What is the difference between compost and soil?

Question

This question has been asked many times on the net and the information provided is unreliable and mostly wrong.

What I am aiming to understand is what is lacking in compost that we can't just add it to make it the same composition as a rich soil?

• Is it the amount of time that is needed to completely break it down?
• Are there some special ingredients that are built below the surface that can't be organically added to the compost mix?
• Is there some special conversion process that happens underground that makes it different from compost?

Answer 1

As with any complex systems, there are no universally accepted definitions, so let's see the differences from this points of view:

• soil a) = naturally occurring granular covering on the surface of Earth, capable of supporting life
• soil b) = substrat plants are growing from
• compost = partially decomposed organic matter, human-made fertilizer and soil amendment

I hope it is clear that from this point of view, "compost" is one of the things you can add to "soil" to make it "rich" and not the other way round.

But well, if you really wanted to cover the ground with glass and put some compost on top of it, the "things" / components / elements to add to make a rich soil from it would be:

• inorganic matter - plants need more minerals to be healthy than just from compost that contains mostly carbon and nitrogen compounds
• soil life - bacteria that feed from plant's roots, roots themselves, fungi, worms, ...
• keeping the right water / air composition (compost is usually just right except it lacks anaerobic layers with anaerobic bacteria further down from the surface)

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Also soil is much more open, complex and stable system - part of an ecosystem, organic matter is dynamically renewing from dying roots, fallen leaves, ... - than compost - a gardener creates it, it has several stages and finally adds it to soil and it becomes soil.

Answer 2

(Expanding my comment a bit. Disclaimer: I'm not a soils scientist, just a guy who reads about gardening and related topics a lot.)

I commented:

Pure compost would be nearly 100% organic matter. Soils have an inorganic component (minerals, rock particles, etc). You could build "rich soil" by mixing compost, sand, silt, clay, etc in the right proportions, depending on what kind of soil you want to build.

There are tons of different kinds of soils on earth, and they can be divided up in numerous ways. Sand vs Silt vs Clay (i.e. by particle size). Organic matter content (low vs high). For growing purposes, we often also talk about soil pH (acidic vs alkaline).

It's very much a possibility to "design" a soil that suits your needs. See my favorite recipe for potting soil, which mixes garden soil with sand to get the desired grain size (for drainage purposes) and then adds compost (for plant nutrients and organic matter), lime (to raise pH), and mineral fertilizers (plant nutrients).

You could do the same thing on a large scale by having three dump trucks swing by your house and deliver one load each of composted manure, sand, and topsoil(*). Add a few bags of lime and some mineral fertilizers and then all you'll need is a bulldozer to mix it and spread it around your lawn.

(*) "Topsoil" is kind of begging the question, I guess. You could get really specific and order up precise quantities of sand/silt/clay in the desired proportions. When I'm mixing potting soil, I just go by how it feels in my hands so that it's the "right" consistency.

Answer 3

I like to think of this more in terms of functions:

• When you say 'soil', think geology, as in the earth has many types of soils, clay, rock, sand, etc. Largely mineral. The top layer (top soil) "should" be where plants grow and therefore it "should" contain organic/biological matter.

• However, humans have been removing topsoil for the sake of having a firm stable, mostly flat surface for building. Compost is stuff humans make by putting various kinds of organic/biological matter together to replace the top soil we systemically remove. If we didn't remove the top soil and left plants where they grow, we would not need so much compost.

• Compost by itself is not enough for plants to grow in. Compost has a function wherein it contains the invisible microbial lifeforms which can break down the structure of hard, mineral-laden soil (think geology again) and render it into something plants can take up by their roots, leaves, etc. That function would be the work of topsoil if we left it alone, but since we removed it we need to replace it with something like compost.

• Not all compost works as well as top soil because it needs the microbial life to do the job. Some compost doesn't provide a very good home to the microbes and so fails to work as a replacement for topsoil.