Timing of potassium chloride application effect on soil and potato uptake of chloride

Potassium chloride (KCl) is often used to supply K for potato (Solanum tuberosum L.) production; it is the least costly K fertilizer. However, crop uptake of Cl can reduce diagnostic petiole NO3-N and reduce tuber specific gravity. This trial evaluated the effect of K fertilizer source (KCl vs. sulfate of potash [SOP] and sulfate of potash magnesia [KMag]) and fertilizer application timing on Cl, S, N, and K in soil, petioles, and whole plant biomass. Fertilizers were applied at 224 kg K2O ha(-1) at three application timings: 210 d before planting (fall preplant), 14 d before planting (spring preplant), or 35 d after planting (Layby) to a sandy loam soil in Hermiston, OR. Nitrogen fertilizer was applied by overhead sprinklers during the growing season. Potassium source and timing had minimal effect on crop S, N, and K uptake. Crop Cl response reflected a fertilizer source x timing interaction. Fall-applied KCl did not increase plant Cl above that observed for KMag and SOP treatments. Spring or summer-applied KCl increased plant Cl concentrations. The most likely explanation for this response pattern was leaching of Cl below rooting depth with fall fertilizer application. Findings of importance for potato production practices in our region included: (a) fall-applied Cl was not taken up by the crop, and so did not represent a risk to potato tuber quality; (b) elevated concentrations of petiole Cl from preplant or in-season KCl application did not affect petiole NO3-N.