A RAPID AND SENSITIVE FLOW-INJECTION TECHNIQUE FOR THE ANALYSIS OF AMMONIUM IN SOIL EXTRACTS

The steam distillation technique for the quantification of soil mineral nitrogen is not well adapted to estimate very low levels of NH4+ and NO3-such as those found in tropical pastures. In this paper we describe a flow injection analysis (FIA) technique for the direct estimation of soil ammonium in potassium chloride extracts. The technique is based on the salicylate/hypochlorite colorimetric reaction with the resulting colouration read at 647 nm. The speed of the reaction was increased by immersing the reaction coils in a water bath maintained at 70-degrees-C. The technique was capable of accurately quantifying ammonium to concentrations as low as 0.05 mg NH4+-N/L (equivalent to 0.25 mg NH4+-N/g dry soil) and sample thoughputs of at least 60 samples per hour could be achieved. It was found that amino acids would also give a positive colorimetric reaction but that in the ten tropical soils examined this was not a significant source of error.