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Assessing Phosphorus Fertilizer Needs Based on Intensity-Capacity Relationships¹

ABSTRACT

Phosphorus in a pH 4.8 NH₄OAc soil extract, through crop response correlation work, may indicate the degree to which a level of P in a given soil is optimum. But, being an intensity measurement, this NH₄OAc-P value does not indicate the amount of P fertilizer needed to bring a soil, which has a particular P sorption capacity, to its optimum level.

The relationship between added P and the square root of the NH₄OAc-P, measured after incubation of a given acid or limed soil, was a straight line with a characteristic slope for that soil. The value of this slope, termed the Phosphorus Fertilizer Index (PFI), showed a high correlation with the amount of Al extracted by pH 4.8 NH₄OAc (r²=0.94). Inclusion of soil chemical characteristics other than Al in the calculation did not improve the P intensity-capacity relationships significantly.

The usefulness of these relationships in determining P fertilizer needs was verified in an incubation study with 50 acid Ap horizon samples of Vermont soils. This study showed excellent correlations (r²=0.90 and 0.92 for unlimed and limed soils, respectively) between NH₄OAc-P measured in the soils after fertilizer addition and the NH₄OAc-P calculated from the PFI slopes estimated from the Al measured in the same NH₄OAc extract.

¹ Vermont Agric. Exp. Stn. J. Article no. 367. Presented before Div. S-4, Soil Sci. Soc. of Am., 29 Nov. 1976, Houston, Texas.

² Former Graduate Fellow and Professor, respectively, Dep. of Plant and Soil Science, Univ. of Vermont, Burlington, VT 05401. Y. S. Lee is now working for the Instit. of Agricultural Science, Office of Rural Development, Suweon, Korea.

Received for publication January 24, 1977. Accepted for publication March 21, 1977.

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