Abstract
Alternative techniques to evaluate soil potassium (K) availability for processing tomato production were compared to the standard ammonium acetate extraction procedure. Ammonium acetate exchangeable K (Kex), solution-phase K (Ksol), and K fixation potential (Kfp) were evaluated on soils from 40 California fields in which processing tomatoes were grown in 1996–98, and on soils from six K fertilization trials conducted in 1997–98. Ksol was determined in the supernatant of 1:10 mixtures (w/v) of soil and 0.01 M CaCl2, either extracted after an initial 30-minute shaking, or after 7 days of incubation at 25°C. Kfp was estimated by enriching dry soil with 1.0 cmol K kg−1 as aqueous KNO3, air-drying, then either extracting immediately with 1 M NH4Cl or after a 7-day incubation in 0.01 M CaCl2 solution. In all tomato fields, the determinate cultivars ‘Halley’ or ‘Heinz 8892’ were grown. Crop K status was evaluated at full bloom growth stage by K concentration in whole leaves, and at harvest by K concentration in fruit dry mass, total fruit K content (kg ha−1), and the incidence of yellow shoulder (YS, a fruit color disorder affected by soil K availability). Kex ranged from 0.17 to 1.33 cmol kg−1, leaf K from 18 to 43 g kg−1, and fruit K from 26 to 70 g kg−1. Ksol and Kfp were highly correlated with Kex (r=0.94 and 0.86, respectively). The Ksol and Kfp techniques utilizing K measurement after the initial shaking/extraction step were highly correlated with results from the procedures utilizing a 7-day incubation period (r=0.99 and 0.98 for Ksol and Kfp, respectively). Kex was more highly correlated with plant K status than either Ksol or Kfp, but correlations ranged from only −0.35 (for YS incidence) to 0.57 (for fruit K content). Potassium application reduced YS in 2 of 4 fertilization trials in which the disorder was prevalent, but none of the measures of soil K availability were predictive of either YS incidence, nor of fruit yield response to fertilization.