Laying new topsoil, what if anything to mix in for worms or to kickstart healthy ecosystem?

Question

For reasons that would take too long to go into (tl;dr version - a combination of mass gradual soil movement needing soil engineering, bad development, old paving/topslabs, + ground levelling to address long term ground issues) the entire top 20-80cm (8-30 inches) of old paving, sand and soil under it is being stripped from a garden (approx size 6 x 14m), and will be replaced with new topsoil when structural and soil movement issues are remedied. Because of the nature of the issues, its not practical to keep the old soil and reuse it (tl;dr #2 - no storage space for such a volume while works going on, economics prevent doing it bit at a time). The ground below is a deep clay soil. The old ground level was sloped, so the depth removed will vary from place to place. The location is close to London, England.

The work is underway, so whether a better way might have existed is pointless now, this is how it is. So the end result will be a new topsoil layer. The work should prevent soil movement in future but water may still tend to flow in the soil in one direction, as adjoining land will be higher in one direction than the other.

My question is about the choice of new topsoil. We get to choose what to put back when the work is done. I don't know if there are different kinds of topsoil, or some that have organic matter mixed in for worms, or whatever else is needed to allow a healthy soil ecosystem to be established. Perhaps some topsoils are "new" and sterile and others are "recycled" and more varied, but I don't know if so. I have the possibility to choose a specific kind of soil, or to premix it with nutrients. While the soil will mostly be laid with grass seed, we would like a fair bit of fruit plants and maybe a fruit tree or two in it, but even if we didn't, we'd like to help the soil to be established with a helping hand toward a thriving subsoil, that is also sustainable (ie reduce nutrient leaching and help nutrients to replenish), and a good soil, whatever that means.

What should we do / what are the options?

UPDATE:

@stormy - see reply to @Bamboo below, the original question contains links to photos before the work. The garden was originally on a slope and had been cut away. While the houses are in good condition for over 100 years, all the gardens on that slope have subsidence/soil movement, and show cracks/bulges/failures in retaining walls and some have substantial retaining concrete. Because of other circumstances time was very pressured and foundations needed redoing in 2 places anyway, so the ground was dug and a new engineer-designed retaining wall will go in. Before the heavy equipment all goes, and while I still have access for large items > 1.5m wide, it's a good time to get into the garden, whatever will be the new top layer of soil.

I haven't heard of seed spraying services, can you point me to any (I couldn't find links). The main reason for seed vs sod is cost, after so much unexpected work the economics override most other things. If seed can be done, then the cost saving will be a big help. Water in this area flows towards the houses and into water drains rather than into next door gardens, but because gardens are at different levels there may be flow of subsurface water from higher to lower gardens as well. I can put in trenches easily.

There isn't a chance to design the beds at the moment, there's too much else to do! So I want to get the topsoil "about right" and the hardest work done. I might need to leave it idle and covered (or even in its large bags) until the Spring season while we fix the rest of the house structural issues, then come back to it in a few months and remove any weeds, plant seed, decide beds/ditches and anything else. But it will be easier to get the soil in now, while I have access for large/heavy items.

Update 2

It turns out that a 2.5 ton digger makes a superlative soil breaker if the bucket is jiggled and the soil allowed to scatter out of it. The soil looks a lot better than I expected, now that the broken glass, rocks and weeds/debris are mostly dealt with. A lot will have to be removed for structural reasons and levelling, but it should make a good soil. I may need to mix something or other in, or put a few inches of topsoil over it so the original soil is not near the surface (I found quite a lot of broken glass throughout, and no way to remove it all). Essentially the digger and crude removal is getting quite a nice soft crumbly mix. Probably needs fixing somehow as its clay and will quickly clump again and be non-draining, but I'm fairly confident now that I could mix in whatever's needed to help with that, in a matter of a few days. I'm down to bare soil throughout, and whats left to do is trenches for structural concrete work, levelling and putting in structurally viable subground retaining walls against soil movement (originally sloped/cutaways at boundaries), and returning the garden back to being a garden not a building site.

So my previous assumption that it's not practical to keep the old soil has been changed. If I sift+keep the needed amount of the existing soil instead of getting new, and I want to modify it to be better in terms of nutrients, drainage, or other qualities, I can probably do it instead of buying new, and mix in whatever is needed instead - but what is it that I should be doing?

Answer 1

Pity you didn't ask before all this work. To remove the top layer of soil beneath that concrete is just sad. But people commonly do this anyway. Purchasing topsoil the composition is never an exact product nor do they come with ingredient lists. I am curious as to what you are talking about with 'soil movement'...erosion? What kind of drainage, what % slope do you have, where are you directing excess water?

Topsoil is topsoil. Do not try adding anything to the soil. You probably have a fair bit of clay over there (btw, Bamboo is right in your neighborhood)! If you could think about sod versus seed, now that would be a great expenditure. Grass seed is a tough way to grow a lawn and in London, space is a premium. Sod eliminates weeds, holds the soil in place so erosion is not a worry, comes already started with fertilizer.

