I think the big question here is when you say "compost", what compost? If comparing to bagged purchased stuff, it's probably just going to be lower quality - lots of bulky filler because that's how bulk compost sales works. Yes, vermicompost (which I'm assuming here means mostly worm castings, not just compost with some worm castings in there) will be more concentrated, but perhaps this advantage is moot because it's frankly hard to get in very large quantities. I'd say use whatever vermicompost/worm castings you can get, but don't pay a whole lot or expect magic.

My overall answer is 'much preferred' only really works if you can get the quantities you need; might be worth it in small plots or pots, certainly worth trying. The other frequent claim about worm castings is that they contain a lot of active bio-organisms (good ones) that are good for soil. I think one can allow that argument but still be cautious that some of the claims might be overstated - it's good stuff, but it's not magic. (Worth noting that vermicompost will often have cocoons mixed in that will hatch when conditions right and that may be a separate soil/biological benefit if they're residing in the mulch layers and doing their thing).

The claims made about pathogens in vermicompost seem to be that the worm digestion process mostly neutralizes pathogens (the worms have some biological defenses) or that 'good organisms' dominate in the worm gut and castings and pathogen levels low (which is largely how healthy soil works too, there are some pathogens but 'good ones' dominate). I haven't seen any studies on this - probably both factors are at work to some degree - and there don't appear to be widespread complaints about 'pathogenic worm compost.' My own personal guess is that a big part is by avoiding anaerobic conditions that benefit pathogens in the worm bins (very bad for worms - mass die-off bad - and the worms aerate some by physical action; if it was anaerobic, you'd smell it immediately).

My parents kept worm bins for years and they and friends praised the worm castings for plants - but they didn't have many other sources of good compost. I did too when didn't have the space for a compost bin and enjoyed it. Now I'm lucky that our backyard compost bin is a megapolis of compost worms and requires no work; I don't try to separate in any way. I think that's ideal - as much bulk volume as needed and it wouldn't make sense to 'add' vermicompost, it's in there in the finished compost anyway. But I'm lucky with that - not everyone will have the yard space.

(By the way the other advantage of this approach - almost all of the things that vermifans say are bad for the worms or to avoid don't apply in an outdoor bin; worms leave those things alone until they've rotted enough. They don't like fresh citrus peels or onion bits, for example - but do just fine with them when rotted.)

Hope this helps.

@armoured I buy compost by the square yard from reputable garden wholesalers. I don't buy it by the sack. Almost all of those wholesalers describe the product as a hot composted product of garden waste. That's a bacterial decomposition process, not reliant on any organism like a worm.

What would be the "filler" that they would add to this product and how do I ask about that?

I think the main point on pathogens is that a hot composting process kills most pathogens by heat alone. Vermicomposting is not a bacterial process, and therefore if you start out with pathogens nothing is there to kill those off as the worms process the material.

This may all be mute if there is no good commercial source for vermicompost in bulk. But I am still interested in the effect of each type of compost on growth of plants when used as a top dressing.

For effect on plant growth - I think you like experimenting, best thing would be to try.

By "filler" I mean mostly the usual stuff, shredded wood and bark and whatnot. Nothing wrong with that stuff - but it's different input from the worm diets, less concentrated.

I'm not going to argue details of what's bacterial vs fungal vs 'organisms' - but a lot of what digesting worms do is bacteria in their guts and what's in the pile (and their castings contain bacteria to speed up the process of pre-digesting for them, too - they create their own buffet, so to speak). That is, worm composting/digestion is also bacterial, and fungal, and partly mechanical. Hot composting just has a cycle where heat-loving bacteria dominate for a while, and then after that, there's a curing phase that I understand is mostly fungal, but also bacterial.

I'm not clear what pathogens
you mean. If you're worried about pathogens dangerous to humans, that's
one issue; if you're worried about specific plant blights, etc., that's
different. If you're getting compost from a facility that deals with
yard and wood waste and you don't want your plants to get an endemic
local blight - you definitely want hot composting.

But 'worm compost' is more often from household food waste or some specific product (spent grains or must/pomace, for example), not leaves and stems from yards. So there
may not be any of the yard pathogens you're concerned about to start with.
Does that make sense? At any rate, neither of these are sterile; vermifans want more of the 'good' microbes and think worm compost has a lot of them.

