The effect of K fertigation through subsurface irrigation lines on processing tomato (Lycopersicon esculentum Mill.) fruit yield and quality was evaluated in four field trials in California from 2002–04. Fields had exchangeable soil K between 0.48 to 0.85 cmol·kg–1, with high exchangeable Mg (10.6 to 13.7 cmol·kg–1) and a history of yellow shoulder (YS, a fruit color disorder) occurrence. K treatments evaluated included seasonal amount applied (0 to 800 kg·ha–1), fertigation method (continuous versus weekly), and timing (early, mid or late season); foliar K treatments were also included in the 2002 trial. In two fields total and marketable fruit yield were significantly increased by K fertigation, and fruit color improvements were observed in all trials. Among color parameters improved by K fertigation were YS incidence, blended color, and L*, chroma, and hue of the shoulder region of fruit. K fertigation did not affect fruit soluble solids concentration. Yield increased only with fertigation treatments initiated during early fruit set. The effects of fertigation method and rate were inconsistent. Foliar K application was ineffective in increasing either fruit yield or quality.
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