Soil Biology Testing for Crop Management
Fact Sheet | Soil Biology Testing for Crop Management
WHAT ARE SOIL BIOLOGY TESTS?
Soil biology tests measure the living components of the soil in order to understand the soil’s overall health and how it will supply plant nutrients. The information in a soil biology test can be used to make management decisions that can: increase particular soluble nutrients, manage soil-borne diseases, confirm the soil management impacts on soil organic matter, and observe other long-term trends.
In organic farming, where natural soil nutrient cycles rather than synthetic nutrient inputs provide more of the plant’s needs, living organisms do a lot of the work to convert nutrients from insoluble minerals locked within soil particles and the living cells of soil biology, into soluble plant-available nutrients. For more than 100 years, farmers and ag professionals have tested soil chemistry in order to understand a soil’s general characteristics, and specifically which nutrients are present and which need to be applied to the field. But, to understand a soil more completely, we observe it through three characteristics: texture, chemistry, and biology.
Soil chemistry tests help farmers understand the nutrients that will be soluble in the soil for plant use. But, we also know that soil biology is also crucial for making soil nutrients from organic fertilizers available for the plant to use. For example: one pound of synthetic 10-10-10 fertilizer will release 10 pounds of N into the soil within a few weeks, whereas that same amount of N contained in two pounds of 5-5-4 chicken manure will not be completely released for at least an entire year. This is because water dissolves the N in synthetic fertilizers, but soil biology makes available the N in organic fertilizers. Knowing how biologically active a soil is can help predict nutrient availability and help track the impact of management decisions on soil health and crop yield.
Testing for soil biology is still a newer type of analysis, and it can take too long to get results in order to make in-season management decisions. For comparison, the turnaround time for soil chemical testing is often rapid and might return results in five days, while the wait times for some soil biology tests are as long as four to six weeks. Also, the costs may be high compared to chemical soil testing. For these reasons, soil biology tests have been used to understand the growth of a soil over time and its longer-term trend, rather than to advise on short-term inputs, as one might use a chemical soil test - though this may change as methods and experiences improve.
We understand a soil most completely as a combination of three characteristics: Texture, Chemistry, and Biology. Soil Health emphasizes the biological element of soil and its impact on crop growth and soil management.
WHAT KINDS OF SOIL BIOLOGY TESTS ARE AVAILABLE?
Soil biology testing is in its infancy, and the agriculture community is still developing different testing methods and translating test results into management decisions in the field. More research by farmers, industry, and university researchers and extension is needed to support their interpretation. For example, one major limitation of these tests is that while soil biology testing labs might identify each functional group of soil microbes (e.g. phosphorus solubilizers) or even specific microbes, they have trouble providing management recommendations for how to increase specific microbe populations.
The Haney Test
One thing that soil chemical tests don’t do well is estimate Nitrogen - because it is so mobile in the soil, coming and going through water and soil biology. Its presence in the soil varies widely and can change soon after a sample is collected. The Haney Test measures three forms of soil N: nitrate, ammonium, and organic nitrogen. Understanding these three forms of soil N provides a bigger picture understanding of the N available to soil biology and the crop. For example, all forms of soil N are not equal - plants prefer particular forms of N and use them preferentially. Additionally, soils that have more carbon and are more biologically active convert more ammonium and amino acids into plant-available nitrogen, and compared to nitrates, ammonium and amino acids can be stored in organic matter longer.
PLFA: Phospholipid Fatty Acid Test
All microbial species are composed of phospholipid fatty acid membranes (just like we are). Think of this as a measurement of the “fat” that is extracted from these microbial species in a soil. The fat is analyzed and categorized into species: bacteria, protozoa, and fungi. This test gives farmers knowledge of the type of microbes in their soil. For example, because fungi feed nutrients to plant roots, farmers often want their management decisions to increase the fungal populations in their soils.
Most soils are bacterially dominant when managed conventionally. The PLFA test can provide insight on whether fungi or protozoa increase over time in response to management practices, such as an organic system. The information provided by PLFA tests only identifies the percentage of each microbe group that makes up a sample: bacteria, fungi, protozoa. In order to identify specific microbe species, a farmer will need to use microscopy or DNA sequencing/genomics.
Genomics or DNA Sequencing
The benefit of DNA sequencing is understanding the presence of specific functional groups within a soil. Such functional groups include nitrogen-fixers, soil aggregators, disease suppressors, phytohormone producers, and phosphorus solubilizers. Each lab uses different ways to show which functional groups are high and which are low, so that the farmer can understand which soil functions are impaired or healthy.
Because access to the computing power needed for soil genomics is newly available, the research is just beginning for us to understand how changes in soil management can impact microbial communities and their associated soil-plant services (see the resources at the end for more on this).
Different labs use different methods of testing. You may hear the term “metagenomics”, which is a specific sequencing technique where high-powered computing allows all the genes in the sample to be identified. There are also other types of genomic tests used to identify microbes using DNA sequencing, and while these tests don’t identify every type of microbe present, they can give a good-enough average at a cheaper price.
Microscopy
Microscopy is a methodology that uses a microscope to identify species of soil microbes. It is time intensive, as a practitioner is examining a slide of soil biology through a microscope, and identifying and counting individual microbes. Similar to PLFA interpretation, microscopy will easily confirm that most soils are bacterially dominant. But beyond that, it often takes too much time to identify the other species that exist in much smaller quantities such as fungi and protozoa. Because this test requires only a good microscope, along with training and hours of practice to identify particular microbial species, it is accessible to farmers wanting to dive deeper into understanding their soil.
Soil Respiration
Soil respiration tests measure the amount of carbon dioxide (or CO2) released by the biological organisms in a soil sample. The amount of CO2 produced by a soil is an indirect measurement of microbial activity, since microbes breathe it out, just like we do. This test gives a snapshot of the soil’s microbial activity and can also estimate the amounts of nutrients a soil will release in response to soil disturbance (such as tillage), as well as how much carbon a soil can sequester. Soil respiration can be highly variable due to conditions that affect microbial activity: air exchange, moisture, temperature - all of which can vary within a different part of the field or time of sampling.
OTHER BIOLOGICAL TESTS
There are other physical tests used to understand a soil’s biology, and many of them can be done quite inexpensively by any farmer on the farm. For example, an Aggregate Stability test (also known as a Slake test) measures how well soil particles adhere to each other in that fine crumb structure that farmers prize, since healthy microbial communities engineer their soil home into this nice crumb structure. Tests like these do not give detailed knowledge about a soil’s biology or prescriptive advice, but they do help farmers compare their soils and track changes over time. The “Cropland In-Field Soil Health Assessment Guide” published by the NRCS contains detailed procedures for qualitative soil health assessments.
TAKE AWAY
Soil biology testing is an evolving field and more research is needed to help connect test results with appropriate management decisions.
Research reported in this publication was supported by The Organic Center and the Foundation for Food & Agriculture Research under award number TOCFFAR-EXT-002. The content of this publication is solely the responsibility of the authors and does not necessarily represent the official views of The Organic Center and the Foundation for Food & Agriculture Research.