Advice needed - seeding a new lawn on clay - how deep my topsoil? or can I do without?

I want to seed a new lawn this autumn in an area that was partly covered by decking and partly planted (sparsely) I have typical London clay - sticky but just about workable. I was planning to dig the area over well and 'tamp' it (tamping clay could be a challenge....) before applying a layer of topsoil ready for seeding. My question is to WHAT DEPTH should I apply the topsoil? Topsoil and turfing companies recommend 10 cm or so however the cynic in me suspects this might be with a view to selling more topsoil .... ? wouldn't 5cm suffice? or might this shallow depth somehow create a sump effect? or might I even be better off not using topsoil at all? (tho' trying to create a workable level tilth with this clay could be very challenging) there is plenty of information out there about seeding a lawn but not necessarily when dealing with less than perfect soil. i don't need a bowling green standard lawn, i just want to 'do the right thing' and I don't want to go to ridiculous lengths if something simple will suffice ....

Re: Advice needed - seeding a new lawn on clay - how deep my topsoil? or can I do without?

Reply from Marissa Zoppellini

Hi, digging the ground over and levelling will be hard work and I think it is a job worth doing properly to avoid problems later. If it is not overwhelmingly heavy clay, I would add 10cm of soil improver and dig it in, which should give you a good topsoil which will be free-draining and moisture retentive for a healthy lawn. Once you have finished digging and tamping, I would advise waiting a while for the ground to settle before a final levelling off ready for sowing, otherwise you are likely to end up with dips. I hope this helps and wish you all the best, Marissa

Re: Re: Advice needed - seeding a new lawn on clay - how deep my topsoil? or can I do without?

Reply from Dockyrph

thanks Marissa, i've started digging already and it's not quite as bad as it could be. think i'm going to bring in a load of soil improver rather than topsoil per se like you say. I'm also going to experiment with some horticultural gypsum that I have recently sourced. i know the jury's out on gypsum but as it doesn't affect ph level too much and over the long term is allegedly pretty effective at flocculating clay soil i'm keen to give it a go. just dislike the idea of sticking a completely unrelated topsoil over the top of a subsoil which could then act as a sump. here goes then. thanks for the input Marissa!

Re: Advice needed - seeding a new lawn on clay - how deep my topsoil? or can I do without?

Reply from AM

Hello Dockyrph, Like you I am going to put down my first lawn and live in London thus the same clay sticky soil. I am planning to use Gypsum as well. I understand it is a treatment that one has to do for 3 years (once every year). Has your soil improved this year compared to last year? Where did you source your gypsum. Looked around local B&Q and they dont have any.

Re: Re: Advice needed - seeding a new lawn on clay - how deep my topsoil? or can I do without?

Reply from Dockyrph

Hi there - not sure if this link will publish however product is Rootwise and I have it in both pelleted and in powder form, seemed easy enough to apply but couldn't tell you about the outcome yet as the turf has only been down for a couple of months therefore impossible to tell how it might have improved the drainage. Further applications of gypsum can be made in powder or pellet form - pellet probably best once turf is down otherwise it simply blows away. I found Debbie at Rootwise very helpful - see what you think . http://www.rootwise.co.uk/gypsum-root-gyp can't remember what I paid however it ain't supercheap but it's worth a punt. I dug over soil once to a good spit and then later applied and forked in the gypsum into the top 10cm or so ..... My fingers are crossed and I can only trust that the product will do what it says on the bag .... I have done a lot of research on improving clay generally and it seems to be a long term project whichever way you spin it. The RHS helpline suggested that for borders I was better off digging in composted bark fines to improve the texture however have also read that mushroom compost can also do wonders tho' beware that unlike gypsum, the lime content commonly found in mushroom compost may change ph. I am about to plant a load of trees/shrubs and herbaceous and am still dithering as to the best way to improve the soil. all treatments are costly and time consuming and the message is continual application of as much organic matter as you can chuck at it and let worms do as much as possible! I have seen gypsum on sale in decorators merchants in West Norwood - presumably to be used in constrution of stud walling however gypsum is gypsum is gypsum I might assume? Good luck and hope this helps - once can only but try!