An agronomic and physiological re-evaluation of the potassium and sodium requirements and fertilizer recommendations for sugar beet

Abstract

SUMMARYThe potassium (K) and sodium (Na) requirements of sugar beet were re-examined in a 6-year series of experiments between 2000 and 2005 using reference plots with a wide range of long-established differences in exchangeable topsoil K (Kex). Two groups of plots with a topsoil concentration Kex range of 40–550 mg/kg were used, each situated within an individual field, one on a silty clay loam at Rothamsted and the other on a contrasting sandy loam at Woburn. The interactions between topsoil Kex and applied N, K and Na fertilizers were studied at Rothamsted. Under these well-defined conditions, maximum yields of 55–71 t/ha of adjusted clean beet were achieved with a topsoil Kex concentration of 120–150 mg/kg, i.e. at Soil K Index 2–, with a small difference between the two soils being accounted for by differences in exchangeable soil Na and subsoil Kex. There were no yield responses to freshly applied fertilizer K, even on low K plots where responses might be expected. It is concluded that the existing recommendations for K fertilizer use on UK sugar beet do not need to be adjusted to allow for the higher yields of modern crops.There were no yield responses to NaCl fertilizer at any level of topsoil Kex at Rothamsted (where the soil contained 15–20 mg Na/kg), but yields were increased on low Kex plots at Woburn whose sandy loam contained only 5–10 mg Na/kg. The uptake of Na from the applied NaCl fertilizer was strongly influenced by the exchangeable K and Na status of the soil. On the low Na soil at Woburn, almost all of the applied Na was taken up by sugar beet grown on plots with low concentrations of topsoil Kex and half of it on plots with adequate concentrations of topsoil Kex compared with two-thirds and one-fifth, respectively, on the higher Na-content soil at Rothamsted.Plants partitioned 0·75 of their K and 0·95 of their Na to the shoot and the balance to the storage root. This pattern of distribution was consistent across sites, seasons and soil K supply. The physiological interactions between K and Na were studied by examining their millimolar concentrations in the tissue-water (mmol/kg) of the shoots and storage roots. The tissue-water concentrations of K in the shoot increased asymptotically with the concentration of Kex in the topsoil, and the increase in K concentration was accompanied by a corresponding decrease in the tissue-water concentration of Na. Maximum concentrations of K in shoot tissue-water (and minimum concentrations of Na) were achieved when the topsoil contained a minimum of 200 mg Kex/kg. The optimal physiological tissue-water concentration of Na in shoots was estimated to be c. 90–100 mmol/kg; maintenance of this level required a minimum of 25 mg/kg of exchangeable Na in the topsoil. When not limited by soil Kex, plants maintained a total tissue-water concentration of c. 300–350 mmol/kg of K+Na within the shoot. This was achieved with 80 mmol of Na and 230 mmol of K/kg of tissue water on the high Na-content soil at Rothamsted, and with 40 mmol of Na and 275 mmol of K/kg tissue water on the low-Na soil at Woburn.Significant correlations were established between measurements of beet K made in the factory tarehouse and those made using standard laboratory chemical analyses and between factory estimates of the concentrations of K in the tissue-water of delivered beet and the topsoil Kex. The uses of these relationships to estimate the off-takes of K in the harvested beet and provide feedback to growers on the K status of their soils, and the implications of the study for the use of K and Na fertilizers on UK sugar beet are discussed.