Sulfur diagnostic criteria for French prune trees

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Abstract
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 μM (8 ppm as SO4).
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