Estimating soil potassium availability for processing tomato production

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.