"Soil biology is really considered a last biotic frontier. We know more about life in outer space and in the deep sea than we do in our own backyards. Only about 10% of the species in soil have been described," said Dr. Deborah Neher. Neher is a semiretired professor of plant and soil science at the University of Vermont.
Neher discussed novel methods of testing biological activity in soil as part of the Soil Health Indicator Series put on by UVM's Soil Health & Research Extension Center (SHREC).
Neher's research team has been driven by farmer input. Farmers requested tests that were better at reflecting and are more sensitive to management practices than they had been receiving. They also requested more biological tests which can measure the quantity and variety of organisms in soil.
Why Test Soil Biology?
"I like to start out by reminding everyone that soil is actually a living entity. Soil is alive. And it's not that we have organisms just living in soil, but they're actually components of the soil, and they can alter the appearance of the soil due to their biochemical and physical processes," Neher said.
Soil is made up of myriad organisms - think of it as a food web - which can contain bacteria, protozoa, fungi, nematodes, mites, springtails, rotifers and tardigrades, insects, myriapods, spiders, diplurans, potworms and earthworms.
Neher's research shows that more complex food webs result in greater plant productivity. As biological diversity is added, plant yield increases. For example, a plant grown in a sterile medium will only be so productive. Add some bacteria and the plant's productivity will increase. Then add some protozoa, and productivity will further increase. The plant will continue to respond as soil biology diversifies.
Testing soil biology can reveal the quantity and variety of organisms in soil and potentially allow farmers to see how their management practices change the composition of the food web. For example, disturbances such as tillage impact the food web. If a farmer reduces their tillage, they may see changes to that food web which may make their crops more productive.
What Are Some Biological Tests?
According to Neher, testing soil biology is laborious and therefore expensive. Only recently, due to the advent of molecular tools and nano-size instruments, have detailed assessments of soil communities and their ecological function been possible. Because of the cost and complexity, the SHREC lab has decided to focus on three tests.
"We don't want to measure three things that tell us the same thing. We're trying to use the three-legged stool, so collectively they tell a good story. But it takes more than one to tell us the story," Neher said.
The first test measures biologically active organic matter (also known as the active carbon pool). The active carbon pool is made up of small organic matter particles that are usually plant-derived, between 53 and 250 micrometers. Soil microorganisms use the active carbon pool as food.
More specifically, this test measures the permanganate oxidizable carbon (POXC) - why it's often called the POXC test.
The active carbon pool portion of soil organic matter is actively involved in nutrient cycling and responds when cropping systems are managed by reducing tillage, diversifying crop rotation or incorporating cover crops. Higher active carbon scores indicate a trend toward more organic matter building up in the soil through biological activity.
"The big reason for choosing it is we found that it's much better suited to track management practices than the traditional measure of just simple percent organic matter," Neher said.
A second test - the autoclaved citrate extractable (ACE) protein index - is an indicator of organic nitrogen. Traditional soil tests only provide information about inorganic nitrogen which is then used to make fertilizer recommendations.
"This has been a missing piece of the nitrogen budget," Neher said.
Soil contains decaying plant material that contains proteins. These proteins are broken down into amino acids, and then the amino acids are converted to ammonium. The ACE protein index is an indicator of the amount of organic nitrogen in the form of protein and protein-like substances. Soils with less tillage tend to have higher organic nitrogen.
The third test measures phospholipid fatty acids (PLFA) to quantity the microbial biomass and general categories of organisms in a given soil sample. Bacteria, fungi and other microorganisms have unique fatty acid chains, allowing those in the lab to isolate and quantify their relative abundance in soil.
The test is used to get an estimate of the ratio of fungi to bacteria. This is important because generally the presence of more fungi represents a healthier soil. Again, it's a valuable test for farmers who are wondering if their management decisions have improved their soil's biology.
"It's all about what I would call ecological succession. It's kind of a measure of disturbance. When we cultivate a field or add a large load of fertilizer or manure, this is going to create a very simple food web dominated by bacteria and bacterial grazers. But if this is allowed to remain somewhat undisturbed, we start to see that fungi and fungal grazers can eventually colonize. So, if we have higher fungal to bacterial ratios, it's telling us we have less disturbance - a more complex food web," Neher said.