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

ABSTRACT

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.

¹ Contribution from the Dep. of Agronomy, Colorado State Univ. Exp. Stn. and published as Scientific Series Paper no. 2947.

² Graduate Student and Professor, respectively, CSU, Fort Collins, CO 80523. The Senior Author is now Soil Scientist at Prompt and Precise Lab, Oshkosh, WI 54901.

Received for publication September 7, 1984. Accepted for publication July 15, 1985.