This experiment was carried out in the experimental field , Horticulture Department , College of Agriculture , University of Baghdad , Abu-Ghraib , during fall 2006 and spring 2007 seasons using Desiree cultivar potato seeds class (A) in fall season and class (Elite) in spring season. In this experiment the effect of potassium spray and companion ion on tuber content of N , P and K and quality characteristics of potato tubers were tested. Three potassium fertilizers (K2SO4 , KNO3 and KCl) at three concentration (0 (control) , recommended 5 g K2SO4 / l, 5.80 g/l KNO3 and 4.28 g/l KCl, 1.5 times of the recommended concentration were used where 5 g K2SO4 / l was used as the recommended concentration. Solution of above mentioned salts was foliar applied twice , first 45 days (vegetative growth stage) and second 65 days (tuber bulking stage) after planting. Randomized Complete Block Design was adopted with three replicates. Least significant differences (L.S.D) at 5% probability was used to compare the means.
In fall 2006, potassium nitrate treatment at recommended concentration resulted in a highest content of N and K in tubers (1.24% , 2.25% respectively) compared to the control (0.86% , 1.65% respectively) . Potassium sulfate treatment at recommended concentration increased P% in tubers (0.46%) as compared with spraying potassium chloride 6.42 g/L which reached lowest P percentage (0.39%) . In spring 2007, S1 treatment significantly increased N content in tubers (1.24%) as compared with the control (0.91%) while spraying potassium chloride at recommended concentration resulted in highest K% in tubers (3.14%) as compared with the control (1.89%) . In fall 2006 , recommended 5 g K2SO4 treatment had highest percentage of dry weight , starch and specific density (16.51% , 10.75% and 1.062 g / cm3 respectively) compared to the control treatment (14.38% , 8.82% and 1.052 g / cm3 respectively) . In spring 2007 , recommended 5.8 g KNO3 / L treatment resulted in highest percentage of dry weight , starch and specific density (21.05% , 14.76% and 1.084 g / cm3 respectively) as compared with the control treatment (18.75% , 12.72% and 1.073 g/cm3 respectively).