Soil sample storage conditions affect measurements of pH, potassium, and nitrogen

Soil quality monitoring schemes are a useful tool for assessing the potential of soils to perform desired services such as agricultural productivity. When researchers or other stakeholders wish to compare results between different schemes or studies, failure to consider differences in soil sample storage conditions presents a significant potential for error. Here, we compared levels of nitrogen and potassium, as well as pH, in agricultural soil samples stored under three different conditions (refrigerated, frozen, and oven-dried). All tests were performed after 7 and 24 weeks of storage. Nitrate decreased significantly in dried (p < 0.001) samples. When refrigerated, nitrate first increased (p < 0.01) and then decreased (p < 0.001). Nitrate levels where unchanged at Week 7 in the freezer but decreased significantly at Week 24 (p < 0.001). Nitrite and ammonium increased after drying (p < 0.001) and when frozen (p < 0.001 and p < 0.05) but remained stable when refrigerated. There was no significant difference in potassium levels between the fresh control and Week 7 in the freezer, but potassium had increased at Week 24 (p < 0.05). Potassium concentration increased in refrigerated samples (p < 0.001) and fluctuated up and down in dried samples (p < 0.01). pH measurements fluctuated significantly in refrigerated and frozen samples (p < 0.001 and p < 0.01, respectively) but were unchanged in dried samples. We suggest that soil monitoring schemes standardize their sample storage, and we encourage researchers to clearly report soil sample storage conditions in publications, to improve transparency and reproducibility.