Potassium Application and Leaf Sufficiency Level for Fresh-market Tomatoes Grown on a Midwestern United States Fine-textured Soil

Abstract

Tomato (Lycopersicon esculentum) response to potassium (K) fertilization on a well-drained, central Iowa loam soil testing low in exchangeable K was evaluated over a 3-year period. Each year the experimental design was a factorial, split-plot randomized complete block with K rate as the whole unit (0 to 332 lb/acre). The subunit was cultivar, either `Mountain Spring' (determinate growth habit) or `Jet Star' (indeterminate growth habit). Fruit harvest began the first week of August and continued weekly for 5 to 8 weeks. For all years there was a significant K rate and cultivar effect for all parameters, but no interaction except for marketable fruit size and unmarketable fruit produced. Increasing the K rate to 103 lb/acre increased fruit size of both cultivars to a maximum of 8.9 oz, but year accounted for greater fruit size difference than the choice of cultivar. Maximum marketable yield for both cultivars occurred at 220 lb/acre K with `Jet Star' producing 13% more fruit than `Mountain Spring', 359 vs. 319 cwt/acre, respectively. Cullage was high, mostly as a result of blotchy ripening disorders, with `Jet Star' consistently producing more culls than `Mountain Spring'. Increasing K rate did not reduce the percentage of culls, which remained constant at about 29% of total yield. Whole-leaf K and leaf petiole sap K levels linearly increased with additional K rate for the two sample periods at flowering and mid-harvest. The whole-leaf K sufficiency level for both cultivars at the flowering stage of growth was determined to be 3.15% and dropped to 1.30% K by mid-harvest. Critical petiole leaf sap K values (using a dilution of 1:1 sap to water) could not be determined at flowering, but at mid-harvest the critical value was about 2200 to 2800 ppm K.