SULFUR DIAGNOSTIC-CRITERIA FOR FRENCH PRUNE TREES

One year old Marianna 2624 (Prunus cerasifera X P. munsoniana) trees g rafted with French prunes (P. domestica) were grown in the greenhouse in pots containing vermiculite and supplied with graded amounts of sul fate-sulfur (SO4-S) to study the methods of evaluating sulfur (S). Tre e fresh weight gain was recorded. Leaf samples were analyzed for diffe rent S fractions. Chlorophyll concentration of leaves was better than fresh weight gain as an indicator of tree S status. Sulfate in recentl y matured leaves is a better diagnostic index for prunes than total-S, organic-S, or N:S ratio which are not suitable for this purpose. Usin g chlorophyll as an indicator of growth potential, critical SO4-S leve l of recently matured leaves is 97 ppm. Sulfate-sulfur as a percent of total-S can also be used as a diagnostic tool. Critical level for thi s index in the recently matured leaves is 7.85%. External critical S l evel is 80 muM (8 ppm as SO4).