Topsoil, compost and organic matter

Latimer · February 2021

Hi all,

I'm still trying to wrap my head around where and how these three elements fit together.

From my understanding, topsoil is inert and needs organic matter mixed into it to give it the nutrients that plants need and improve the structure.

In the ground, I always see advice to lay on our dig in manure but generally not to use compost because the compost somehow "disappears" ie the soil level would drop.

In pots I see it recommended to use a mix of topsoil or garden soil with compost, particularly when the pots are going to be a permanent feature, I'm assuming shrubs or trees. So why would you not treat it like it's in the ground and mix soil and manure?

I supposed the question then is, what really is the role of compost? It's it just an intermediary for younger plants that perhaps can't take the the "strength" of manure?

I'm not being very articulate, for which I apologise, but I'm just trying to understand the function of each of these things.

Thanks, and good luck deciphering the above mind dump! Lol

Palustris · February 2021

Topsoil is far from inert. Subsoil may be, but definitely not topsoil which is usually teeming with all sorts of micro-organisms.

Never seen any advice not to use compost on soil. Depends on what you mean by compost. Potting compost is not the same as compost made from decaying plant materials. Adding it to soil is good as it feeds the micro-organisms. It cannot lower the level of soil if it has physical bulk. Peat based potting composts are not as useful as a soil conditioning material as they are generally sterile. However they would still add bulk to the soil.

Manure is not generally added to potting mixes as it is a bit concentrated in terms of nitrogen etc. and could 'burn' plant roots.

The only reason why soil in a garden would drop is if you grow lots of bedding plants which are removed at the end of the season taking soil with them. Mind that would be put back if the remains were composted and that compost re-used on the soil.

steephill · February 2021

Topsoil is far from inert, it should be teeming with life. It is home to all sorts of microorganisms, invertebrates, fungi, bacteria etc. They will break down and consume compost which is why it disappears in the soil over time.

Long term pot planting needs a good structure which compost on its own will not provide. A compost only fill is also harder to keep properly hydrated.

Seed and cutting compost has very little in the way of nutrients, its job is mainly to provide an open structure to help roots establish. Adding fresh manure would cause problems due to the chemical reactions it undergoes as it ages. Well rotted manure would be more stable but seeds and cuttings don't need the nutrients yet. I think advice not to use it is partly based on making better use of it elsewhere in the garden.

It is worth remembering that many seeds get distributed ready wrapped in its own manure package courtesy of our birds and other animals but those seeds germinate the following spring so aren't sitting in fresh manure any more.

BobTheGardener · February 2021

The advice about 'soil level dropping' only really refers to using commercial multi-purpose compost to fill large planters and raised beds, as it is not really suitable for that type of use. If you do use MPC for those purposes, it will indeed lose 'bulk' over time and so the 'soil' levels in the planters drop and need regular topping-up. The nutrients which MPC manufacturers add when they make it, runs out after about 6 weeks, so you then have to also regularly feed anything planted in it.

Home-made garden compost is a different thing entirely, and it's a real shame that it is also referred to as simply 'compost' and this causes confusion. When you add either home-made or MPC to the surface of the soil, it will be taken down by worms and other soil organisms over time, so the thickness of the mulch layer you put on top of the soil will slowly reduce, but it won't lower the original soil level at all (in fact it will raise it slightly.)

Hopefully, that little tidbit together with the excellent info from Palustris and steephill above will make things clearer.

Latimer · February 2021

Ok, thanks @Palustris @steephill and @BobTheGardener.

I think perhaps my confusion does come from the interchangable use of terminology. So I'll try and ask the question a little differently as there's still a little confusion in my mind.

There's topsoil you buy in bags from the garden center. Is this the same as "topsoil" from the garden?

And as @BobTheGardener says, MPC from the garden center is not that same as what I have in the compost heap. So mine is a mix of plant, veg, cardboard etc left to break down over a year. I thought what was in the bags from the GC was the same except the use heat to speed up the process. Is that not the case? Is the MPC from the garden center billed up with other stuff (coir, for example, I think I've read) and also artificially topped with nutrients?

BobTheGardener · February 2021

MPC is generally sterile and may contain all sorts of things to provide the bulk, such as the traditional peat, or coir and, these days, a lot of shredded wood (which is treated to look black.) Slow-release nutrients are then added (these often look like tiny round eggs.)

Topsoil from GCs is also usually sterile and is made by mixing sand, silt and clay, so approximates natural soil but without the all-important microbiology present. To add the microbes, you add well-rotted manure or home-made compost, both of which are full of the microfauna and flora which makes a soil self-fertile and self-sustaining.

Edit: I'll just add that the diverse microbiology present in home-made compost makes it unsuitable for raising seedlings as there are also 'baddies' in it, such as the fungi which cause damping-off. That is why most of us use MPC and other more specialized composts for this.

Latimer · February 2021

@BobTheGardener so essentially you do have 4 different things all being referred to by 2 names! No wonder I'm confused but things are starting to make sense.

So, let me get this right: MPC is mostly a sterile substance such as coir or wood which, over time, is going to break down and therefore diminish in volume. The nutrients are purely provided by the chemicals introduced by slow release pellets. I could therefore "replace" MPC with a bulking matter, say topsoil, and my own home made compost to provide the nutrients, correct? (For more established plants, not for seeds or seedlings).

BobTheGardener · February 2021

Essentially, yes. Home-made compost doesn't contain a lot of nutrients (this happens later, as it is broken down by the soil ecosystem) so this is where adding well-rotted (ie already composted) manure comes in, as that contains a lot of nitrogen and phosphates etc. which is immediately available to plants, as well as containing those micro-organisms. The manure would be the equivalent of the slow-release pellets used in MPC, but will last longer.

Latimer · February 2021

Thanks @BobTheGardener, I think it's starting to make sense now.

BobTheGardener · February 2021

If it helps, I use a mixture of my own garden topsoil, home-made compost and well-rotted manure for all of those shrubs, trees and perennials which are grown in containers and can't think of anything which would be better. Anything which is only in a pot for a few months usually gets just MPC.

Latimer · February 2021

@BobTheGardener that makes complete sense and it's much clearer to me now looking at the coming year how I would use these different materials.

Thanks so much for your time and patience.

Topsoil, compost and organic matter

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