Is Another Soil Test Needed?

In the quest to assess and evaluate “soil quality” or “soil health” for the purpose of determining if a soil has the biological activity needed to promote optimum crop growth as well as preserve the “biology” of the soil itself, maybe it’s time to ask “Is another test needed when we have soil tested?”

In an earlier article entitled “Soil Health”, I presented a short narrative on how microbial activity in soil is an integral part of soil health. This article included a link to a video presentation that described the various types of soil microbes and how microbial activity is involved in mineralization and nutrient turnover in soil. The article also contains a link to a description of a test that can be used to measure microbial respiration, which can be used as an indicator of soil health.

A recent article entitled Should Soil Testing Services Measure Soil Biological Activity by Dr. Alan J. Franzluebbers provides further details about this test and how it can be conducted to obtain meaningful results. He also presents strong support for its being incorporated into soil testing procedures. Major points in this article follow.

●   Conservation production practices are known to promote soil organic matter (OM) accumulation.

●   Total soil OM changes slowly, but active fractions are more dynamic.

●   Soil health vs. soil quality emphasizes the biological component of soil because health refers to something that is living.

●   A key indicator of healthy soil is potential biological activity, which can be measured rapidly by soil testing services using a test that involves measuring the CO₂ flush following rewetting of dry soil.

●   The flush of CO₂ is related to soil microbial biomass Carbon, and is strongly related to Nitrogen mineralization during standard aerobic incubations.

●   Evaluation of soil fertility and health could be elevated by testing soil for biological activity, such as that done by the above-referenced flush of CO₂, because testing for and measuring inorganic nutrient availability alone does not provide a complete assessment of soil fertility or of how soil biological properties affect crops and the environment.

●   Key features of the flush-of-CO₂ test that make it amenable to use are 1) uses dried soil, 2) involves a relatively rapid analysis time, 3) has strong correlation with other important soil health indicators, and 4) uses the inherent biological conditions of the soil.

●   Actual soil microbial biomass Carbon level needed to achieve a given level of soil health across diverse soils has not been determined.

●   Calibration of the CO₂ flush test to Nitrogen availability in the field is critical in utilizing the test as a predictor of biologically-available Nitrogen. This knowledge could fill a critical void since most soil testing laboratories lack a test for biologically-derived Nitrogen. Research needed to quantify this relationship is underway.

●   A test to measure soil biological activity should be based on irrefutable, verifiable calibrations and correlations so that recommendations resulting from such a test are grounded in sound scientific principles. Studies to establish these base points are being conducted.

Dr. Franzluebbers describes the procedures and sampling requirements for the test, as well as developments that make it amenable to being used by soil testing laboratories. He also provides a list of laboratories that offer the commercialized test.

The value of the above test for Midsouth soils that are inherently low in OM is yet to be determined. However, since rotation of soybean with grain crops is now more common in this region, it is probable that it will become valuable as increased residue from grain crops leads to increased soil OM.

Composed by Larry G. Heatherly, Apr. 2016, larryheatherly@bellsouth.net