unsulfonated residue (USR or R)

as defined in the ASTM-D-483 test, the percentage of the oil sample that did not react with 37 normal sulfuric acid. The phytotoxicity of agricultural oils has been shown to increase with an unsulfonated residue content below 90%. An oil with an unsulfonated residue content of 92% minimum is required for spraying orchard crops in the leaf or bud state. Above that level, there appears to be little or no added advantage in terms of reduced toxicity.

Also see orchard spray oil.

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