Influence of chemical fertilizer on yield and fruit quality of Late Valencia sweet orange in Ghana

Studies were conducted to investigate the influence of inorganic fertilizers on the yield and fruit quality of Citrus sinensis [L] Osbeck cv. Late Valencia at the Agricultural Research Station, Okumaning near Kade (ARS-Kade). The trees which were 30 years old, had been budded onto either Rough lemon, C. jambheri Lush (RL), Cleopatra mandarin, C. reticulata Blanco (CM), or Rangpur lime, C. limonia (RGL), rootstocks. The fertilizer treatments common to all the rootstocks were as follows: T1 (control, no fertilizer); T2 (96.4, 40.2, 201 and 70 kg N, P2O5, K2O and S/ha, respectively); T3 (204, 80.4, 402 and 135.6 kg N, P2O5, K2O and S/ha, respectively). In treatments T2 and T3, the fertilizers applied were 15-15-15, sulphate of ammonia, and muriate of potash. For RL, an additional treatment (T4) was applied as in T3 but without S. The effects of fertilizer application rates on the pH, percent juice content, juice volume, total soluble solids (TSS), titratable acidity (TA), and TSS/TA ratio were investigated. Fertilizer application increased yield significantly, irrespective of rootstock. The increases ranged between 89 and 122 per cent for RL; 23 and 72 per cent for CM, and 26 and 67 per cent for RGL compared to the no fertilizer application. For RL, ammonium sulphate as N source tended to increase yield more than urea. Increasing fertilizer rate resulted in significant yield increases, irrespective of rootstock. While the pH and TSS/TA ratio of the juice was increased, titratable acidity was decreased by fertilizer application. Except for CM, the highest juice volume was observed from T2. In addition, the TSS was highest at T2 while ammonium sulphate application increased the TSS content of the fruit more than urea, suggesting the importance of sulphur in improving fruit quality. Fertilizer application reduced the maturity period of the fruit as measured by the TSS/TA ratio. A minimum TSS/TA ratio of 10 is recommended for acceptance as maturity index for sweet oranges in Ghana. Ghana Jnl agric. Sci. Vol.31(1) 1998: 27-3