Soil mass and volume affects soil-test biological activity estimates

Authors: Franzluebbers, Alan

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/2/2020
Publication Date: 5/10/2020
Citation: Franzluebbers, A.J. 2020. Soil mass and volume affects soil-test biological activity estimates. Agronomy Journal. 84:502-511.

Interpretive Summary: Soil testing for potential biological activity has become a valuable tool to assess soil health under a diversity of soil types and management conditions. Variations still exist in how this test is performed, and therefore, further methodological assessments are needed. A scientist from USDA-Agricultural Research Service in Raleigh NC evaluated the effect of a wide range of soil weights and size of containers on the accuracy and precision of soil-test biological activity estimates. Five soils were selected from Georgia and North Carolina to obtain a gradient in soil texture and organic matter content. Both accuracy and precision of soil-test biological activity were optimized with a moderate weight of soil from about 50 to 100 g. Random variations were greater at soil weights much less than this target range. Soil testing labs are encouraged to standardize protocols for soil biological activity measurements, and this study showed that a minimum of 50 g of soil should be considered for most consistent and reliable results. These results will benefit commercial and research soil-testing laboratories and the clientele they serve.

Technical Abstract: Soil biological activity is a key indicator of soil health assessment. Methodological details may affect accuracy and precision of this indicator. Accuracy and precision of soil-test biological activity estimates were determined from 10 replications of 10 mass/volume treatments under standard laboratory conditions using an alkali trap technique with acid titration. Five soils varying in texture and organic C and N concentrations were used to assess a gradient of soil mass/volume conditions on C mineralization following rewetting of dried soil during 0-3, 3-10, and 10-24 d of incubation at 50% water-filled pore space and 25 °C. Soil type explained >90% of variation in C mineralization, but soil mass/volume treatments also had significant effects on estimates (p < 0.001). In sets of treatments with the same surface area exposed, increasing soil depth (volume) led to lower C mineralization across soil types. Entrapment of CO2 in soil pores may have led to lower C mineralization, but similar responses continued with longer incubation periods suggesting the contribution of an undefined factor. Treatments with 50 to 100 g of soil were closely associated with median estimates for each soil. Precision of C mineralization estimates was greatest when soil mass was >50 g of soil. A consistent methodology is needed with sufficient soil mass to account for potentially enhanced random variation of organic-enriched surface soils.