Grit I have used in pots for mediterranean plants is a fine grade horticultural grit. Good for top dressing of succulents etc too.
In the raised bed, I used a courser grade grit. I suppose it also depends on how long you are willing to wait before your soil improves with your continued efforts and how much of a budget you have..
Fallow, a forks depth of decent soil isn't that bad, in fact pretty good. 75% of my customers garden on clay, so a lot of my time is spent improving soils. For most plants a forks depth of soil is enough to allow plenty of root growth and good health. I suspect your builders filled your bed with subsoil, which by it's nature is usually a cloying mix of clay and compacted matter, usually about 12 -18 inches below soil level. So far you have about a foot of decent soil, so that's not bad, many people garden on a lot less.
Top soil incorporated into clay doesn't break the clay down, the soil particles aren't big enough to break the clay particles up, they just combine and can make things worse.Thats why you need grit, organic matter, sharp sand to incorporate, as their particles are bigger than the clay particles, and the organic matter is food for worms and micro organisms which help break up the clay particles and allow air into the soil which when all combined gives you a good loamy top soil.
If you add more organic matter every year, you will find that the depth of the top soil will gradually increase, but you'll never get more than 18 inches of loam as worms are only really active in the top 6-10 inches of soil, anything below that is the result of micro organisms at work.
From your later description of your bed, I'd get hold of as much organic matter as you can(if it's shop bought, a bag per square metre) and dig it in. Then repeat every Autumn by top dressing with the same again for a few years. By then you you'll get more than a forks depth of decent loam.
"decent sharp sand is full of grit. with very little sand."
Well just buy grit...as you said above, sand is not "big enough to break the clay particles up, they just combine and can make things worse" Hence there is no reason to ever add sand to clay soil.
Sand is also good at keeping water, so terrible in winter when around roots - you can build sand castle with damp sand because of this property.
Gravel and bark and compost is best thing to break down clay soil.
You can take out some of the clay but be careful not to create a sump. I think the most important thing is to break up the surface of the clay base with your fork. You will find that no matter how much mulch you pile on, it soon settles down to the original level and you will be tipping on more. I don't know where it all goes, but it does.
if you can break the clay into lumps and let the frosts at it, the clods with break up nicely as the ice in them breaks them up, then straight after some hard frosts mix organic matter and sharp sand/grit in, this will help the clay stay broken up.
Thanks for all your advice. I have dug a hole of about 1foot depth. I filled it with water and it took about 2 minutes to drain. I tested the PH and it is alkaline.
The compost you see is what the builder added when I complained about the soil quality. when you get to the bottom of the hole it looks like liquid concrete and I am very worried about my ability to cope and grow things in it. Is there somewhere I can get advice on it? I am wondering if this is fit for purpose? Am I worrying unnecessarily?
Given i am currently overhauling my (predominantly) clay soil through the whole of my garden, id say that you don't have a drainage issue if that volume percolated within 2 minutes!
I've dug out all my lawn borders to at least 300 mm, and disposed of material. Im going to incorporate manure within the top 100mm of existing, and import new soil. I'm going to town with the garden, so might as well start as i mean to go on.
Had i not have gone to that extreme, in your position, i would turn over soil to a minimum depth of 300mm, remove any large clods, incorporate any form of soil improver and manures etc, and job done.
I feel pretty sure that if it is a raised bed and you add lots of good stuff it will be fine. When I started trying to improve my clay soil I dug out and turned over and exposed to frost and all the rest. It nearly killed me. On later projects I just broke up the subsoil so that it wouldn't form a 'pan' and dug in every bit of compost, grit and manure that I could get my hands on. You wouldn't be able to tell the difference, now, but it took a fraction of time and pain.
Posts
Grit I have used in pots for mediterranean plants is a fine grade horticultural grit. Good for top dressing of succulents etc too.
In the raised bed, I used a courser grade grit. I suppose it also depends on how long you are willing to wait before your soil improves with your continued efforts and how much of a budget you have..
Fallow, a forks depth of decent soil isn't that bad, in fact pretty good. 75% of my customers garden on clay, so a lot of my time is spent improving soils. For most plants a forks depth of soil is enough to allow plenty of root growth and good health. I suspect your builders filled your bed with subsoil, which by it's nature is usually a cloying mix of clay and compacted matter, usually about 12 -18 inches below soil level. So far you have about a foot of decent soil, so that's not bad, many people garden on a lot less.
Top soil incorporated into clay doesn't break the clay down, the soil particles aren't big enough to break the clay particles up, they just combine and can make things worse.Thats why you need grit, organic matter, sharp sand to incorporate, as their particles are bigger than the clay particles, and the organic matter is food for worms and micro organisms which help break up the clay particles and allow air into the soil which when all combined gives you a good loamy top soil.
If you add more organic matter every year, you will find that the depth of the top soil will gradually increase, but you'll never get more than 18 inches of loam as worms are only really active in the top 6-10 inches of soil, anything below that is the result of micro organisms at work.
From your later description of your bed, I'd get hold of as much organic matter as you can(if it's shop bought, a bag per square metre) and dig it in. Then repeat every Autumn by top dressing with the same again for a few years. By then you you'll get more than a forks depth of decent loam.
I hope that makes sense!
Dave
"decent sharp sand is full of grit. with very little sand."
Well just buy grit...as you said above, sand is not "big enough to break the clay particles up, they just combine and can make things worse" Hence there is no reason to ever add sand to clay soil.
Sand is also good at keeping water, so terrible in winter when around roots - you can build sand castle with damp sand because of this property.
Gravel and bark and compost is best thing to break down clay soil.
You can take out some of the clay but be careful not to create a sump. I think the most important thing is to break up the surface of the clay base with your fork. You will find that no matter how much mulch you pile on, it soon settles down to the original level and you will be tipping on more. I don't know where it all goes, but it does.
if you can break the clay into lumps and let the frosts at it, the clods with break up nicely as the ice in them breaks them up, then straight after some hard frosts mix organic matter and sharp sand/grit in, this will help the clay stay broken up.
Thanks for all your advice. I have dug a hole of about 1foot depth. I filled it with water and it took about 2 minutes to drain. I tested the PH and it is alkaline.
The compost you see is what the builder added when I complained about the soil quality. when you get to the bottom of the hole it looks like liquid concrete and I am very worried about my ability to cope and grow things in it. Is there somewhere I can get advice on it? I am wondering if this is fit for purpose? Am I worrying unnecessarily?
Given i am currently overhauling my (predominantly) clay soil through the whole of my garden, id say that you don't have a drainage issue if that volume percolated within 2 minutes!
I've dug out all my lawn borders to at least 300 mm, and disposed of material. Im going to incorporate manure within the top 100mm of existing, and import new soil. I'm going to town with the garden, so might as well start as i mean to go on.
Had i not have gone to that extreme, in your position, i would turn over soil to a minimum depth of 300mm, remove any large clods, incorporate any form of soil improver and manures etc, and job done.
when it starts to warm up you could add some worms, they'll help get the organic material through the clay eventualy
I feel pretty sure that if it is a raised bed and you add lots of good stuff it will be fine. When I started trying to improve my clay soil I dug out and turned over and exposed to frost and all the rest. It nearly killed me. On later projects I just broke up the subsoil so that it wouldn't form a 'pan' and dug in every bit of compost, grit and manure that I could get my hands on. You wouldn't be able to tell the difference, now, but it took a fraction of time and pain.