Does potassium fertilizer form, source, and time of application influence potato yield and quality in the Columbia Basin?

Potassium (K) fertilizer form (liquid or granular), source (chloride or sulfate), and time of application (preplant or in-season) were evaluated for effects on potato tuber yield and quality in Washington's Columbia Basin. Potato yield and quality did not respond positively to potassium fertilizer although pre-plant soil-test K values were in the range where adding K fertilizer is recommended in the current (1999) Washington guidelines. In one of three years, total tuber yield was reduced with in-season fertilization with potassium chloride and with 75% in-season liquid K application. While the results were inconsistent over growing seasons, a few of the in-season, liquid K fertilizer treatments resulted in a significant reduction in percent marketable yield. Thus, the study results uphold the current practice of using granular K fertilizers pre-plant. In addition, the results of this study strongly indicate that K source does not affect tuber specific gravity. The limited response of yield and quality factors to K fertilizer on sites that Washington's current soil-test and K fertilizer guidelines indicate should be K responsive suggest that further research is needed to fine tune the guidelines.