Soil N mineralization potential - What it is and why we should measure it

 By David Burton

With fertilizer costs and environmental concerns on the rise, accurately accounting for soil-derived nitrogen is more important than ever. Researchers are evaluating practical ways to measure and predict soil N mineralization, helping producers and their CCAs make better-informed N management decisions. 

This article focuses on Canadian research and production systems; however, the concepts and tools discussed are relevant to crop advisers throughout North America. Earn 0.5 CEUs in Nutrient Management by reading this article and taking the quiz.

With the price of nitrogen (N) fertilizer and crop values fluctuating so dramatically, the efficient use of inputs has never been so important. The determination of the “right rate” of N fertilizer needed to maximize profit requires that we quantify all N sources, including the soil itself. The soil is one of the largest and least quantified sources of N to the crop. The mineralization of soil organic matter supplies approximately half the N the crop takes up. Soil N mineralization potential varies dramatically from field to field depending on soil type, cropping system, and past management. If we are to improve N management to increase efficiency, control costs, and reduce environmental impacts, it is important that we have a means of routinely quantifying soil N mineralization potential and are able to use this information in the determination of the rate of supplemental N fertilizer.

We have understood the process of N mineralization and its regulation for decades. It occurs as a result of the decomposition of crop residues and soil organic matter. Net N mineralization occurs when N in those substrates is in excess of the microbial community requirements for biosynthesis. We have developed numerous methods for measuring this potential, but unfortunately no one methodology has gained broad acceptance. As a result, few commercial soil test labs in Canada offer a measure of N mineralization potential as part of their routine soil-testing package. This is unfortunate as N is one of the most expensive inputs in agriculture and use in excess of requirement has significant environmental impact.