Modification of the NH₄HCO₃‐DTPA soil test to omit carbon black

Abstract

Abstract Recently Soltanpour and Schwab3developed an NH4HCO3 ‐DTPA (AB‐DTPA) soil test to simultaneously extract NO3, P, K, Zn, Fe, Mn, and Cu. In this test carbon black was used to clear the extract for the chromotropic acid method of nitrate determination. However, use of carbon black was inconvenient due to its staining and sometimes contaminative properties. The purpose of the study reported here was to determine the effect of the elimination of carbon black from the AB‐DTPA test on the extractable levels of P, K, Zn, Fe, Mn, and Cu. The results indicated that carbon black adsorbed Fe‐, Mn‐, and Cu‐DTPA, but did not affect the levels of P, K, and Zn in the AB‐DTPA extracts. Regressions of the modified AB‐DTPA test (no carbon black) on the DTPA test for Zn, Fe, Mn and Cu were run. These regressions showed that the levels of AB‐DTPA‐extractable Zn, Fe, Mn, and Cu are highly correlated to the levels of these nutrients determined by the DTPA test. It was concluded that carbon black should be eliminated from the AB‐DTPA test not only because of its staining and sometimes contaminative properties but mainly because it adsorbs metal chelates and will introduce rather large errors if its particle size is not kept constant.