Modification of the NH4 HCO3‐DTPA soil test to omit carbon black

Abstract

Recently Soltanpour and Schwab developed an NH4HCO3-DTPA [diethylene triaminepentaacetic acid] (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 effect of eliminating carbon black from the AB-DTPA test was determined on the extractable levels of P, K, Zn, Fe, Mn and Cu. Carbon black apparently 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. Carbon black should be eliminated from the AB-DTPA test not only because of its staining and sometimes contaminative properties but because it adsorbs metal chelates and will introduce rather large errors if its particle size is not kept constant.