The Effects of Foliar Potassium Applications on Table Grape Quality

About 99% of all U.S. grown table grapes come from California and over 100 million boxes were shipped to over 60 countries in 2012. Potassium (K) is the most abundant nutrient in mature grapes and has a significant effect on increasing fruit quality compounds. Foliar K applications are often used since soil K is not always sufficiently translocated for optimum crop quality. In 2013 two foliar applications were made using either water or 1.3 g/L of K at veraison and three weeks later at California State University, Fresno. Samples were collected weekly until harvest. Soluble solids content, berry K content, juice pH and titratable acidity were measured at each sampling. Berry firmness, diameter, weight and color were also measured at harvest. Potassium caused significant increases in soluble solids in both cultivars including increased berry K content, firmness and color intensity, while berry size and firmness decreased significantly. Foliar K applications after veraison appear to be a promising approach to obtain earlier harvest. In addition, five different K salt solutions were tested. The salts included K bicarbonate, K potassium phosphate, K phosphite, K sorbate, and a commercial potassium product Metalosate^® K. These products were applied at the onset of veraison and again three weeks later to clusters of ‘Sweet Scarlet’ and ‘Scarlet Royal’ table grapes. Per liter, the solutions contained 1.3 g of K and 0.3 g of surfactant B1956. The clusters were immersed for 10 seconds into the solutions. Five berries from each sample cluster were collected weekly for three weeks and soluble solids and K concentrations were determined. The fourth and final sample consisted of 20 berries per cluster and in addition to soluble solids, berry color, firmness, berry size, and juice pH and acid content were determined. The soluble solids of untreated ‘Scarlet Royal’ and ‘Sweet Scarlet’ berries were significantly lower than the Metalosate^® K treated berries. Responses from K bicarbonate and K sorbate applications were similar. The three potassium applications that increased soluble solids: 1) did not alter berry color; 2) generally reduced berry firmness; 3) reduced the size of ‘Scarlet Royal’ berries but not those of ‘Sweet Scarlet’; and 4) did not alter acid content or pH. Results indicate that the application of these salts could be used to increase soluble solids to facilitate earlier harvest, or to mature a larger crop on the vines.