Effects of NPK compound fertilizer on stalk, juice and sugar of sweet sorghum [Sorghum bicolor (L.) Moench]

Sweet sorghum has been widely recognised as a potential alternative source of bio-fuel because of its high fermentable sugar content in the stalks. In Botswana, little has been done to determine its NPK compound fertilizer requirement which may increase the stalk juice and sugar concentration. A field study was conducted in Francistown, Botswana during the 2010-2011 planting season to assess the effects of different levels of NPK compound fertilizer 2:3:2 (22). Fertilizer treatments used were: Control, 400, 800, 1200, 1600 and 2000 kg ha(-1). Compared with the control, the applied fertilizer levels increased stripped stalk weight by 56-97% and except for 400 kg ha(-1), the fertilizer levels also increased the juice volume by 54-94%. With regard to sugar concentration, fertilizer levels, except 400 kg ha(-1), reduced it by 10-17%. There were moderate linear relationships between fertilizer treatments and stripped stalk weight (R-2=0.664), juice volume (R-2=0.715) and sugar concentration (R-2=0.723). In conclusion, results indicate that 800 kg ha(-1) of fertilizer could be recommended as a starter fertilizer since it was equivalent to results obtained at higher rates (1200-2000 kg ha(-1)) but significantly higher than the control.