NITROGEN AND POTASSIUM FERTILIZATION OF POTATOES - YIELD AND SPECIFIC-GRAVITY

Potassium and N fertilization is often required for maximum potato (So lanum tuberosum L.) production. Nitrogen, K, and K-sources (KCl, KSO4) are known to affect yield and quality of potatoes but N and K interac tions as affected by K-source have not been defined. This study evalua ted the NK and K-source interactions on Russet Burbank tuber yields a nd specific gravity (SG) in two irrigated field experiments. Nitrogen rates of 0, 112, 224 or 336 kg ha-1 were combined with selected K rate s of 0, 112, 224 or 448 kg ha-1 as either KCl or K2SO4 in an incomplet e factorial. A multiple linear regression model was fit to the data an d used to predict yield and SG for a complete factorial for each K-sou rce. Both N and K applications increased yields independent of K-sourc e. Nitrogen decreased yields at the 336 kg ha-1 rate. Potassium increa sed yields up to 448 kg K ha-1. Both K-sources decreased SG a similar amount with N application; without N, KCl decreased SG but K2SO4 did n ot. Nitrogen also decreased SG. Petiole NO3-N and K concentrations wer e positively related to yields and negatively to specific gravities. T he petiole K concentration 100 days after planting should be above 4.5 for highest tuber yields. The NK*K-source interaction was important for yields at low available N and for SG at adequate N availabilities. This study showed that N or K fertilizers can be applied according to their respective soil test concentration and the crop's requirement, generally without consideration of K-source.