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This article in SSSAJ

  1. Vol. 41 No. 4, p. 710-712
    Received: Jan 24, 1977
    Accepted: Mar 21, 1977

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Assessing Phosphorus Fertilizer Needs Based on Intensity-Capacity Relationships1

  1. Yong S. Lee and
  2. R. J. Bartlett2



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.

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