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Extractable Soil Boron and Alfalfa Uptake: Calcium Carbonate Effects on Acid Soil

USDA-ARS, U.S. Salinity Lab., 4500 Glenwood Drive, Riverside, CA 92501

L. J. Evans, T. E. Bates and G. A. Spiers

Dep. of Land Resource Science, Univ. of Guelph, ON, N1G 2W1, Canada

*Corresponding author.

ABSTRACT

Since soil tests for B in fine-textured soils give inconsistent predictions and the amendment of acidic soils with CaCO₃ is a common practice, the suitability of extractants for plant-available B in a CaCO₃-amended clayey soil was studied. This research investigated the pH effect on B adsorption, the changes in the amounts of hot-water-extractable B (HW-B), mannitol-extractable B (MANN-B), and soil solution B (SOLN-B) as a result of B and CaCO₃ addition to a silty clay soil (a clayey, mixed, mesic Typic Humaquept) with an initial pH of 4.2. The relationship between the amounts of extractable B and B uptake by alfalfa (Medicago sativa L.) was also studied. Amounts of extractable B in soil were in the order HW-B > MANN-B > SOLN-B, decreasing with increasing pH and after growth of alfalfa. Addition of 2.5 and 5.0 g CaCO₃ kg^–1 increased alfalfa dry matter yield; whereas addition of 0.5 and 1.5 mg B kg^–1 had no effect. In the absence of added B, the B concentration in alfalfa shoots decreased with increasing pH. Linear correlations between B concentration in alfalfa shoots and extractable B in soil were significant for all three cuts and for all three extractants. Total uptake of B was significantly correlated with all forms of extractable B except HW-B for the third cut only. Inclusion of pH improved the relationship for the other cuts. The dilution of B in plant shoots by CaCO₃-stimulated growth may result in B deficiency.

Received for publication October 18, 1993.