MONITORING THE POTASSIUM STATUS OF GREENHOUSE TOMATOES USING QUICK PETIOLE SAP TESTS

'Celebrity' tomatoes (Lycopersicon esculentum Mill.) were grown in peatperlite under greenhouse conditions in two experiments with five levels of K (25, 50, 100, 200, and 300 mg.liter-1) in irrigation waters. In both experiments, aqueous petiole sap K concentrations (milligrams K per milliliter) were monitored weekly with colorimetric paper test strips. Variation in petiole sap K concentrations among plants was low among three-plant replicates (CVS of 9% to 11%). Petiole sap K concentrations varied considerably from week to week, but remained relatively constant over time at levels determined by external-feed K concentrations. Petiole sap K levels and marketable-fruit yield increased quadratically with increasing external K concentrations. Maximum yields were produced with 190 to 200 mg K/liter in external-feed solutions. Maximum marketable yields of almost-equal-to 2.75 kg/plant occurred with almost-equal-to 5.9 mg K/ml in petiole sap. Quick petiole sap K tests appear to be suitable for assessing the K status of greenhouse tomatoes.