Sulfate adsorption and activity in solution of selected soils in relation to the growth, yield and nutrient uptake of lowland rice [Philippines]

Results of the adsorption isotherm studies, using the linear Langmuir equation indicate extensive SO4 = adsorption in an acidic soil (Lucban). Soil properties associated with this soil relate well with its adsorbing capacity for sulfate. In the determination of sulfate adsorption isotherm, higher maximum concentration used for equilibration, longer equilibration time and lower soil to solution ratio resulted in higher adsorption maxima suggesting higher bonding energy or a stronger pull of the soil surface for the ion towards it. In a laboratory study on the competitive adsorption of phosphate and sulfate, results indicate phosphate adsorption to be possible in all the soils. Only Lucban soils have both reaction occurring at the same time but more phosphate ions were adsorbed due to the higher bonding energy imposed by Lucban soil to phosphate ion. Slight response to S of IR64 plant was observed in Bongabon soil. In Lucban, the application of P alone has provided positive effect on some agronomic parameters and S uptake suggesting perhaps that P application could make available the native S in the soil. A greenhouse study involving S-deficient soils was done to evaluate the effect of S application and relate the effect of ion-pairing of SO4 = in solution on plant growth. Reduction in activity of sulfate was evident after correction for ion pair formation. Only Ca and Mg ions showed extensive ion-pairing with sulfate. Of the four soils studied, Cabagan had the least Ca and Mg content. The three other soils namely Bacnotan, Sta. Maria and San Juan have relatively high Ca and Mg concentration in the soil solution. The non-responsiveness of the rice plant to S additions in Bacnotan and Sta. Maria soils may be attributed to the marked reduction in the activity of the sulfate in the equilibrium solution of these soils. The relatively lower activity reduction in Cabagan and San Juan soils is apparently related to the observed significant responses of the plant to S addition.