Comparing Extractants for Calibrating Potassium Rates for Tomato Grown on a Calcareous Soil.

Potassium (K) application rates based on a reliable soil testing method are critical for maximizing crop yields and minimizing fertilizer costs. Vegetables grown on calcareous soils, however, have no soil test K (STK) interpretations in Florida. Therefore, the objectives of this study were to compare Mehlich-3, ammonium bicarbonate-DTPA (AB-DTPA), and water to estimate K availability and calibrate K rates for tomato (Solanum lycopersicum L.) grown on a calcareous soil. The experiment was conducted during the winter seasons of 2014 and 2015. Potassium fertilizers were applied through preplant dry fertilizer and drip fertigation at total rate of 0, 56, 93, 149, 186, and 223 kg K ha^-1. Regression models were performed to calibrate K rates by tomato relative yield with total K input (initial STK plus full-season K rate). Concentrations of STK were significantly correlated among the three extractants and the highest correlation occurred between Mehlich-3 and AB-DTPA. Due to the high variability, water was ineffective to estimate K availability. Significantly linear relationships were found between total K uptake (TKU) and total K inputs using both Mehlich-3-K and AB-DTPA-K. The low STK levels were predicted from 85 to 150 and 70 to 120 mg K kg^-1 and the corresponding required K rates ranged from 307 to 151 and 271 to 151 kg K ha^-1 using Mehlich-3-K and AB-DTPA-K, respectively. Thus, both Mehlich-3 and AB-DTPA can be used to predict K availability and K rates needed to produce optimal marketable yield for tomato grown on calcareous soils in Florida.