These units were depressed in soil for nearly their full length. The capillary contact with the subsoil induced good drainage and permitted good root development throughout the container. A complete nutrient solution, described by HOAGLAND and ARNON (5), was applied twice weekly for the first month. By May 28 the trees all showed new growth and were assigned treatments in four blocks of three trees each. The rate of boron used in the nutrient feeding was the only differential variable for the succeeding three years. The lowest boron level was that which was supplied as impurities in the C.P. salts and the lake water used as a water source. During the second year of growth obvious boron deficiency symptoms (9) appeared at this lowest level and boron was thereafter applied at the rate of 0.01 p.p.m. A medium boron level of 0.5 p.p.m. and a high of 2.0 p.p.m. were maintained as the other two treatments. After 18 months the basal nutrient solution was modified as follows: P lowered from 31 to 10 p.p.m.; K from 235 to 180 p.p.m.; Mg increased from 47 to 72 p.p.m.; and the micro-nutrients, Mn, Zn, and Cu increased five-fold over the relatively low levels recommended for the Hoagland solution. Solution was applied twice weekly throughout the period of study at the rate of 2 liters per application for the first year, 2i liters for the second year, and 3 liters during the third. Water was applied between nutrient feedings in amounts that induced leaching. Leaf samples were collected each year and analyzed for various major elements, as previously described (11). The mico-nutrient elements were determined by A.O.A.C. methods (1), with the exception of Fe, which was determined spectrographically.