Evaluation of the Solvita Test As an Indicator of Mineralizable Nitrogen in Minnesota Soils.

In most corn (Zea mays L.) cropping systems, nitrogen (N) is the most limiting nutrient. While extractable soil N is readily and cheaply measured, determining potentially mineralizable soil N (PMN) is expensive and laborious to ascertain. Accurate, low-cost soil test estimation of PMN could improve fertilizer recommendations and increase N fertilizer use efficiency. Traditional laboratory incubation is recognized as the standard method for predicting PMN, but this procedure cannot supply in-season recommendations due to the long incubation time required. A quick test for CO₂ flush after soil rewetting has been proposed as an indicator of microbial activity and PMN. The commercial version of this test, Woods End’s Solvita CO₂-Burst test, evaluates the CO₂ flush in 24 hours after rewetting dry soil and is a relatively simple tool to estimate potential soil N contributions. Some previous studies have shown high correlations between soil respiration rate from this test and the estimated mineralizable N pool from incubation experiments, but this method has not been tested for Minnesota soils and conditions. The recommended standard rewetting method (20 ml water added to 40 g soil) resulted in high variability for duplicate samples, and greatly underestimated respiration in coarse-textured soils. In a trial experiment, alternative rewetting methods were tested and the 50% water filled pore space method  significantly decreased variability, with 87.5% of duplicate samples tested differing from one another by less than 10% compare to 51.3% with the standard method. After two years of testing, our results have shown that the Solvita test is better correlated with PMN estimated from a 64-day aerobic incubation than SOM is (r=0.38 vs. 0.29, respectively). This suggests that the test kit may be a better predictor of required fertilizer N than SOM would be, but neither measure is well correlated.