By Sally Brown, University of Washington

Abstracts of these resources are available in the searchable Information Portal offered to Northwest Biosolids members. 

  1. Compost: It’s role mechanism and impact reducing soil borne plant diseases.
  2. Disease-suppressive compost enhances natural soil suppressiveness against T soil-borne plant pathogens: A critical review
  3. Had compost biochar added during process added value over biochar or compost increase.
  4. Exposure and risk assessment of Salmonella in recycled residuals
  5. Cascading effects of composts and cover crops on soil chemistry, bacterial communities and the survival of foodborne pathogens

I got my second shot of Moderna today and feel like a huge weight has been taken off.  Hope all reading this are well on their path to immunity as well.  Based on the state of the world and a new paper that has Kate Scow’s name on it, I decided that it would be appropriate to have a library focused on how organics can help fight against soil borne pathogens.  Composts as the Moderna for the soil.  Just an aside here- I got the Moderna because that is what they gave me.  Compost could easily be the J&J or Pfiezer for the soil. 

Sally Covid Shot
Right after by 2nd vaccine

The vast majority of the work on compost and pathogens has been on the bugs that hurt the plants.  That is the focus of the first three papers.  The new stuff (from UC Davis) goes into how this can work on the bugs in soil that can harm people).  That is the focus of the last two papers.  Skip to the review of Paper #4 if that is the part that interests you.


Plant pathogens

I’ll start with plant pathogens.  Although none of the articles in the library are authored by Harry Hoitink, it has to be recognized that he is the scientist that started this whole field (  Dr. Hoitink, now an emeritus professor at Ohio State University, was the first person to study the disease suppressive properties of certain composts.  Instead, the first two articles are reviews that both site Dr. Hoitink’s work.  The first review from 2014,  is more basic than the second (2020)and provides good background.  The authors talk about the range of pathogens and the range of studies that have shown that composts can suppress them.  They quote one study suggesting that the overall success rate in disease suppression is 54%- so a little worse than the AstraZeneca vaccine.  They then go into detail on the potential ways that this happens. 

  • First is competition- the good guys that show up and outcompete the bad guys for nutrients, essentially starving them out.    
  • Second is that the good guys have special weapons to get the bad guys (technical term here is antibiosis like antibiotics) and the weapons here are not special lasers but rather secreted compounds.   
  • Third is direct attack- good guys against bad guys (the technical term for this is hyperparatism)
  • Fourth is system acquired resistance (SAR) that reads a lot like having antibodies

A diagram from the study gives a nice and easy to follow version of this:

diagram 1

They summarize the process:

In conclusion, the various mechanisms involved in compost based disease suppression suggest an important role of the microbial communities present in composts.

The article then goes on to discuss the different types of microbes involved it the different stages of composting and then methods to differentiate different microbes.  A good basic primer.

If you read the second paper, I would recommend more skimming than in depth reading.  The author appears to know his stuff and presents some excellent information but English is clearly not his first language and the paper is filled with not quite sentences that ramble.  Not quite a page turner.  The author points out that the market for disease suppressive compost is largely driven by organic farmers.  He also points out that this presents a great opportunity for those concerned with developing a circular economy.  He then notes that one of the primary mechanisms here that make this approach successful is that it results in a large and robust community of organisms in soils that leaves much less room for the bad guys.   

picture 1
Dead lettuce from the Sclerotinia fungus.  I saw this first hand during my MS degree

He points out that the compost has to reach thermophilic temperatures to achieve pathogen and weed seed kill.  This will also kill all of the microbes in the material that would survive in a soil.  Microbes will seed the material during the curing phase that may survive in the soil.  When added to the soil however, the compost will do a number of things that make the soil more hospitable for the good guys.  Stuff you know about- better structure, better water holding, better air flow in addition to more nutrients and also an increased food supply.  A soil with compost would rate 5 stars on the soil microbiota VRBO equivalent.  Without- maybe 2.  This goes for the plants as well as the microbes.  The healthier soil environment means the plants grow bigger and stronger and are better able to fight off pathogens.  Showing how much we don’t understand about this process- he points out that efficacy can vary based on application rate, frequency of application, particular pathogen and compost properties.  Basic conclusion is that it usually works and it works for a variety of reasons.  Sometimes it doesn’t.  There is a three- page table listing a range of studies that reads almost like a recipe book.  It includes the recipes for exotic composts, the biocontrol agent (weapon of choice), target bad guy, and the host plant.  There is also a discussion of techniques used to measure microbial actors and diversity. 

From reviews in #1 and 2, we go to ‘Can char make it better in article #3.  Here the authors tested biochar and compost separately as well as in combination to see what the impacts would be on basal rot for lettuce and potato cyst nematodes for (you guessed it!) potatoes.  Short answer here is adding the biochar to the compost made the compost less effective. 

Soil pathogens and human health

Pathogens in soils don’t always hurt plants.  Sometimes they can hurt people.  Much of the focus on composts and pathogens has been on the potential for pathogens to regrow as compost is cured.   Kill them during the thermophilic phase and then a few mammal droppings later, they come right back.  Paper #4 is a risk assessment about salmonella regrowth in biosolids and biosolids composts.  Awhile back the issue of regrowth got people really worried.  Here some of the real pros in the field (Pepper and Gerba) go into detail about regrowth potential for stored biosolids and biosolids composts.  Both direct contact and airborne particles are evaluated.  Short answer is don’t worry unless you store your compost in a pool (under anaerobic conditions). As for animal manures that have not been properly composted- that is a different story.  The library from December 2014 goes into detail about human illness as a result of improper manure applications.

picture 6
Just one of the many produce recalls due to pathogen contamination 

Finally, we get up to paper #5.  Here the authors monitor pathogen survival in soils amended with composted poultry manure where cover crops are also used to increase soil health.  These are the same plots that showed that carbon storage only occurred through the soil profile when compost was included (October 2019).  Cover crops alone were not sufficient to store carbon.  Neither was fertilizer addition.  Here we see a different benefit of the compost.  The authors monitored survival of Salmonella enterica and Listeria monocytogenes in these soils.  The authors justify their study by referencing the high number of illness that have been associated with these two pathogens (and E coli) when untreated animal manure is used on soils.  They point out that many farmers have stopped using both untreated animal manure (OK by me) and compost because of fears of contamination.  They point out that using compost- rather than being a source of illness, could kill human pathogens in a similar way that it has been shown to kill plant pathogens. 

The group collected soils from the field trial and brought them to the lab for testing.  The samples were inoculated with the pathogens and then monitored for changes in both pathogen numbers and microbial populations over time.  The soils were also characterized for total organic matter and available nutrients.  This is what they found (if you’ve made it this far in the library, you rate bullet points instead of text):

  • Nutrients were highest in the compost/ cover crop treatment
  • Microbial populations were more diverse in the compost/ cover crop treatment
  • Populations of the pathogens were lowest in the compost/ cover crop treatment at the 10 day time point
  • Over time, the differences in pathogen populations in the compost/cover crop treatment and the fertilizer treatment disappeared


In other words, the same way that compost enriched soils defeats plant pathogens, it also defeats human pathogens.  Appropriately treated composts are a way to prevent disease, not a cause of disease.  These days when we are all hoping for herd immunity in the human population, we can take some solace in knowing that our appropriately used organics can provide that to soils.