Soil Analysis\nby Mobile Multinuclear NMR: Quantification\nof Phosphorus, Aluminum, and Sodium

Soil analyses are essential to ensure economically and\nenvironmentally\nsustainable crop production while maintaining the soil fertile for\nfuture use. Unfortunately, common soil analyses may be highly demanding\nin terms of time, chemicals, and costs. This applies, in particular,\nwhen total quantities of elements are desired. As an easy and fast\nalternative without consumption of chemicals, we here present mobile <sup>31</sup>P, <sup>27</sup>Al, <sup>23</sup>Na, and <sup>1</sup>H NMR\nfor quantification of phosphorus, aluminum, and sodium contents in\nsoil. This enables accurate on-site analysis and is suitable for direct\nmeasurement on fresh, undried soil samples since the water content\nis quantified as well. For demonstration, 40 various Danish agricultural\nsoil samples were analyzed using a mobile NMR sensor, and the results\nwere compared with external laboratory analyses for P, Al, and Na.\nThe laboratory analyses were conducted with ICP-OES after four-acid\ndigestion, which additionally were compared with aqua regia digestion,\nshowing inadequacies in the performance of the latter. Good agreement\nbetween NMR and laboratory analyses (correlation coefficients 0.91\nfor P, 0.98 for Al, and 0.90 for Na, in the concentration ranges 250–1200\nppm P, 1.4–5% Al, and 0.3–1% Na) were obtained with\nhigh accuracy using NMR measuring times of 20 min to 1 h for P, 4–12\nmin for Al, and 6–20 min for Na. Additionally, the NMR measurements\nprovide information on the amount of P associated with paramagnetic\ncenters (e.g., Fe<sup>3+</sup>). Good correlations were also observed\nto other parameters such as the clay content, which is predictable\nfrom the intensity of the more fast-relaxing of three <sup>27</sup>Al NMR components.