Double-digging clay soil

rocket · #1 04/20/06, 02:25 PM

I have a very flat yard with heavy clay soil. I've always built my raised beds up on top of the soil rather than digging down into it so that I don't end up with a clay "sink" that holds water and drowns my plants.

But I was recently digging a large hole in the yard (to transplant a tree) and found that about 12 to 18 inches down, the soil transitions from the heavy, brown clay to a more crumbly, yellow soil. This weekend I'm going to dig a test hole and see how well water drains through this subsoil. If it drains well, I'm thinking of trying to double-dig my vegetable beds through to this layer. It would be really nice if my veggies could easily put roots down several feet, instead of the one foot depth of my raised beds.

So, I'm wondering if anyone here has faced a similar situation and whether or not any of this is a good idea.

SquashNut · #2 04/20/06, 02:55 PM

In case you don't know the yellow soil is sub- soil. You don't want to incorporate it into you top soil. That is the way my garden is. You need to break the crust of the sub soil with out mixing the 2 layers.
You may find it is easier said than done. If you have a wheel barrow remove the first block of soil and save it to put back at the end of the job. Then start turning the next batch of soil on to the beggining of the bed. That is how I double dig my beds.

rocket · #3 04/20/06, 03:02 PM

Thanks, Squashnut. And yes, I referred to it as sub-soil. But could you explain the reason for not wanting to incorporate the subsoil into your growing bed?

SquashNut · #4 04/20/06, 04:14 PM

It's silly but i cann't explain it, other than to say that the sub-soil will dilute the good nutrients that the top soil has. Maybe some one else can explain it scientifically for you.
We wrecked a spot in my gardenwhen we put up a trellis. The sub and top soil got mixed together and 3 years later it will still not grow much of any thing.

VALENT · #5 04/20/06, 04:52 PM

Squashnut, have you added much organic matter to this area? Normally, lots of organic matter will make up for most all mistakes.

SquashNut · #6 04/20/06, 05:54 PM

Yes I have amended it twice a year for 3 years. with compost, fresh kitchen garbage and horse manure. While it is getting better I have a way to go. It's just best not to mix your sub soil in to top soil if you can help it. I put a few bean seeds there last year and they showed signs of nitrogen shortage.

Cyngbaeld · #7 04/20/06, 07:42 PM

Instead of double digging, why don't you make the beds a foot higher? Lots less work! (I'm all for less work).

Paquebot · #8 04/21/06, 09:54 AM

In this case, mixing the subsoil with the surface clay would be a benefit since the soil structure would be vastly improved from what it is now. When it would be a problem, as Squashnut mentioned, is when the situation is reversed and it's the clay which is the subsoil. Double dig, add lots of organic matter throughout the entire tilled depth, and you'll have some super soil. If you have a river nearby with a handy sandbar, incorporating 15% river sand into the mix would make it even better!

Martin

VALENT · #9 04/21/06, 01:53 PM

squashnut, have you ever used an innoculant with your bean seeds. The beans should be able to "fix" their own nitrogen. However, if the rhizobium are not present in the soil,and you dont add them, the ability to "fix" nitrogen is hampered enormously. If there is clover (or other legumes) growing in your vicinity, the rhizobium may be present already.

rocket · #10 04/21/06, 03:51 PM

Well, I started my test hole in the garden last night. I got down through 18 inches of heavy clay and hit a hard black layer. I brought out the pick axe and broke through a 1 to 2 inch layer of the stuff. When I broke it, it smelled like a freshly paved road. I took a match to a chunk of the black stuff, and it started melting! I think I have a tar deposit in my yard!

Underneath the tar (or coal or asphalt or whatever?) it still looked like clay, but now the clay if filled with small, sharp-edged rocks. Doh! I'm almost 2 feet down so I hope I hit a crumbly, good-draining subsoil soon.

And Martin, your point about using the sub-soil to improve the structure of the clay is what I had been hoping for. Easier and cheaper than trying to bring in sand.

rocket · #11 04/21/06, 03:56 PM

Oh, and Cyngbaeld... I have been slowly trying to increase my bed depth. But I can only make so much compost and my flat yard doesn't provide many good spots to try to take soil from. To get any immediate increase in depth, I'd have to sacrifice bed area.

MELOC · #12 04/21/06, 06:00 PM

could that be an old road from long ago?

rocket · #13 04/21/06, 06:14 PM

If it were closer to the surface I would have thought the same thing. I doubt a man-made road would have had time to accumulate 18 inches of clay over it. It also didn't look the same as man-made asphalt.

MELOC · #14 04/21/06, 07:08 PM

the clay could have been fill dirt over a road...perhaps.

SquashNut · #15 04/21/06, 08:11 PM

Yes I do use an innoculant, so I am sure that is not the problem.

Cyngbaeld · #16 04/21/06, 09:17 PM

Careful, you may find a mastadon or sabre tooth tiger in there! LOL

Is there a stable anywhere near you? They frequently use coarse sawdust for bedding and it is free for the hauling. I made a beautiful garden with it when I was in colorado.

jnap31 · #17 04/22/06, 01:13 AM

What about municipal compost cant you get it from the city?

woodspirit · #18 04/22/06, 02:38 PM

Subsoil has no nutrient value. It does hold minerals, but no nutrients. The municipal stuff will contain disease pathogens like wilts, powdery mildew, and leaf spots. If it were sterilized it wouldn't. The best stuff I've found for clay soil is good old fashioned cow manure. It will allow the benefits of worms to improve your soil. Cay soil generally has little to no nutrients in it because there isn't any air in it. Over time the clay turns sour. Adding sand to clay will make some great bricks for building projects like schools and hospitals. I've tried using peat moss, raised beds, drip irrigation, and gypsum to improve my clay. It did create a great environment for insect outbreaks. It wasn't until a farmer friend nearby came over with a spreader full of composted manure that my garden turned into Eden. You gotta wonder why we don't check with farmers first before we make all our mistakes. Now I use chicken manure evry year from our chickens who eat our crickets and grasshoppers, and I can grow anything in that soil. It's no longer clay based gardening. It's black crumbly soil.

swamp man · #19 04/22/06, 04:44 PM

Without a proper analysis,advice on whether or not to incorporate it would be premature.The sand is a great idea,but only if you also till in organic matter of some kind,to improve structure.Don't sweat nutrient levels too much, that can be corrected after the structure is satisfactory.I ran into a similar situation when building my raised beds-just couldn't come up with enough compost to top 'em off.My solution-I used what compost I had, and tilled it in with soil mix.The soil mix I get from the local nursery goes for $25 for a heapin' front end loader bucket full.You are wise to be wary of creating a plot that will hold water.I'd just build the beds as high as you want 'em.

woodspirit · #20 04/22/06, 09:41 PM

The problem with a "proper analysis", is that it changes day to day, and even changes from time of day. If you were to go the commercial fert. method then you would use a balanced mix like 10-10-10, 0r 20-20-20. However the use of composted cow manure cannot be overdone even if it were ten feet high. Unless you use it on acid loving plants like rhody's or blueberries. Forego the analysis and add composted cow manure. And it also precludes the neccesity of adding organic material since it is organic material. No need for sand.