Abstract
Sorghum is an important fodder for animals in Pakistan. Phosphorus is a quality nutrient for fodders. The use of
P for sorghum is negligible. Thus a field study was conducted to evaluate the effect of model based P fertilization
to improve yield and quality of sorghum fodder on an Ustochrept soil (clay loam, calcareous). Adsorption
isotherm was constructed by equilibrating 2.5 g soil with 25 ml of 0.01 M CaCl2 solution containing 0, 20, 40, 60,
80, 100, 200,300,400 and 500 µg P mL-1 as KH2PO4 and shaking for 24 hour at 20 0
C. P fertilizer doses were
computed by using empirically derived Freundlich equation to adjust soil solution P levels of 0.05, 0.10, 0.15,
0.20, 0.25, 0.30, 0.35, 0.40, and 0.50 mg P L-1. It was observed that different P fertilizer doses were required to
adjust different soil solution P levels. Adsorption of P increased with P application rate but marginal adsorption
decreased significantly. Yield and quality of sorghum fodder were improved with P application. Yield increased
upto T8 where P was applied @ 83 kg ha-1 but quality traits (P concentration, dry matter, crude protein, crude
fiber and ash contents (%) improved with the highest rate of P use i.e. 98 kg ha-1. P application showed
nonsignificant affect on NDF and ADF contents (%). External and internal P requirements of sorghum fodder to
obtain 95 % relative yield were 0.31 mg L-1 and 0.26 %, respectively
