NITROGEN AND POTASSIUM FERTILIZATION OF POTATOES - SUGARS AND STARCH

Considerable evidence exists that both N and K influence potato (Solan um tuberosum L.) yields and quality. The impact of nutrients on tuber quality parameters under field conditions should be identified so opti mum management practices can be utilized. We evaluated the N and K fer tilization by K-sources effects on concentrations of nutrients, reduci ng sugars, sucrose and starch in the stem and apical tuber ends from t wo irrigated field experiments with the Russet Burbank cultivar. Nitro gen rates of 0, 112, 224 or 336 kg ha-1 were combined with selected K rates of 0, 112, 224 or 448 kg ha-1 as either KCl or K2SO4 arranged as an incomplete factorial. A multiple linear regression model was fit t o the data and used to predict the response surface for a complete fac torial for each K-source. Nitrogen applications increased or decreased reducing sugars in the apical and stem ends, respectively. Potassium decreased reducing sugars in both tuber ends. Sucrose was higher in th e apical end than in the stem end but was not appreciably affected by fertilizer treatment. Nitrogen and potassium applications reduced dry matter and starch concentrations in both tuber ends. Potassium had a s maller effect on the apical end when starch was expressed on a dry wei ght basis, indicating that increased water content was a factor in the K effect. Both N and K concentrations in the tuber ends were negative ly related to starch concentrations, but the relationship was differen t for the apical and stem ends. Tuber Cl concentration indirectly affe cted starch concentrations in the stem end when KCL was applied. The K fertilization effect on specific gravity depended upon the K concentr ation in the harvested tuber and was independent of K fertilizer sourc e. These data illustrate the effects of preplant N and K fertilization rates on final tuber quality parameters. Additional studies are neede d to further define the effects of nutrient concentrations at differen t plant growth stages.