PHOSPHORUS REQUIREMENT OF CORN AND SUNFLOWER GROWN ON CALCAREOUS SOILS OF PAKISTAN

Determining phosphorus (P) fertilizer requirement through sorption isotherms is considered more accurate than conventional soil testing Phosphorus sorption isotherms were constructed by using the Missa (Typic Ustochrept), Gujranwala (Udic Haplustalf), Abbotabad (Aquic Entrochrept), and Rasulpur (Ustochreptic Camborthid) soil series which varied in their calcium carbonate (CaCO3) and clay contents. Quantities of fertilizer-P (mg P/kg soil) required to adjust the standard soil solution P concentration (0.2 mg P/L) were: Missa, 83; Gujranwala, 51; Abbotabad, 50; and Rasulpur, 8. Langmuir model conformed better with the adsorption data than Freundlich model and indicated that the order of maximum adsorption capacity (b) was: Missa > Abbotabad = Gujranwala > Rasulpur. The P sorption curves were used to develop the soil solution P concentration in the Gujranwala and Missa soils varying from native to 0.5 mg P/L. Soil solution P requirement of com and sunflower for near-maximum (95%) biomass production was approximately the same, 0.32 mg P/L. Internal P requirement in whole shoots (less-than-or-equal-to 30 cm tall) were 0.29% for com and 0.39% for sunflower.