Assessment of Two Chemical Extraction Methods as Indices of Available Nitrogen

Abstract

Abstract A soil test for estimating mineralizable N in soil is needed to improve fertilizer use efficiency. Our objectives were to: (i) test the effectiveness of two chemical indices in estimating potentially mineralizable N (N 0 ) in soils, and (ii) determine if these indices could be used to differentiate the impact of cultural practices on the N‐supplying capacity of soils. We collected samples from the 0‐ to 0.15‐m depth of 42 soils representing all agroecological regions in Saskatchewan, Canada. We determined (i) N mineralized (N min ), N 0 , and the rate constant ( k ) by aerobic incubation at 35°C for 24 wk, and (ii) the NH 4 ‐N extracted by (a) 2 M KCl heated at 100°C for 4 h, and (b) a steam‐distilled phosphate‐borate solution buffered at pH 11.2. The association between N 0 and the hot 2 M KCl extracted NH 4 ‐N was close ( r 2 = 0.78, significant at P < 0.001); the relationship between N 0 and the phosphate‐borate NH 4 ‐N was slightly less precise ( r 2 = 0.73, P < 0.001). When NH 4 ‐N extracted with cold 2 M KCl was subtracted from the NH 4 ‐N extracted with hot extractants, the association with N min was weaker ( r 2 ≤ 0.50). The value of k for the 42 soils was generally constant (avg. 0.067 ± S x t 0.05 = 0.040 wk −1 ). In a second experiment, we compared the two chemical extractants vs. the N mineralized in a 24‐wk incubation regarding their effectiveness in differentiating the effects of eight 37‐yr crop rotation treatments on N‐supplying capacity. These three indices performed effectively in this regard. For example, the 2 M KCl NH 4 ‐N was closely associated with N mineralized in 24 wk ( r 2 = 0.92), as was phosphate‐borate NH 4 ‐N ( r 2 = 0.88; both significant at P < 0.001). We concluded that the chemical extractants may provide a quick test for assessing N‐supplying capacity of the soil.