I do disagree on saying that there's 'nothing to kill off those [pathogens]' in cold composting/vermicomposting. There's no heat to do so, but nature has plenty of ways to kill/reduce them (mostly competition, they get eaten by other organisms/microbes, but also many don't do well in aerobic conditions); whether that's sufficient for your purposes/comfort is a distinct question. It's definitely not a sterile output, of course.

Of course, weed seeds are a different matter. No simple answer on that - you'll see some vermifans claim that most seeds don't make it through a worm bin viable, but some do (may depend on inputs as most worm bins aren't being used for yard wastes with weed seeds). Tomato seeds for example definitely stay viable. If the worms aren't being fed yard waste though, probably doesn't matter.

@westes - I apprehend that my 'filler' comment may have sounded pejorative, wasn't meant that way. It's just the way hot composting works - what's left is going to be the tougher woody waste, that doesn't break down in hot composting. Thermophilic bacteria can't break down lignin. Worms can't eat that stuff either. It's also good stuff for soil, just different.

@armoured The compost I buy is usually a pure powder. I guess they are filtering it by size and discarding the large pieces that did not or could not process.

@armoured I think you are still missing the essential component of a hot versus cold process. In hot compost, the temperature is often above 130F, and this is where many bacteria quickly die off. At a minimum, they get outcompeted by other bacteria that can survive those temperatures. So - for example - if there is a local blight in the feed material for the compost, that gets killed. This affects most of the pile, so it is a very comprehensive kill-off.

In a cold process using worms, those bad bacteria or fungus may never get ingested by the worms. They may simply find an ecological niche to stay alive somewhere in the pile. Because the pile stays cold, the bacteria or fungus may stay alive for a long time.

You have made the point that vermicompost is going to come from the organic waste inside the home, so that issue is avoided. But if I were able to find an outside supplier for vermicompost, I would have no such guarantee.

There are beneficial fungi and bacteria that fight plant disease that are killed off in a hot pile at the same time.

@westes, thanks, I do get the point and the process. Hot composting definitely kills off more pathogens and bacteria. But worm composting also kills off pathogens. So does (aerobic) composting generally, even cold. The question is, how much remains and is it a problem? What types of pathogens? There are plenty of pathogens present in soil usually, but in numbers that don't represent a risk (depending on the specific risks of course and environment for them, etc). That's the point that vermifans make - done right, conditions are good for good microbes, bad for pathogens, and what remains is 'safe'. You may or may not find that convincing - my point is it's basically a balance of risks. If you're really concerned, go for the hot composted product.

You're quite right that if you get vermicompost, you may not have a good idea of what was in there and whether you should be concerned about it. The 'protection' is that the vermicompost process doesn't lend itself to composting yard waste with the types of blights you'd be concerned about - but you're right, that's no guarantee unless you get reassurance about how it's made.

There seems to be the impression sometimes that hot composting is akin to
sterliization or pasteurization - I'm just pointing out that of course
there are lots of bacteria and fungi that survive, and then after hot
composting there is typically a cure period of a month or more - and of course that pile gets recolonized. Does that include 'pathogens'? Probably. Probably not enough to worry about. Probably not of the specific types you might be concerned about, like various yard blights. But it's by no means a sterile product.

Come to think of it, perhaps pasteurization is a reasonable analogy - kills off most microbes but not all. At the first opportunity (temperature, exposure to air, etc), they'll be back. Even sterilized products will get recolonized very quickly.

@annpat makes the point that hot composting kills off both beneficial fungi and bacteria, too - it's a good point. That beneficial aspect is part of what vermifans claim.

I think some of those claims are a bit overdone, but fair perhaps to say that those who use vermicompost don't typically find pathogens to be a big problem.

Something that always catches my attention with worm castings is the claim that they are more nutritious or somehow have a higher benefit compared to compost (managed decomposition). Neither route is absolute, in that composting happens in worms bins and worm action (all sorts of wild action) happens in a compost bin. But if we said all things were equal and you filled a bin of chopped leaves, coffee grounds, banana peals, and some old cheerios and let hot composting occur to decompose the material… and then fed the exact same materials to worms... what difference would there be in the final product?

I could see some off gassing occurring during the hot composting process but in general… my brain would say that the final completely broken down product is identical between the two. Worms can't add anything to the process physically. I would also think that anything that develops uniquely during either process (different bacterial or fungal growth) will soon disappear once either process is completed and the material will find a natural state.

It may well be that in large garden composting, there's a lot of time spent on small differences; welcome to the internet.

I agree with you overall that mostly, in the end, compost is compost and gets to more or less the same place. And some of the claims about vermicomposting are overdone, with some magical thinking involved (true about other compost type claims, too, of course).

