The effect of different N and K sources on tuber nutrient uptake, total and graded yield of potatoes (Solanum tuberosum L.) for processing

A field experiment was conducted for 3 consecutive years (2002-2004) on loamy sand on an organically managed farm near Osnabruck, Germany. Four replicates of four fertilizer treatments and a control were established annually in a split-plot design for two maincrop potato cultivars (Solanum tuberosum L. cv. Agria and cv. Marlen). The application of fertilizers followed a soil test of available potassium at the onset of vegetation. Fertilizer treatments were deep litter cattle manure, potassium sulphate (40% K), potassium sulphate + horn grits (14% N) and horn grits, all supplying equivalent quantities of K (175-215 kg ha-1) and/or N (100-145 kg ha-1), respectively, and a control with no fertilization. Deep litter cattle manure was analysed for total N, K and other elements and -- just as the other fertilizers -- applied in spring just before ploughing. Soil content of NO3-N was determined from samples taken at defined growth stages of the potato crop. Furthermore, total and graded tuber yields, tuber concentration as well as uptake and concentration of N and K were assessed. Results of soil nitrate-N content confirmed that organic cropping systems are dominantly nitrogen limited and this is likely to affect crop utilization of K. Available K content of soil and K in tuber dry matter could be increased through application of either cattle manure or potassium sulphate. Total yields depended strongly on the individual year and varied between 27.9 and 35.3 t ha-1 (cv. Agria). Results indicate a strong influence of fertilizer treatment on total tuber yields and those relevant for processing into crisps (40-65 mm) or French fries (proportion of tubers >50 mm in yield >35 mm). Highest yields were obtained after application of the combined mineral K (potassium sulphate) and organic N (horn grits) source. The response of tuber yield to cattle manure was not consistent over the growing seasons, which confirms that cattle manure is generally a very insecure source of plant available N in the year of application. Possibly, the positive yield response in 2004 was due to K rather than N, since only tuber K concentration and uptake were significantly affected. Overall, the results suggest that in organic potato cropping the correlation between available K -- as determined with the common soil test procedures -- and yield response may be low. Response of tuber yields graded for crisps and French fries production confirmed that cultivars have to be chosen carefully to secure adequate tuber yield of the required size grades.