Soil Biological Activity and Soil Health
Soil is an ecosystem that not only holds and provides nutrients and water for plant growth, but also provides habitat for soil microbes that are an integral part of the soil’s interaction with plants that are growing in it. These microbes are key to decomposition of chemical and plant residues and resultant nutrient cycling, as well as being integral to soil components that affect soil structure, aeration, porosity, and water holding capacity.
A White Paper on this website provides an in-depth discussion of Soil Health and its importance in the production of an economical crop. A portion of that article is repeated below.
The USDA-NRCS defines soil health (also referred to as soil quality) as “the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals, and humans”. They further state that “this definition speaks to the importance of managing soils so they are sustainable for future generations”. In a recent publication, the case was made that soil health vs. soil quality emphasizes the biological component of soil because health refers to something that is living; thus, the two are not necessarily synonymous using this criterion.
Understanding soil health involves assessing and managing a soil’s inherent properties of fertility, structure, microbial activity, etc. so that it functions to support optimal plant growth, both now and in the future. This means that changes in soil health must be monitored so that soil is not degraded, and conversely is managed using a set of practices that are both sustainable and promote soil sustainability for the long term.
Soil biological activity is increasing in importance because it is considered an indicator of “soil health” as well as an indicator of just what a particular soil can contribute to plants. It indicates the quality of the microbial habitat and thus its ability to optimize nutrient cycling as well as the potential for carbon sequestration. Two recent articles provide a furtherance of how this activity can be measured and how those measurements can be made uniform across varied testing environments.
The first article titled “Soil mass and volume affect soil-test biological activity estimates” appears in Soil Sci. Soc. Am. J. 84:502-511 (2020) and is authored by Alan J. Franzluebbers. A summary of the contents of this article follow.
• Accuracy and precision of soil-test biological activity (STBA) may be affected by methodology used in its determination. Thus, standardized methods are needed so that results from STBA tests can be compared across diverse laboratories.
• Objectives of the research reported in this article were to 1) compare estimates of carbon (C) mineralization as affected by mass and volume of the soil sample used for the test, 2) determine the variation caused by laboratory operator conducting the test, and 3) estimate random variation among soil samples that vary by mass and volume.
• Tests were conducted to gain an objective understanding of how variations in lab protocols might impact results from and assessments of STBA tests.
• Soil from two locations in Georgia and three locations in North Carolina were used to conduct tests to meet the above objectives.
• Soil texture at the five individual locations was silt loam (2.36% OM and 0.21% N), coarse sandy loam (1.52% OM and 0.095% N), clay loam (4.2% OM and 0.387% N), loamy sand (0.56% OM and 0.033% N), and sandy loam (0.43% OM and 0.026%N).
• Sample sizes of 5, 10, 20, 50, 100, 200, and 500 g, and 23 and 74 ml of oven-dried soil were prepared for processing.
• The five soil types provided a reasonable diversity of both physical and biochemical conditions.
• Mass and volume treatments of 5, 10, and 20 g, and 23 ml had the least exposed soil surface.
• Each soil sample was wetted to an approximate 50% water-filled pore space by applying water to the top of each soil sample.
• Soil type explained ≥90% of the total variation in C mineralization on a per unit of soil mass basis.
• At the defined 0-3 day incubation period, C mineralization responded to soil mass and volume treatment similarly across soil types.
• STBA in the mass and volume treatments with intermediate surface area (50- and 100-g treatments, and 74-ml treatment) averaged the same as in treatments with lower and greater surface area across soils.
• Soil mass was the single most important factor that influenced C mineralization rate across mass and volume treatments at 0-3, 3-10, and 10-24 day periods.
• Low soil mass treatments had greater inherent variability than higher soil mass treatments.
• Since large soil mass with small surface area-to-volume ratio limited accuracy of C mineralization estimation, and low soil mass limited precision of estimation, these results indicate that a moderate soil mass of 50-100 g in a defined volume should be used to optimize estimations. Thus, standardized approaches/protocols are needed/should be used to obtain consistent and reliable results of STBA estimates that can be compared across diverse laboratories.
The second article titled “Soil carbon and nitrogen mineralization after the initial flush of CO2" appears in Agric. Environ. Lett. 2020;5:e20006 and is also authored by Alan J. Franzluebbers. A summary of the contents of this article follow.
• STBA and its role in N mineralization are key components of soil health evaluation, but the role of STBA in long-term soil N mineralization (NMIN) is not documented.
• This study was conducted to evaluate short- and longer-term C mineralization (CMIN) and NMIN in five soils from Georgia (2) and North Carolina (3) (same soils used in above study) to determine associations between STBA and NMIN.
• The objective of the study was to evaluate how short-term incubation of a wetted soil to determine STBA relates to longer-term CMIN and NMIN.
• As expected, cumulative 1-day CMIN values were greatly different among soils (Clay loam > silt loam > coarse sandy loam > loamy sand = sandy loam).
• Instantaneous NMIN among soils was also greatly different and the differences followed essentially the same pattern as those for CMIN.
• A shift in dominance from NH4-N during the first 10 days of incubation to NO3-N thereafter suggested that nitrifying microbes were present but delayed in their activity.
• STBA after 3 days of incubation provides a good indication of forthcoming soil NMIN.
• The results from this study determined that STBA can be used as an indicator that shows a strong association with soil NMIN for up to 150 days. Utilization of this tool to estimate soil NMIN gives producers a reliable tool for predicting N fertilizer requirements and avoiding unnecessary N fertilizer inputs.
The subject matter of these two cited articles and the above summaries may appear to be of dubious value to soybean producers. However, in the present environment of producers striving for consistent and sustainable high yields, they are encouraged to consider testing for soil biological activity as important as testing for soil fertility because this component of soil health should be considered an integral part of the soil’s support system for achieving and sustaining those high yields. And the results from the above two studies indicate that accurate assessment of soil biological activity requires that standard procedures be used by all laboratories that test for such activity.
Composed by Larry G. Heatherly, May 2020, larryheatherly@bellsouth.net