Conditioning clay soils


Old 07-03-05, 09:59 PM
Thread Starter
Join Date: Jul 2002
Location: Ca
Posts: 743
Conditioning clay soils

Seems like they hid the forum search function- I know this is probably asked every few days and I also searched the web and got some limited info but here goes...

In my area the native soil is virtually 100% clay. The location that I'm currently preparing for perrenial flowers had some existing plants but the soil wasn't maintained so it's still partial clay down to 8" and virtually 100% clay below that. I have added about 4" of compost which has helped but I'd like to cover all bases. The compost I added is primarily bark based.

As I understand it there are 2 issues here, long term prep and short term (for next spring)-

Long term:

Assuming I have composted enough, should I also add some sand to further lighten the soil. It's not too bad now but it's also not very wet and I have had 12" deep holes in other parts of the garden hold water for days. I don't know how sand is graded but most of what I have read recommends a somewhat larger grade for clay soils.

Also can't hurt to add some gypsum from what I have read.

Short term:

Interestingly I understand that compost actually takes years to reach full effect so I should probably add some fertilizer either now or at planting time. Any particular ratios or should I test first?

Also, I understand that the bark can tie up the nitrogen so some ammonium nitrate or sulfate may be in order.

Thanks for any advice.

Last edited by AlexH; 07-03-05 at 10:39 PM.
Sponsored Links
Old 07-05-05, 08:14 PM
Join Date: Nov 2001
Location: Taylors, SC
Posts: 9,483
You might have a soil test performed before adding gypsum. This will tell you what else might be needed as well as what might not be needed. You can get a soil test kit from the box stores or nurseries.

Compost improves the friability of the soil immediately as well as improving moisture retention. The improvement in moisture retention for clay would be to allow the moisture to percolate out quickly. Compost has some long term benefits, but will decompose further quickly. Compost can usually be added annually without a problem. Part of the feeding effect of compost comes from the readily available nutrients from the composting process. Long term, the establishment of earthworms in the bed will improve the soil and spread nutrients as the worms move through the soil.

What I have done in beds of dubious quality is to prepare each plant's hole and feed the plant itself. This will encourage the plant to thrive and its root system to spread. Adding mulch to the surface of the soil will help control weeds, retain moisture, and provide a continuing source of decomposing organic material for the benefit of the soil and the use by the worms. As the mulch decomposes, the soil improves. Add new mulch from time to time to keep it up to a thick enough level to keep weeds down and control evaporation. Most beds require additional mulch each year to replace that which has decomposed. Over time, you will notice that the soil turns into a pretty good imitation of plain potting soil due to the decomposition of the wood mulch.

Nitrogen is bound in the decay of uncomposted organic materials especially wood. If the bark has been composted, then this should not be a problem. I have used raw wood mulch to kill weeds in developing beds. It binds almost all the available nitrogen, starving the weeds to death under the mulch.

Hope this helps.
Old 07-06-05, 10:04 AM
Thread Starter
Join Date: Jul 2002
Location: Ca
Posts: 743
Originally Posted by chfite
I have used raw wood mulch to kill weeds in developing beds. It binds almost all the available nitrogen, starving the weeds to death under the mulch.
Thanks, the soil is looking pretty good after working it.

What you said above may explain a planting failure that really stumped me. A few years ago, I planted a 10'x10' area with annuals the goal being a carpet of bright flowers. I took no chances, removing about half of the existing soil and adding potting soil and manure. The plants did very, very well with lots of bright flowers. The only issue was because of all the bright sun on hot days the surface soil would dry out in hours and the shallow root annuals would start to suffer.

So the next year I added a little more manure and replanted the same plants but this time I put down redwood bark after planting to help retain moisture. The plants did terrible, dieing back to almost nothing. I was at a total loss. The only thing I thought could have happened was over fertilized but from your telling me it may have been the bark (of course soil testing would have told me for sure - definitely have to start doing that).

So what kind of mulch do recommend to help retain moisture when necessary?
Old 07-07-05, 09:00 AM
Join Date: Dec 1999
Location: United States
Posts: 2,535
Just keep adding the compost. One mix I've found that I like is called mushroom compost. You'll have to get it at a nursery, landscaping store, or mulch store. You won't find it at the local big box.

BTW - do NOT add sand. Adding sand will not help at all and may actually make it worse as the clay binds to the sand.

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread
Display Modes