This is the its-too-good-to-be-true column about the benefits of compost I’ve always wanted to write. So, cut it out and paste it in your gardening scrapbook. For years, I’ve been adding compost to our gardens on a regular basis and if you’ve been reading this column for any length of time, you’ll have seen the regular advice – add compost – to solve many gardening problems.
Why Do So Many Gardeners Have Problems I Never See?
I’ve never really been able to understand why so many people have problems I never have with pests and diseases. The answer is in the soil and in the large amounts of compost we add every year. This column and the next one is going to detail some of the research on what exactly compost is doing down in the soil.
To begin with, Dr. Harry Hoitink, a plant pathologist at Ohio State University is responsible for much of the research in this field; this is not some wool-tie-dyed hippie out in the garden dispensing advice but is research based on scientific studies.
Secondly, much of the soil’s health and disease resisting properties are found in the micro-organisms(good guys) that feed on soil-borne pathogens (bad guys) and promote plant health.
How Do We Get A Healthier Soil?
The question is invariably asked, how do we get more of these little micro-organisms or mycorrhizae in our gardens. Compost, my friends, compost feeds and nurtures these tiny creatures and the more compost you feed your garden soil, the more mycorrhizae you’ll have.
The more mycorrhizae you have, the healthier your garden plants will be. You see, the mycorrhizae are already in your garden but if they have nothing to feed on, (compost) their activities will be limited. To increase their numbers, you simply have to feed them and they’ll do the rest naturally.
Compost Suppresses The Bad Guys
Some pathogens such as Pythium and Phytophthora (two of the most common for stem rots and damping off diseases) are affected in other ways. These are suppressed by competition for nutrients. The more beneficial microflora you create by adding compost and organic matter (peat moss is particularly effective against Pythium) to your garden, the fewer problems you’ll have with diseases created by Pythium. The term for this is “general suppression”.
You add the compost and in the resulting competition for its nutrients, the bad guys lose to the hungrier good guys so the bad guys don’t thrive and bother your plants.
Compost also suppresses diseases by producing antibiotic type compounds. Some of the biocontrol agents that colonize composts include bacteria like Bacillus, Enterobacter, Flavobacterium balusstinum, and Pseudomonas; actinomycetes like Streptomyces; and fungi like Trichoderma and Gliocladium.
Now I confess I wouldn’t recognize an Enterobacter if it came up and bit me but I can guarantee you that when it gets into your soil, it is going to do some damage to the pathogens trying to eat your plants.
So, not only does compost help the good guys to compete against the pathogens, it also actively suppresses some forms of pathogens. Some of these compounds are also growth promoters as well as pathogen suppressers.
So, by adding compost, you not only get rid of the bad guys, you add a natural growth promoter to the plants.
Different Composts Work In Different Ways
One of the surprising things the research is showing is that different composts and the methods used to make them influence their effectiveness.
Compost varied in both the degree of suppression and the length of time this suppression lasted.
In many ways, homegrown compost produces the best material for the garden.The research showed that mature composts – those that are allowed to complete their cycle after the heating phase – were more suppressive than composts packaged or used just after the heating was finished.
Compost Bins Open or Shut?
Compost piles that are in the open are more effective than either plastic bins or covered bins because the microbial organisms can colonize them easier.
Those piles with open bottoms and tops that are next to trees (tree roots and leaves support fantastic microbial colonies) have some of the best levels of microbial action of any production system.
Does adding compost to containers do the same thing?
The short answer is yes it does.
With container growing, adding compost to the soilless mix (if you’ve been reading my column you’ll not be using real soil in your flowering containers) creates the same level of pathogen-fighting activity that is found in garden soil.
I do know growers who add up to 50% of the soil volume as compost but the majority seem to stay down around the 10-20% range.
I add compost (about 10-20% of volume) to the soilless mix every year so I reuse the mix in my containers.
Do not sterilize your compost if you are adding it to the container.
Sterilizing compost kills all the beneficial organisms and defeats the entire purpose of adding compost to the mix.
If you are mixing your own soilless mix for containers using peat moss and compost –throw in 10% perlite and you have a complete mix (80% peat, 10% perlite and 10% compost)– also do not sterilize the peat or composted bark chips.
Again, sterilization kills the beneficial microorganisms.I understand that it is likely only the nurseries that will be making their own potting soil for containers, but if I was purchasing a soilless mix for mine, I’d buy the best soilless mix I could find and then add my own compost to it to make it perfect container soil.