EFFECTS ON N, P AND K AND THEIR INTERACTIONS ON YIELD, TUBER BLIGHT AND QUALITY OF POTATOES

Maximum and optimum fertiliser rates based on main effects data from 33 and 43 factorial experiments were calculated and compared with the values obtained by taking interactions into account. High‐nitrogen dressings depressed yield in several instances. Increased phosphorus applications overcame this on some sites, which indicated the probability of it being a physiological effect rather than salt damage. The size distribution of tubers into large, medium and small was influenced most by potassium, which increased the proportion of large tubers while decreasing the other grades. The trends from nitrogen were similar but of lesser magnitude. Phosphorus had no consistent effect. Fertilisers had no influence on organoleptic tests as reflected by taste panel estimates of colour, flavour and texture of boiled potatoes. The level of reducing sugars was influenced most by potassium, which reduced their content. N and K decreased dry matter content, while P had the reverse effect. Phosphorus had a pronounced influence on the distribution of blighted tubers on sites with a higher than normal incidence of blight. On the average of the sites where the effect was significant, its application reduced blighted tubers from 20 to 12%. Although the trend from nitrogen was highly significant, the actual increase in the proportion of diseased tubers was low. The interaction effects found in a combined variance analysis of all sites were categorised. The most prevalent interaction affecting tuber yield was a decreased response to any one element in the absence of the other. In the case of percentage dry matter there was a consistently adverse NK interaction, while on a limited number of sites nitrogen or potassium depressed dry matter content most in the absence of phosphorus. Copyright © 1969 John Wiley & Sons, Ltd