Bucknell University is a private liberal arts college in Lewisburg, Pennsylvania with excellent research facilities and innovative teaching. Students get the opportunity to work closely with professors in their chosen field.
Students in the Biology 203, Integrative Concepts in Biology, laboratory have a unit all about soil. The students visit the Bucknell Farm to learn about the properties of healthy soil. They then pick a location on campus to study. Students study the health of the soil in different conditions, such as soil with native flowers growing compared to soil under a tree. They measure microbial biomass, soil respiration rate, and various other soil properties to determine the overall health of the soil.
“The microBIOMETER® test allows students to quickly and easily measure microbial biomass and the relative amounts of bacteria and fungi in the soil. It is easy to use for non-experts with very quick results! We have measured huge differences in the microbial biomass at locations across Bucknell’s campus and have been surprised to have very high levels of biomass in the grassy areas, too!” – Rebekah Stevenson, Director of Core Course Laboratories – Biology Department
There are many soil tests on the market so it can be difficult for farmers to ascertain whether or not they’re choosing the right one. The truth is, there are pros and cons to every soil test. Therefore it boils down to finding which ones align best with your farming goals and which are easily and readily available to you without needing to stretch your resources too much.
Since microBIOMETER® is a relatively new soil test on the market, a lot of questions are raised on how this test is different from other commonly used soil indicators such as the Haney Soil Test and PLFA test. While all three are soil biological health tests, their methodologies are very different and the tests measure different parameters.
The microBIOMETER® is an on-site soil test that measures the microbial biomass and fungal to bacterial ratio of living and dormant bacteria and fungi. The test process works by measuring the color intensity of the microbial solution created and comparing the color to the test card comparator. This patented, colorimetric analysis process is generated through our microBIOMETER® Reader App and produces results within 25 minutes of starting the testing process. Test prices range from $13.50/sample to $6.75/sample. The low cost, rapid result detection, and on-site testing of living soil are what makes this test stand out against others. The microBIOMETER® has a slightly limited scope, however, as it’s only able to measure the overall biomass of fungi and bacteria. It does not differentiate between microbial species nor does it measure any other parameters.
The Haney Soil Test is a lab test that focuses on assessing a variety of soil parameters such as pH, microbial biomass, water extractable organic carbon and nitrogen, soil respiration, and inorganic plant available nutrients such as NPK. This test uses multiple methods in order to obtain results, including the Solvita CO2 Burst test to indicate soil microbial respiration and biomass, and the use of unique soil extracts to determine organic and inorganic nutrient availability. While this test offers a large array of soil parameters, there is controversy in the science community about using the Solvita CO2 Burst test methodology as a way of accurately predicting microbial biomass. This is because the soil is dried then rewetted to trigger a release of CO2 to measure microbial activity. Drying soil decreases microbial biomass, and while rewetting it will increase biomass again, it doesn’t necessarily repopulate back to the original microbial composition. The Haney Soil test is offered at several labs throughout the country and recommendations are included with results. Generally, this lab test costs about $50/sample and takes about 3-4 weeks to receive results.
The PLFA Soil Test is a lab-based technique that analyzes phospholipid fatty acids (PLFA), which are found in the cell membranes of living organisms, to determine an estimation of living microbial biomass, fungal to bacterial ratio, and to identify the general presence or absence of microbial functional groups in bacteria, fungi, and protozoa. For this test, labs first dry the soil overnight then use multiple solvents to extract fatty acids from the sample. Then, mass spectrometry is used to identify the sample’s microbial composition based on specific PLFA biomarkers. This testing process takes a few days to complete and generally costs about $60/sample depending on the lab. It is one of the most utilized testing methods since it gained popularity in the late 80’s. Since then, it was discovered that some of the PLFA biomarkers used for identification aren’t limited to one microbial group, therefore making it difficult to determine the accuracy of some results.
The value of each of these tests is to determine a baseline assessment of your soil health. The information obtained from any of these tests will help you gain a better and more rounded understanding of what’s happening in your soil.
Both microbial biomass and respiration are parameters used to assess soil health. Soil respiration is the measure of the carbon dioxide produced by the microbes in a given weight of soil while microbial biomass is the measure of the mass of microbes- both active and dormant.
Microbial biomass (MB) is an excellent predictor of soil health because the size of the microbial population correlates with the available nutrients in the soil. Interestingly, MB is low in soil treated with high levels of mineral fertilizers. Research has shown that the stimulus for the plant to grow a microbial population is its need for nitrogen and phosphorus. If these nutrients are artificially supplied, the plant is not being stimulated to feed the microbes that usually provide these nutrients to the plant. This can alter plant-microbe interactions and cause an increased need for pesticides in order to protect the plant, as microbes play a fundamental role in the function of the plant’s immune system.
Microbial respiration measures the amount of carbon dioxide (CO2) produced by the microbes in a given weight of soil. The soil is dried and then rewetted and put in an airtight jar that allows measurement of the amount of CO2 produced over 24 hours. The CO2 is produced by the activity of the microbes in the rewetted soil. Between 20% and 70% of the microbes die during drying, but their dead bodies often provide nutrition for the survivors to use and regrow the population to its original level. Respiration reflects the regrowing work that is being done. The respiration level is often mistakenly believed to predict microbial biomass, though it doesn’t.
People often assume a high respiration rate is good because it means there is a lot of microbial activity occurring. However, it doesn’t necessarily mean the soil is healthy. Microbes in a low pH or toxic soil have to work harder, and therefore their respiration rate is higher, just as your respiration rate in the gym is higher than when you are watching TV. High respiration rates can indicate an unstable microbial population, which, for example, can be seen after excessive tillage occurs. Tillage aerates the soil, so right after there is often a boost of microbial respiration. That increased activity however does not always last, as the other damage done by tillage – disruption of microbial life and destruction of existing plants- can lead to a decreased soil microbial population over time.
The use of soil primers stimulates an increase in soil organic matter (SOM) decomposition, which temporarily increases microbial respiration. Excessive decomposition of SOM can cause a loss of stored soil carbon and other mineral nutrients, allowing for the increased production of CO2. Basically, when you stimulate the soil using a fertilizer or biostimulant, it’s an all-you-can-eat buffet for the microbes. It wakes them up and they start growing and reproducing. But whether they can continue to grow depends on the continual supply of existing nutrients and plant life in the soil. It’s very important that there be sufficient food for the microbes after stimulation. For most soils, this requires that the fertilizer have the correct C:N ratio for the soil and crop. A fertilizer with too high a C:N ratio will cause the microbes to harvest some of the stored carbon, nitrogen and other nutrients in the soil, boosting respiration. This means the stored carbon is being depleted and released into the atmosphere as CO2, the microbes won’t be able to nourish the plant and build soil structure as needed. Adoption of less invasive management practices, such as select-till and reduced chemical fertilizers can reduce CO2 emissions from agricultural soils by retaining soil organic matter.
Priming can be a good way to understand the difference between and uses of respiration data and microbial biomass data. Testing for both initial respiration and long term microbial biomass population can tell you if the priming worked and if the increase in microbial activity led to increased soil microbial biomass and therefore increased soil health and fertility.
