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Inorganic Phosphorus Fractions and Their Relationship to Residual Value of Large Applications of Phosphorus on High Phosphorus Fixing Soils¹

ABSTRACT

Large applications of P to a high P fixing soil were rapidly converted into Al and Fe-P. Over an 8 year period a decrease in Al-P with an increase in Fe-P occurred. Reductant soluble Fe-P occurred 3 and 5 years following P applications at 1,371 kg/ha (1224 lb/acre) and 685 kg/ha (612 lb/acre).

Soil test P was highly correlated with Al-P. Regression analysis indicates that increasing rates of P application, or increasing Al-P level, up to the reported P adsorption maximum, resulted in greater efficiency in increasing the soil test P. At a 1.371 kg/ha rate of application, which was above the reported 719 kg/ha (642 lb/acre) adsorption maximum, the efficiency was greater than at 343 kg/ha (306 lb/acre) rate of application but slightly less than at 685 kg/ha application rate.

A method of investigating long-term field equilibrium conditions in a relatively short period, by boiling, was investigated and resulted in equilibrium conditions requiring several years under field conditions. It is suggested that this method when coupled with other data may be used as a means of estimating P applications which would result in sufficient available P for maximum yields over an extended period.

¹ Contribution from the Dep. of Soil Sci., North Carolina Agr. Exp. Sta., Raleigh. Published with the approval of the Director of Research as Paper no. 2376 of the Journal Series.

² Assistant Professor of Soil Sci. Mountain Horticultural Crops Res. Sta., Fletcher, N. C. and Professor of Soils and Plant Nutrition, Univ. of California, Riverside, respectively.

³ Acknowledgment is made to Mr. W. H. Rankin, Associate Professor Emeritus, who applied the initial treatments, and to Drs. E. J. Kamprath and F. R. Cox, Professor and Associate Professor, respectively, for assistance in obtaining soil samples.

Received for publication May 24, 1967. Accepted for publication October 2, 1967.