The petiole (leaf stalk) of a young fully expanded strawberry leaf indicates the nitrogen content of the strawberry plant when it is tested either by the “quick test” with diphenylamine reagent or by a laboratory analysis using either the phenoldisulfonic acid procedure or the new nitrate electrode method. In the quick test, if a drop of diphenylamine reagent placed on the cut surface of the petiole (cut at a slant) turns blue immediately, the strawberry plant is well supplied with nitrogen and fertilizing with nitrogen at that time is usually not warranted. If, however, the test is negative, as indicated either by a lack of blue color formation or by a browning of the cut surface, fertilizing with nitrogen is indicated·–provided, of course, that there is sufficient growing weather remaining for the crop to benefit from nitrogen fertilization.

The California Strawberry Advisory Board supported this research.