That said: 'all things being equal' is doing a lot of work there. The truth is, things aren't always equal - they're different processes with different advantages and disadvantages, and (usually) somewhat different inputs.

As I understand it, there are good reasons to think e.g. worm compost/castings have somewhat higher nutrient load (possibly only marginally though), and different mixes of beneficial microbes that can be good for soil - as well as containing some live worms, cocoons and other critters that have some benefit as well. Some of these differences are even measurable (nutrients more so than microbes) - although inconsistent because each worm farm and garden is different, too.

But way more important than this is what you're comparing to what - dried peat is a a thin gruel and a different matter than a varied backyard or municipal compost, etc. Wood chips are great, bulky, durable, and have the lignin that sticks around (other compsots disappear much more quickly) - but clearly have close to zero nitrogen.

So hey, I'm not ideological on this. Try different things and see if it works for you. Some are just different aesthetically. Personally I don't worry about this that much - I use what I have, mostly leaves, wood chips and household compost bin (some yard stuff) where the worms rule. You can feel and smell the difference quite easily - the bin compost is dense and clearly loaded, it goes in beds for our favoured plants. Leaves go a bit of everywhere when shredded, drier ground and lawn and under various plants. Wood chips - everywhere but the lawn really but preferably top layer, bare ground, under bushes, places where durability matters more.

@westes - interesting that the compost you buy is almost a pure powder. I presume they either start with very fine inputs (or that break down quite fine like leaves), or mechanically shred at various points, or (perhaps most likely) filter out/separate for different purposes/markets. Most compost operations I'm aware of make use of a lot of shredded wood/wood chips - to keep smells down, get the hot composting right, and aeration - and 'end product' usually has at least some visibly larger bits in it. At least that I've encountered, not that I've done a whole lot of comparison shopping (mostly use what I have to hand).

A question: are you most concerned about plant pathogens that would be bad for your garden, like specific blights etc., or a more general concern like pathogens that could be bad for humans?

I use compost mostly to improve soil texture and ability to retain moisture,castings more so for neutrents and neither as mulch. I seldom give patagens and bactiria a secound thought. I had just responded to a newbie composter before seeing this discssion. Here's what I told him.

" I place material in a 3'x3'x4' wire cage as material (mostly browns) become available. Just toss it on top,sprinkle with water and go about my business. About once per week I remove rotted material from bottom of wire cage and add to a 2'x2'x3' bin where I mix in greens,kitchen scraps , small quinities of worm castings and maintain moisture. Once per month,I dumo barrel into a wheel barrow then refill barrel from bin. Worms move away from surface within 2 or 3 hours so that a layer of compost can be removed to secound wheel barrow,bags or buckets. By day's end layers have been removed leaving a cluster of worms in a gallon of compost to be sold or dumped into bin. Sound's like a lot of work but it require's less time than rolling half a dozen barrels every day."

So i guess I'm on the fence trying for best of both but that's just my nature. The only thing I've had problems with is potatoe diseases carried over in compost. I've often used store bought potatoe peels to grow new potatoes in the compost bin before seed potatoes came available. Worked great until one year vines wilted just as they bloomed. The next two crops failed as did those planted in garden where compost had been incorprated. I have only planted a few in years since just to see if the disease was still in the garden and after 6 years,it is. I once bought several bags of compost at a neighbors request to use in his new bed. He backed out and left me with the bags which I stacked outdoors with plans to resale. The bags deterioated and nutsedge sprang form every crack in bags. I believe that prove's commercial com[post is like Forest Gump's box of choclates and reason enough to make your own.

@armoured I would be worried about passing diseases to any of the plants in my yard.

I just needed to add some more compost to my yard, and before I ordered I wanted to be sure I was not missing a better alternative. The fact that vermicompost is not easy to source in large amounts by itself rules it out for my yard. I buy a few cubic yards of the hot compost and that lasts me for at least half a year.

Yes would be pretty hard to get that kind of volume, I should think. Keep your eyes out for some to experiment maybe, or as a modest supplement, but wouldn't easily replace what you do now.

My parents used to sell some at church charity rummage sales; ziploc bag for a dollar or two. Not saying that was a real market but volumes were a different order of magnitude entirely. I guess people used them on their potted flowers and the like.

Gardengal, you are an incorrigible cynic. When one buys magic beans, only a handful is needed to ascend the beanstalk unto the heavens.