Investigation and synthesis of sweet sorghum crop responses to nitrogen and potassium fertilization

Optimizing nutrient management practices in cropping systems is essential to improve economic and environmentally sustainability. Sweet sorghum [Sorghum bicolor (L.) Moench] is a crop that is capable of high sugar, grain, and lignocellulosic biomass yields, but a synthesis of published nutrient response curves reveals gaps in our knowledge. We therefore evaluated responses of sweet sorghum on a marginal North Florida sandy soil to ranges in nitrogen and potassium application rates and combined the nitrogen data with literature datasets to provide a broadly useful assessment of the relationship between nitrogen uptake and yield. There were no significant effects of the application of potassium observed. In response to nitrogen, leaf SPAD and photosynthesis were less with 20 kg N ha−1 than with more nitrogen (60, 100, and 140 kg ha−1), reflecting end-of-season crop responses in total biomass, estimated sugar, and grain yield. A linear plateau model indicated that application of 70 kg N ha−1 was sufficient to optimize total biomass yield at 20.8 Mg ha−1, although aboveground nitrogen uptake reached 120 kg N ha−1. These results are aligned with other reports on this topic showing similarly small optimal nitrogen fertilization rates and a strong capacity to extract soil nitrogen. In a combined analysis of this and three other published studies, crop nitrogen uptake reached approximately 127 kg ha−1 at the plateau in biomass production (19.7 Mg ha−1) with a slope of 0.13 Mg kg−1. Sustainable management plans for sweet sorghum should include strategies to replenish the often considerable soil nitrogen removed by the crop.