The objective of this research was to determine the optimum nitrogen fertilizer rate for producing sweet sorghum (a promising biofuel crop) juice, sugar, and bagasse on silt loam, sandy loam, and clay soils in Missouri. Seven nitrogen fertilization rates were applied, ranging from 0 to 134 kg N ha À1 . Regardless of the soil and year, the juice content of sweet sorghum stalk averaged 68.8% by weight. The juice yield ranged from 15.2 to 71.1 m 3 ha À1 . Soil and N rate significantly impacted the juice yield (P < 0.0001). The pH and the density of the juice were not affected by the soil or N. The sugar content (Brix) of the juice varied between 10.7% and 18.9%. N fertilization improved the sugar content of the juice. A negative correlation existed between the sugar concentration and the juice yield. In general, the lowest sugar content was found in the clay soil and the impact of the N fertilization on juice sugar content was most pronounced in that soil. The juice sugar yield ranged between 2 and 9.9 Mg ha À1, with significant differences found between years, N rates, and soils. N fertilization always increased the sugar yield in the clay soil, whereas in loam soil, a significant sugar response was recorded when the sweet sorghum was planted after corn. The average juice water content was 84% by weight. The dry bagasse yield fluctuated between 3.2 and 13.8 Mg ha À1 with significant difference found with N rate, soil, and year. When sweet sorghum was grown after soybean or cotton, its N requirement was less than after a corn crop was grown the previous year. In general, a minimum of 67 kg N ha À1 was required to optimize juice, sugar, and bagasse yield in sweet sorghum.
Nitrogen fertilization and nutrient removal are some of the greatest challenges associated with the production of bioethanol from corn grain in the United States. The project was conducted in Missouri from 2008 to 2009 in a silt loam soil to determine the effect of N fertilization on the nutrient removal by corn grain. Corn grain removed 31 to 201 kg N/ha, 10 to 102 kg K/ha, 3 to 20 kg P/ha, 0.6 to 11 kg Ca/ha, 0.7 to 2.7 kg Fe/ha, 0.045 kg Mn/ha, and 0.003 kg Cu/ha, depending on the N rate. Our results suggest that as farmers add N to their soil to improve the grain or starch yield, they will also be increasing the nutrient removal, therefore affecting the fertility of their fields if the nutrient removal is not offset.
Sweet sorghum is a C4 grass which is traditionally cultivated for making syrup from the sugars in the stalks. Sweet and grain sorghum are in the same species, Sorghum bicolor (L.) Moench. In optimum conditions, sweet sorghum can grow 4.5 meters tall and produce 45 to 110 Mg of fresh weight biomass per hectare with less N and water than maize. Ethanol can be produced from sweet sorghum stalks by extracting the juice and fermenting the sugars with yeast. Bagasse remaining after extraction can be fed to livestock or converted to useable energy by burning to produce steam for generating electricity, anaerobic digestion to make methane, or reacting with oxygen at high temperature to produce synthetic gas. Sweet sorghum can be grown in most climates. Some cultivars will grow as far north and south as the 45° latitude. It is normally grown as an annual crop. But, in warm climates, a single seed planting can be managed for two or three years by leaving lower stalks and roots at harvest to produce new tillers for the next growing cycle. Rotating sweet sorghum in alternate years with a non-grass crop is an effective management tool for reducing insect, weed, and disease pests. Nitrogen fertilizer can often be reduced when sweet sorghum is planted after a legume crop such as soybean. Most of the sugar for syrup and ethanol from sweet sorghum is currently produced with open pollinating cultivars. Recently, hybrid sweet sorghums with increased sugar content and biomass yield have been developed in India, China, and the United States.
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