Effect of Tillage and Fertilization on Agronomics and Nutrient Uptake of Sweet Sorghum and Soil Test Extractable P and K after Four Years of a Monocrop Production System

Sweet sorghum [Sorghum bicolor (L.) Moench] (SS) has been grown in the southern United States to produce syrup for many years. There is an interest in SS as a biofuel feedstock due to its high sugar content and high total biomass. Currently, little is known about the nutrient demand for SS or how it responds to tillage and fertilization. The objectives of this study were to: 1) evaluate the effects of tillage and phosphorus (P) and potassium (K) fertilization on SS agronomics, 2) evaluate nitrogen (N), P, and K uptake and nutrient partitioning in SS, 3) determine P and K maintenance fertilization rates for sugar and cellulosic ethanol production, and 4) evaluate the effects of tillage and maintenance fertilization on soil test extractable P and K at three depths after four years of a monocrop system. A split-plot, randomized complete block design with four replications was used to evaluate the effects of two tillage treatments (no-till system (NT) and conventional tillage (CT)) and two fertilization treatments (with “maintenance” (MF) and without “maintenance” (NMF)) on SS production from 2012 to 2015. The MF applied 45 and 67 kg ha-1 of P2O5 and K2O, respectively. The CT decreased days to 50% heading and increased the initial plant population. The NT increased the number of harvestable stalks which were derived from tillers. The MF increased plant height, stalk diameter, total biomass, and stalk biomass. The NT increased the P removal rate in green leaves. The MF application increased K concentration of stalk, green leaves, and the total K removal rate of the whole plant. The MF increased the P removal rate in the stalk. A 75 Mg ha-1 of SS would remove 40 and 145 kg ha-1 of P2O5 and K2O, respectively, when only the stalk is harvested. When the whole plant is removed, approximately 78 and 193 kg ha-1 of P2O5 and K2O would be removed, respectively. The MF application increased soil test exactable P at the 15 to 30 cm soil depth. Soil test extractable K was not affected by tillage and fertilization across the different soil depths