Where does soil/ compost go?

[Lady BlackThumb]

Possibly a dumb / Let Me Google That For You question, but it's gotta be asked.

This is in relation to soil/ compost/ organic matter, any granularish "growing medium" you add to your raised bed/ garden, which for brevity I'll just call soil.

Does the mass/ volume of soil decrease each season as you grow veg/fruit/flowers? Say if you had a raised bed, filled it evenly to the brim to grow in, by the end of the year does the level of soil drop quite a bit?

Asking because i'm thinking, I have X amount of space in beds, and if I keep composting garden/ kitchen scraps etc every year and add it to the beds, will I eventually end up with an overflowing mound in the beds? But then again, surely all those nutrients used by the plants must take up some kind of physical space in the world, you can't make something out of nothing..... but then again it's not like the even lawn in your garden will keep sinking lower and lower each year (since we don't add compost to lawns).

THis is today's procrastination-instead-of-working question. Thoughts from all you seasoned growers?

[Martin H]

Organic stuff will rot down and disappear. Of the rest of the soil, almost all of it stays where it is. Only a tiny proportion dissolves to feed the plants. Most of the mass of a plant comes from the air.

So if your beds are mostly soil, the only drop in level comes from settlement.

It's a good job really, or England would end up below sea level before too long!

[Thelma Sanders]

What he said!!

When I used to dig properly, every winter, after digging, the soil level was about 2ins higher than before. Adding a couple of bags of compost or manure each year to each bed, never really changed the level much either, 'cos it would all soon settle down after a few weeks.

[annie8]

I have noticed that the level in some of my raised beds has gone down. Even adding compost there is no risk that it’ll overflow. Think it’s a combination of the soil settling and probably me pulling out soil along with the veg when I’m harvesting.

[Lady BlackThumb]

Thanks y'all. My curiosity is sated.

[Jay-ell]

Organic stuff will rot down and disappear. Of the rest of the soil, almost all of it stays where it is. Only a tiny proportion dissolves to feed the plants. Most of the mass of a plant comes from the air.

So if your beds are mostly soil, the only drop in level comes from settlement.

It's a good job really, or England would end up below sea level before too long!

Either that or taller than Everest.

Soil is made up of organic matter and minerals - the compost and decomposing roots, leaves etc forming the the organic matter and stones, gravel, grit, sand, silt and clays forming the minerals. The different types of mineral components are graded by the size and the shape of the particles that make them up.

In addition to this there's the biological - worms, nematodes, fungi, bacteria, etc.

Soil will naturally "settle down" - gaps in between the different particles being squeezed together by the weight of the soil and by actions of water running through it. This reduces the amount of air in the soil and compacts it.

Several of the biological entities in the soil will eat the organic matter and give off carbon dioxide which will also lead to the soil level going down as some of the solid matter is converted to a gas.

The rate of this happening depends on the amount of oxygen the organics can get so a light airy freshly dug bed will have more air (and therefore more oxygen) trapped in it for the biologicals to use so freshly dug soil can have it's consumption of organic matter accelerated in comparison to soil that hasn't been dug.

In nature the majority of organic matter is added at the top surface and then distributed by the biologicals to the top level of soil -= the sub soil doesn't usually get much of this organic matter pulled into it as the biologicals like to live where there is air to breathe and food to eat so the sub soil tend to be more mineral in stucture.

The mineral component of soil breaks down at a far slower rate. It is weathered by rain and chemical reactions in the soil but it will take years/decades/centuries/millennia for a grain of sand to weather away. Organics in the soil can play a role in the weathering (e.g. soil passing through a worms guts or from acidic secretions from a plants roots) but this is still a slow process - if you have sandy soil it will always be sandy soil even with ongoing environment processes.

So how quick and how much will shrink depends on how much organic matter and how much air. Natural soil is only about 5% organic matter but about 45% mineral (the rest being air and water) if it's getting eaten away too fast you might need to raise the percentage of the mineral content (dig up and mix with the subsoil the first year or add sand/gravel/powdered clay/rock dust)

[Madasafish]

Erosion reduces soil levels naturally.

[SarrissUK]

Every time a worm munches on stuff in the soil such as coir and something not quite broken down, it reduces in size.

It's the same for humans if you think of it - we eat a much larger volume of food than we produce the other end lol

[Lady BlackThumb]

It's the same for humans if you think of it - we eat a much larger volume of food than we produce the other end lol

Oh dear lord. ��