Soil Science Society of America Journal Soil Sci. Soc. Am. J. 81:1621–1628 doi:10.2136/sssaj2017.04.0134 Received 27 Apr. 2017. Accepted 12 Sep. 2017. *Corresponding author: (qiangzhuuf@gmail.com). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA. All Rights reserved. Comparing Extractants for Calibrating Potassium Rates for Tomato Grown on a Calcareous Soil Nutrient Management & Soil & Plant Analysis Potassium 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 potassium (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 calcare- ous 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 sig- nifcantly 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. Signifcantly linear relationships were found between total K uptake (TKU) and total K inputs using both Mehlich-3 extracted K (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. Abbreviations: AB-DTPA, ammonium bicarbonate-diethylenetriaminepentaacetic acid; DAPF, days after preplant fertilization; STK, soil test potassium; TKU, total potassium uptake; TSMY, total season marketable yield. S oil testing is used to assess potential availability of soil nutrients to crops. In soil testing, a universal extractant is the ideal option for extracting both macronutrients and micronutrients with a single extraction solution ( Jones, 1990). Soil test interpretation usually includes three categories: low, medium, and high, at which 50 to 75%, 75 to 100%, and at or near 100% of crop yield potential can be expected, respectively, without addition of nutrients (Savoy, 2013). Using this type of rating scale, there are dramatic changes in rates of fertilizer recom- mendations if the soil test values are near the boundaries of adjacent ratings. Such boundary effect might be circumvented by the continuous function curves to cal- culate the required fertilizer with soil test results (Savoy, 2013). Potassium fertilizer recommendations based on preplant soil test are critical in ensuring crop yield and quality and minimizing fertilizer costs (Ozores-Hampton et al., 2012b). Soil K is typically divided into four forms with increasing plant availability: mineral, non- exchangeable, exchangeable, and soluble K (Havlin et al., 2014). Plants absorb K mostly from the soil solution, which is buffered by exchangeable K and ultimately by nonexchangeable forms (Wang et al., 2010). Adopting a soil test method to estimate K fertilizer requirements is an impor- tant component of nutrient management for vegetables. Currently, there are two Qiang Zhu* Monica Ozores-Hampton Southwest Florida Research and Education Center Univ. of Florida 2685 State Road 29 N Immokalee, FL 34142 Yuncong C. Li Tropical Research and Education Center Univ. of Florida 18905 SW 280th Street Homestead, FL 33031 Rao S. Mylavarapu Soil and Water Sciences Dep. Univ. of Florida 2181 McCarty Hall Gainesville, FL 32611 Core Ideas • Water was ineffective to estimate K availability in calcareous soils in Florida. • Both Mehlich-3 and AB-DTPA can be used to calibrate K rates in calcareous soils. • High K level was above 213 and 183 mg kg –1 with Mehlich-3 and AB- DTPA, respectively. Published online January 4, 2018