You can easily detect a K deficiency by watching for leaf symptoms. These symptoms usually appear in midsummer as a fading of green color at the leaf edges and between the main veins. The leaf margins tend to curl upward and become necrotic. Mild deficiencies will not show until late summer, approaching harvest. Severe deficiencies may appear around bloom and may ultimately affect most of the leaf area with chlorosis in addition to leaf margin necrosis.
Adams, D.O., Franke, K.E. and Christensen, L.P. 1990. Elevated putrescine levels in grapevine leaves that display symptoms of potassium deficiency. Am. J. Enol. Vitic. 41:121-125. Abstract
Christensen, L.P. 1975. Long-term responses of 'Thompson Seedless' grapevines to potassium fertilizer treatment. Am. J. Enol. Vitic. 26:179-83. Abstract
Christensen, L.P. 1984. Nutrient level comparisons of leaf petioles and blades in twenty-six grape cultivars on three years, 1979-1981. Am. J. Enol. Vitic. 35:124-33. Abstract
Christensen, L.P. 2000. Mineral Nutrition and Fertilization (PDF). Pages 102-114 in: Raisin Production Manual. University of California, Agricultural and Natural Resources Publication 3393, Oakland, CA. Buy book
Christensen, L.P., Boggero, J., and Bianchi, M.L. 1990. Comparative leaf tissue analysis of potassium deficiency and a disorder resembling potassium deficiency in 'Thompson Seedless' grapevines. Am. J. Enol. Vitic. 41:77-83. Abstract
Christensen, L.P., Kasimatis, A.N., and Jensen, F.L. 1978. Grapevine nutrition and fertilization in the San Joaquin Valley. University of California Division of Agricultural Science, Publication 4087, Berkeley, CA.
Christensen, L.P., Peacock, W.L., and Bianchi, M.L. 1991. Potassium fertilization of 'Thompson Seedless' using two fertilizer sources under drip irrigation. Am. J. Enol. Vitic. 42:227-32. Abstract
Kasimatis, A.N., and Christensen, L.P. 1976. Response of 'Thompson Seedless' grapevines to potassium application from three fertilizer sources. Am. J. Vitic. 27:145-49. Abstract
Williams, L.E., Biscay, P.J., Smith, R.J. 1987. Effects of interior canopy defoliation on berry composition and potassium distribution of 'Thompson Seedless' grapevines. Am. J. Enol. Vitic. 42:118-22. Abstract
2004. Soil Environment and Grapevine Mineral Nutrition Symposium. Proceedings of the American Society of Enology and Viticulture. To Purchase