Critical Management Point: Soil Type

Critical Management Point: Soil Type

Transcript (with additional commentary)

Soil is its own living, dynamic ecosystem composed of mineral and organic solids, gases, liquids, and living organisms, which is capable of supporting plant growth.  There are many types of soils, with properties resulting from the integrated effects of climate and living organisms acting upon parent material, as modified by topography, over periods of time. It can take 500 years to form just one inch of topsoil!

Each soil type has different ratios of sand, silt, and clay that determine the soil texture, and impart various properties to the soil and help determine how it functions. These properties include drainage and aeration. Organic material, like roots and decaying matter, along with other factors, help determine soil structure, water and nutrient holding capacity, and how well it supports the growth of plants.

An acre of soil can hold 5-10 tons of living microorganisms.  One gram of soil can hold as many as 5-7,000 bacteria species, and more than one billion individual microbes!  The bacteria and other microorganisms are necessary to break down dead and decaying matter and turn it into usable energy by living plants.

Soil has several essential roles. Healthy soil holds more water, reducing runoff and evaporation. The productivity of all ecosystems—whether agricultural or natural–depends on soil health.  Soil minerals act as a filter, while soil microbes degrade and detoxify a variety of materials, including some industrial waste products. Nutrients are stored and cycled through the soil. Finally, soil structure provides a scaffold for plant roots, and support for our houses and roads.

In our research, the third critical control point examines the role soil plays in the fate of antibiotics, antibiotic resistant bacteria, and antibiotic resistance genes: Does the kind of soil to which you apply the manure or compost matter?

Previous studies, including research here at Virginia Tech, have shown that land application of manure can increase the persistence of some forms of antibiotics and increase antibiotic resistant bacteria and antibiotic resistance gene levels in soil, but little is known about whether composting can mitigate this effect or if soil type can buffer any impacts.

Some studies show that soil type determines whether antibiotics and genes are degraded or allowed to persist.  However, those antibiotic-soil interaction investigations have relied on adding antibiotics directly to the soil, which is generally not a good representation of the real manner in which antibiotic residues are incorporated into soil as part of manure.

In a bench scale microcosm study conducted by Dr Xia’s team, compost samples from the previous composted manure study were amended to three different soil types and incubated for 120 days. Samples were collected at specific intervals during this period and stored.

In a greenhouse study, lettuce, broccoli, and radish seedlings were grown in loam or sandy soil with four treatments: compost with antibiotics; compost without antibiotics; raw manure with antibiotics; and a control with chemical fertilizer. After growing for 3 months, soil samples were collected, examined, and analyzed.

Finally, the field study, conducted at Virginia Tech’s Urban Horticulture Center, entailed growing lettuce, and broccoli in the same loam soil as the greenhouse study, with plots for each of the same treatments.  Samples of the soil were taken from these replicate plots and analyzed.  Stormwater runoff from each plot was also collected and analyzed for sediment, indicator bacteria, and antibiotic resistance genes to determine whether these were moving downstream to other fields.

State-of -the-art tools were used for each of the studies to determine the fate of antibiotics, antibiotic resistant bacteria, and antibiotic resistance genes.
This study will provide guidance on whether certain soil types amended with composted manure have an effect on the transfer of antibiotic resistance.

Glossary of Terms

  • Antibiotic resistance – Antibiotic resistance is the capability of some bacteria to survive antibiotic treatments. Some bacteria are intrinsically resistant, for example an antibiotic that specifically targets Gram positive bacteria will not affect Gram negative bacteria. However, bacteria can also acquire resistance, either through mutation or through sharing of antibiotic resistance genes, in which much higher doses of the antibiotic are needed to have the desired effect.
  • Antibiotic resistance genes – Pieces of DNA carried by bacteria that encode functions that allow bacteria to survive and grow in the presence of antibiotics.
  • Antibiotic resistant bacteria – Bacteria that are not controlled or killed by an antibiotic with a spectrum that includes that type of bacteria.  An antibiotic spectrum is the range of bacteria susceptible to that drug. (CDC, Antibiotic Resistance,
  • Attenuation of antibiotic resistance genes – Reduction of the DNA encoding antibiotic resistance capabilities with time.
  • Field Study – Data collection in a natural field setting or environment.
  • Greenhouse study – Data collection in an environmentally-controlled greenhouse setting.
  • Microbial ecology – The study of microbes in the environment and their interactions with each other. Microbes are the tiniest creatures on Earth, yet despite their small size, they have a huge impact on us and on our environment.
  • Soil-A living, dynamic ecosystem composed of mineral and organic solids, gases, liquids, and living organisms, which is capable of supporting plant growth.
  • Transfer of antibiotic resistance – A complex process involving sharing of genes between bacteria that code for bacterial processes conferring resistance to the function of an antibiotic.


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