Effect of rock phosphate with and without different levels of humic acid on straw production of wheat. Where RP, rock phosphate; RP+HA-5, rock phosphate with humic acid @ 5 kg ha-1; RP+HA-10, rock phosphate with humic acid @ 10 Kg ha-1 Bars having different letter(s) are significantly different at P ≤ 0.05.