Abstract
The present study was conducted to measure the potassium (K) adsorption capacity of (Bopra Khird, Gujranwala) soil of rice tract of Punjab, Pakistan to compute K fertilizer quantities for field application, internal and external K requirements of the rice. The soil selection was based on clay percentage, calcium carbonate content. The data showed that soil was free from salinity and sodicity, deficient in organic matter and adequate in available potassium. Sorption isotherm was constructed in the laboratory by equilibrating and shaking 2.5 g soil with 25 ml of 10 m M CaCl2 containing 0, 25, 50, 75, 100, 125, 150,175, 200 and 250 mg K L-1 as KH(2)PO(4)for 24 h. Sorption data was fitted into modified Freundlich model and K adsorption capacity was determined from the equation. Theoretical doses were computed from modified Freundlich model for pot application. The studies were laid out in Completely Randomized Design (CRD) with three replications. Different theoretical K doses were applied in the pots to develop soil solution K level of 3, 6, 9, 12, 15, 18, 21 and 24 mg L-1 alongwith a control of native K soil solution. Recommended doses of N and P were applied to the rice crop alongwith recommended agronomic practices. Plant sampling was done at the harvest of rice crop. Maximum rice paddy yield was 9.20 g/pot. K uptake by rice paddy and straw of 54.79 g/kg and 4360.2 g/kg was measured. External K requirement (for 95% relative yield) for rice was 10.5 mg K L-1 at harvest stage.