Private: Evaluation of the Solvita Test to Estimate Mineralizable Nitrogen in Minnesota Soils

Summary

The Solvita CO2 burst test was strongly related to all other soil measurements used in this study, but did not prove to be the best predictor for PMN, as has been found in some previous studies (Franzluebbers et al., 2000, Schomberg et al., 2009; and Culman 2013). We found that POXC and SOM both were more strongly related to PMN than Solvita, although Solvita had as strong a relationship to N mineralized after 64 days as did POXC and SOM. Morrow et al. (2016) also found that POXC was a better predictor of mineralizable N than short term C mineralization. When considering previous crop history, we found that Solvita correlated with SOM but not extractable N in the top 0-15 cm when the previous crop was alfalfa, but was very strongly correlated with extractable N for sites previously cropped to either soybean or corn. Solvita had stronger relationships with other soil measures in the coarser textured soils (clay < 35%) than in soils with higher percent clay. In terms of agronomic performance, SOM, extractable N, and Solvita were equally good at predicting corn grain yield and grain N uptake in the top 15 cm, but SOM and extractable N were better indicators when the top 30 cm of soil were included.

Results from the 2013 growing season showed that PMN was more closely related to the Solvita results than to SOM (r=0.54 vs 0.34, n=19). For the 45 samples analyzed for PMN in 2014, PMN was more highly correlated to SOM and POXC than to Solvita (r=0.46, 0.39, and 0.36, respectively), although all three relationships were significant (p<0.001, 0.01, and 0.05). However, for the N mineralized after 64 days, the correlation coefficients for SOM, POXC and Solvita (0.56, 0.52, 0.50) were nearly identical and all were significant at the 0.001% level. Results so far do not suggest that Solvita is a better indicator of PMN than SOM or POXC, but in some cases may be a more accessible test. For the 120 field corn control plots, SOM, extractable N and Solvita were all equally well correlated with yield (p < 0.001), although SOM was the most strongly related when the 0-30 cm depth was considered. In this study, the Solvita test was well correlated with yield in corn after corn plots (as was extractable N) but not for corn after alfalfa or soybeans.