Effect of potassium fertilizer on the yield, quality and potassium offtakeof sugar beet crops grown on soils of different potassium status

The effect of different rates of potassium (K) fertilizer on the yield and quality of sugar beet was studied in a series of 26 trials on soils of diff erent type and K index between 1992 and 1997. There were few yield response s even though the majority of trials were on soils of low K index, and larg e quantities of fertilizer were applied (0-600 kg K/ha). Potassium offtakes (kg/ha) in the harvested beet increased asymptotically, not linearly, with yield and were much larger for a given yield on high K index soils than on low index soils. Commercially acceptable concentrations of beet K for proc essing are in the range 700 to 1000 mg K/100 g sugar. Concentrations in exc ess of this decrease the amount of sugar crystallized from the extracted ju ice. They were not greatly affected by large applications of fertilizer K b ut were strongly influenced by long-established differences in soil exchang eable K (K-ex) due to soil type, previous cropping or manuring history. The asymptotic nature of the K offtake:yield relationship was confirmed by factory tarehouse measurements relating to the national sugar beet crop del ivered during the 1993-97 UK processing campaigns. Potassium offtakes gener ally increased linearly with yield up to 60-70 adjusted t of clean beet/ha, but increased little beyond that. The amount of K removed by a 60-70 t/ha crop of beet varied from 70 kg K/ha on low K index sandy loams to 120 kg K/ ha on clay soils of K index 3 and above. Further increases in yield decreas ed the amount of K in fresh beet from 1.7 to 1.4 kg K/t on low K index soil s, and from 3.6 to 2.5 kg K/t on high K index soils. An analysis of data from individual fields of commercially grown sugar beet showed that much of the site and season variation in the K content of beet was due to differences in K uptake driven by K-ex, and to differential eff ects of nitrogen (N) supply on K uptake and sugar yield. Regressions on K-e x and total crop N (kg/ha) accounted for c. 30 and 50% of the variance in b eet K content, respectively, and the two together for over 60%. Total N upt ake by the crops ranged from 100 to 550 kg N/ha. The total K content of the crop and the amounts of K in the beet (kg/ha) both increased linearly with crop N over the whole of this range, whereas sugar yield increased asympto tically with total uptakes of N up to 250-300 kg N/ha. Consequently, low yi elding crops grown on soils in which N and K were freely available produced beet of poor K quality. However, the asymptotic relationship between beet K (kg/ha) and yield implies that, in many situations, the processing qualit y of the beet could be improved by increasing yield through better agronomy .