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Evaluation of the Ammonium Bicarbonate-DTPA Soil Test for Assessing Boron Availability to Alfalfa¹

ABSTRACT

The NH₄HCO₃-DTPA (AB-DTPA) soil test of Soltanpour and Schwab was developed to simultaneously extract plant-available P, K, Zn, Fe, Cu, and Mn. The extractant consists of 1.0M NH₄HCO₃ and 0.005M DTPA adjusted to pH 7.6. Although the AB-DTPA soil test was developed primarily to extract the previously mentioned nutrients, its use for determining plant availability of other nutrients may be feasible. The objective of this study was to evaluate the AB-DTPA soil test for determining plant-available B.

Seven soils from Colorado which varied in texture and organic matter content were treated with sodium borate (Na₂B₄O₇·10H₂O) at 0, 1, 3, 5, and 10 µg B/g soil. Alfalfa (Medicago sativa var. ‘Ladak’) was planted in the soils in the greenhouse and then cut three times at the 30% bloom stage. Plant tissue digests and soil extracts (AB-DTPA, hot-water, saturation) were analyzed for B using inductively coupled plasma atomic emission spectroscopy (ICP-AES).

Saturation and AB-DTPA extractable B levels were nearly identical while the hot water soil test extracted significantly ( = 0.05) higher amounts of B. Alfalfa yields were not significantly ( = 0.05) affected by increasing B application rates indicating B deficiency and toxicity conditions were not present.

Plant B was highly correlated to all three extractants for the individual soils; however, the coefficients of determination (r²) for plant B and soil B differed appreciably when the pooled soils were analyzed statistically: r² = 0.51 for AB-DTPA, r² = 0.87 for hot water, r² = 0.76 for saturation extract, indicating the superiority of the hot-water extractant for B. Multiple linear regression analysis also indicated the superiority of the hot-water extractant and denoted that NH₄HCO₃-DPTA soil test required the addition of soil variables (soil pH, % OM, % clay) to be considered an effective test.

¹ Contribution from the Dep. of Agronomy, Colorado State Univ. Exp. Stn., Ft. Collins, CO 80523, and published as Scientific Series Paper no. 2698.

² Graduate Research Assistant and Professor of Agronomy, respectively. Present position of the senior author is: Assistant Professor of Agronomy, Dep. of Agric. Sci., Technol., and Education, Univ. of Southwestern Louisiana, Lafayette, LA 70504.

Received for publication April 6, 1983. Accepted for publication August 11, 1983.