Lainey, in my current garden the base is silt rather than clay--incidentally, many gardeners confuse the two, as I did for a while. I do have some experience with clay along with book-larnin'.

1. The most important point is not to work or step on clay soil when it is wet. If you avoid compacting it and keep it mulched with organic material, clay will develop some structure over time with better aereation and drainage,

2. If you amend, amend the whole planting area uniformly rather than back-filling holes with lighter soil. The latter practice can make drainage worse.

3. Start with a percolation test to see if there is adequate subsoil drainage. If not, raise the bed 6" or 8". When the soil and subsoil are already wet, water should drain from a hole at the rate of 1" per 2 hours (marginally acceptable) or 1" per hour (satisfactory drainage). Pre-wet the hole by filling and draining before testing.

4. Do not add sand. I would dig in 3" of manure to a depth of 10-12". Manure seems to be the most concentrated source of humic acids that help the clay for a crumb texture. Or use finished compost if you have it.

5. What else to add depends on the soil chemistry, so it is best to get a soil test. Check the pH, and if it is below 6, add lime to adjust to 6.2 or 6.5. Calcium helps clay develop a crumb structure. If pH is above 6.8, add sulfur. Be careful to use recommended amounts when adjusting pH.

6. A complete soil test may show that the soil is high in sodium or magnesium. If sodium is high, adding gypsum will help the soil a lot. If magnesium is high, you must avoid ever adding epsom salts or the dolomite form of lime. These problem soils occur from Texas westward more often than in the East. Piedmont clay soils are usually acid and low in calcium and magnesium (both contained in dolomite lime).

Thanks for posting this. Very helpful. I wasn't aware that just amending the hole would make things worse but it makes sense.

Right, Tuggy. Drainage ultimately depends on the subsoil, and if the subsoil won't drain, the only recourse is to raise the bed.

If you put light soil in a hole in impervious clay, during heavy rains water runs off the surround and saturates the soil in the hole.

Is this true for rocky sandy soil also? I pry out big boulders in order to plant anything. I'm on a lava flow hillside. Lately I have been doing more and more trenching rather than just digging big holes. So many boulders come out that I have to amend a lot just to fill up the trench. I notice that these areas kind of sink down even though I water in a lot as I add. I just keep adding more compost and native soil. I can't trench the whole yard but plants can't stretch out much if I just dig a small rock surrounded hole. I will have to say though that everything grows even trees that were put in by landscapers who just jackhammered holes to fit the root balls. Seems that trenching gives more room to spread. The soil drains like crazy. Keep trenching?

Sure, if the sand goes deep, trenching or digging holes wouldn't cause a drainage problem. It's a good idea to remove large rocks down to a depth of 18-24", as you are doing. A good additive for sandy soil is plain clay cat litter at 10%-12% by volume. It retains lots of nutrients and water.

Wow cat litter. I never thought about that. I think that would help slow things down a bit. Thanks.

Mary

Thanks for the info. An interesting observation: I had to remove four big shrub roses with RRD last year. Three I had planted in carefully prepared holes with enriched soil and additives. One I had planted in a hastily prepared hole in mostly clay with good topsoil on the surface. The three in prepared holes had widely spread roots. The one in the unprepared hole had roots that went down to hell, but were not wide spread.