Approximating Phosphorus Leaching from Agricultural Organic Soils by Soil Testing.

Zheng, Z.M., Zhang, T.Q., Kessel, C., Tan, C.S., O'Halloran, I.P., Wang, Y.T., Speranzini, D., and Van Eerd, L.L. (2015). "Approximating Phosphorus Leaching from Agricultural Organic Soils by Soil Testing.", Journal of Environmental Quality, 44(6), pp. 1871-1882. doi : 10.2134/jeq2015.05.0211  Access to full text


Phosphorus applied to soils in excess of crop requirement could create situations favorable to P enrichment in subsurface flow that contributes to eutrophication of surface water. This pathway of P loss can be more severe in muck (i.e., organic) soils where agricultural production is intensive. This study evaluated the suitability of various environmental and agronomic soil P tests initially designed for mineral soils to predict dissolved reactive P (DRP) in subsurface flow from organic soils. Intact soil columns were collected from 44 muck soils in Ontario to provide a wide range of soil test P levels. A lysimeter leaching study was conducted by evenly adding water in an amount equivalent to 5 mm of rainfall. The leachate DRP concentration was linearly related to soil water-extractable P and CaCl2–extractable P with r2 values of 0.90 and 0.93, respectively, and to Bray-1 P and FeO-impregnated filter paper extractable P in a split-line model with a change point. Mehlich-3 P and Olsen P, a method recommended for agronomic P calibration in Ontario, were not related to leachate DRP concentration. All P sorption index (PSI) based degree of P saturation (DPS) values were closely related to leachate DRP in split-line models, with the DPS indices expressed as Bray-1 P/PSI and FeO-P/PSI having the highest correlation with leachate DRP concentration. Because it is desirable from practical and economic standpoints that the environmental risk assessment shares the same soil test with agronomic P calibration, the two PSI-based DPS indices as presented can be considered as environmental risk indicators of DRP subsurface loss from organic soils. Core Ideas: • Leaching loss of P in organic soils can be severe, but soil P testing is not available. • Leachate DRP was not related to Mehlich-3 P or Olsen-P, default soil tests. • Leachate DRP was related to P sorption index based DPS in split-line models. • DPS indices Bray1-P/PSI and FeO-P/PSI had the highest correlation with leachate DRP. • FeO-DPS can be considered a risk indicator of DRP subsurface loss from organic soils.

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