Sulfur diagnostic criteria for French prune trees

One year old Marianna 2624 (Prunus cerasifera X P. munsoniana) trees grafted with French prunes (P. domestica) were grown in the greenhouse in pots containing vermiculite and supplied with graded amounts of sulfate-sulfur (SO4-S) to study the methods of evaluating sulfur (S). Tree fresh weight gain was recorded. Leaf samples were analyzed for different S fractions. Chlorophyll concentration of leaves was better than fresh weight gain as an indicator of tree S status. Sulfate in recently 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. Using chlorophyll as an indicator of growth potential, critical SO4-S level 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 this index in the recently matured leaves is 7.85%. External critical S level is 80 muM (8 ppm as SO4).