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This article in SSSAJ

  1. Vol. 49 No. 6, p. 1437-1440
    Received: Sept 7, 1984
    Accepted: July 15, 1985

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A Comparison of NH4HCO3-DTPA, NaHCO3, CaCl2, and Na2-EDTA Soil Tests for Phosphorus1

  1. V. K. Labhsetwar and
  2. P. N. Soltanpour2



The NH4HCO3-DTPA soil test of Soltanpour and Schwab was developed for simultaneous determinations of availability indices for P, K, Zn, Fe, Cu, Mn, and other elements. In this paper NH4HCO3-DTPA was compared with NaHCO3, CaCl2, and Na2-EDTA soil tests for P in a greenhouse study. Five rates of P: 0, 50, 100, 150 and 200 mg kg−1, were applied to 11 Colorado soils and yield response of alfalfa (Medicago sativa L.) to P was measured. Phosphorus levels in soil extracts were measured using inductively coupled plasma spectrometry. The NH4HCO3-DTPA extracts also were analyzed for P colorimetrically. The inductively coupled plasma spectrometric method gave higher values (1.7 mg kg−1 on the average) for P than the colorimetric method probably due to inclusion of organic P. The Nelson and Anderson analysis of variance method indicated that all tests were equally effective in separating soils that required fertilizer P from non-responding soils with r2 values of 0.52, 0.50, 0.60, and 0.50, for NH4HCO3-DTPA, NaHCO3, CaCl2, and Na2-EDTA soil tests, respectively. However, the NH4HCO3-DTPA is more economical because it is designed to extract other elements simultaneously, thereby saving considerable time. The r2 value was 0.58 for colorimetric method. The greenhouse critical levels of P were 9.4, 27.6, 0.46 (0.23 mg L−1 in solution), and 36.8 mg kg−1 for NH4HCO3-DTPA, NaHCO3, CaCl2, and Na2-EDTA soil tests, respectively.

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