If you are insisting on seed a second option would be a grass spraying service. Comes with a mulch to ensure germination especially if you screw up and allow the germinating seed to dry, just once and you have to begin anew. Also a good baby grass fertilizer is added. These companies are amazingly informative, knowledgeable and precise. Draw your edges and circles around trees, delineate your plant beds and they don't get a seed beyond your lines.

Most importantly is rolling or compacting that soil before seeding or sod! After fine grading to get the initial surface, you have to compact with a water-filled roller, grade again to eliminate low and high spots and then roll again. This is so critical I can not jump up and down enough!!

Plant beds should be a good foot higher than the lawn with a 6"X 6" trench between the raised bed and the lawn to carry excess water away instead of soil and water covering your lawn and walks. Make sure the soil is a good 4" below the bottom of your wooden fences and siding of your home. You don't want your neighbor's water to flow onto your property and you do not want your excess water flowing on their property! Gets a bit tricky with the grading.

I know this field quite well; grading and drainage and lawns and plant beds. Subsoil is never thriving in the normal world. Only the top soils. If one has to make top soil out of subsoil, the only way to improve is by the addition of decomposed organic matter to the surface. This you will be doing every year, every other year to your plant beds. Top dressing your lawn after aerating is the equivalent. (Mow no shorter than 3", aerate once per year, fertilize with extended release not fast release, sharp sharp blades and water deeply allow to dry out before watering deeply again).

To add anything to your topsoil is counterproductive. I am pretty sure you guys in London have clay soils; clay plus sand, gravel, water, gypsum, lime all rotated, manipulated makes concrete. Topsoil is usually full of organic matter because soil on the top gets organic matter raining down on it all of the time, part of the description. When you get a nice, thick, uniform crop of lawn grasses you will then be adding what that crop needs; fertilizer higher in Nitrogen than P and K, extended release, (don't do the fast release synthetic fertilizers) and depending on the fertilizer at least 3X per year, 4X for synthetic, 2-3X for organic/extended release fertilizer per year... cores that disintegrate from core aerating and keeping that crop no shorter than 3"...watering only when you see your footprints staying down on your lawn and watering deeply (4-6" into the soil profile) will make a drought tolerant crop and save tons of money on watering/maintenance.

Please send pictures and make sure you know where every drop of rain will be flowing to. Excellent that you are concerned about creating the proper beds now.

Answer 2

I recall your question a few weeks back about changing levels in your garden - seems like you've gone ahead and had most of the work done professionally, as suggested. With regard to replacement soil, good, loamy topsoil in the UK should be fertile when its delivered, but it does depend to an extent which grade of topsoil you buy, and the supplier you choose. They're divided into three classes or grades here - premium, general purpose (this category may contain more than one type of topsoil, speak to the supplier) and economy. The latter will be variable, probably will contain weed seeds, roots, debris, stones and may actually be motorway or building excavation spoil, and its fertility may be questionable. For areas where you're laying turf, general purpose should be fine - for areas where you want to grow plants and trees, you might want to splash out on the premium grade. If you decide on economy grade, its probably wise to order composted manure or composted material to incorporate with the topsoil, in order to improve its fertility and to 'lighten it' because its often clay, even though its topsoil. You may find you need to screen or seive economy grade topsoil once it arrives, depending on the debris content. More information here https://www.rhs.org.uk/advice/Profile?pid=375

UPDATE:

The extra information in your question doesn't change the answer I've given above, except to say two things; its just a bit late to sow grass seed, and if the soil isn't even in place yet, then a spring sowing would be more appropriate. If you can, it would be better to get the soil in place where its needed and spread out before the winter rains set in, and to give it time to settle properly over winter. If you can't spread it all before winter, then do it during winter at times when the weather isn't very wet or frozen - it needs to be spread and left for a good month or so prior to sowing seed or doing general planting. Some weed growth is probably inevitable, especially if we have a mild winter, but removal of weeds and proper levelling is part of the preparation for sowing grass seed in spring anyway.

Second, there are no seed spraying services available in the UK - I'm afraid it's the old fashioned way of casting seed, by sowing it yourself direct - you can buy a seed spreader if you like, its just a bit of kit on a stick that you roll over the area with the seed in the small hopper, and should give a more even spread. I'm not sure they're worth it, though, because by the time the birds have had their share and there's been rain, the seed will move around anyway. If the area is sloped, there is a risk that seed will be washed down the slope if heavy rain arrives shortly after sowing, but once its germinated and started growing, then that's less likely to happen. Germination in spring will take longer, and you will not be able to use the seeded area 'normally' for three months after it starts growing. 'Normal' use includes walking over it, (though it will be necessary to walk carefully over it when you do the first and second cuts, which cannot be done with a mower, although you could risk using a good, sharp bladed hover mower without the collection box in place for the second cut) as well as anyone playing on it.

The only other thing to add is to say again, get hold of a copy of The Lawn Expert by D. G Hessayon - even a new copy is under £15, and it will give explicit and clear guidance regarding growing lawns in the UK from seed, or laying turf, including full preparation instructions, together with lawn maintenance and procedures as well as problem shooting ongoing.

And a final word about 'eco system' - you need not worry about that, topsoil is not sterile, and over time, a perfectly good ecosystem will establish itself once the soil is spread.