The recommendation to amend individual planting holes has been proven to be a deficit to good plant growth rather than a benefit and is no longer advocated. If amending soil conditions is necessary, it is far better to incorporate the amendments over an extended area - i.e., an entire planting bed - so that the amendments are fully integrated into the native soil. If one is unable to amend a large area of native clay, providing a very wide but shallow planting hole, planting high and backfilling with the removed soil only will work. Amendments can then be used as a topdressing or mulch.

Japanese maples prefer a loose, organically rich, moisture retentive and acidic soil. Adding a good quantity of organic matter in the form of compost will aid in both lightening clay soils, improving drainage and adding to its ability to retain moisture. If you are planting in a container, you never want to use regular garden soil - use a good, bark-based commercial potting medium or prepare your own. Most commercially prepared potting mixes will have pumice or perlite already incorporated - it is necessary to maintain porosity and rapid drainage that a container growing situation requires. Vermiculite is not a great addition to a container mix and should be avoided - while it retains many times its weight in water, it rapidly collapses once saturated and loses porosity, eventually making drainage worse. The Containers forum has dozens of threads with specific container mix recipes you can make, including a number that address maples specifically.

Sorry, I'm a little confused now. It sounds like it really is necessary to amend clay soils with organic matter, even though amending planting holes is a bad idea (first paragraph from gardengal48). Assuming I amend with compost what else can I put in the soil to make it acidic? How much is too much? THANKS.

You have hit on one of my sore spots and you are RIGHT ...I think the 'correct" answer is to amend if absolutly necessary...as in... if you don't your tree will not grow... IE solid clay solid rock sand total muck etc otherwise you are correct it is not a enlightened position ...but amending in borderline soil , I agree may not be your best choice since the tree will grow out of it shotly. David

So are we talking container planting or in the ground? The two situations are vastly different and need different approaches.

If in the ground, what are your soil conditions now? Clay soils do not necessarily equate with poor drainage although they often are related. Have you tested for existing soil pH? The other aspect of clay soils is that they tend to have greater buffering capacity than loamy soils so resist any substantial changes in pH. Although J. maples prefer slightly to moderately acidic soils, like most plants they tend to somewhat tolerant of less than ideal pH conditions. You may be OK in that respect but you won't know unless you test to be sure.

The pH is pretty neutral - but I'm more concerned about drainage while the tree is getting established I guess...

This is what I don't get as far as the new recommendations for not amending soil when you plant a tree or whatever. The tree has been growing in artificial conditions for its entire life if it's in a pot. The soil would be moisture retentive, but well-draining. Now, my question is if you take a maple that has been growing in an optimum growth soil mixture in a pot and put it into soil that you don't amend, how is this better for the tree? This just doesn't make sense to me. It seems like a gradual transition from pot to the ground would always be preferred. Maybe someone can explain the rationale behind not ammending. Granted, it would be a lot easier, but I cannot bring myself to do this when I plant a tree, or anything for that matter.
Kay

Container culture is very different from growing plants inground and you shouldn't try to make comparisons because each of the two situations is distinct. One of the primary reasons not to amend individual planting holes is because of soil interface issues. When you introduce a different soil type, as will happen if you amend a planting hole, it affects the texture and porosity of the soil in that area and when water moves through soils of different textures, the natural flow is altered and drainage is impacted, sometimes radically. The natural porosity of the soil - the spaces between the peds or soil particles - is changed and when water percolating through the soil encounters changes in porosity, its flow is slowed. A similar situation exists with the oxygen penetration of the soil and changes in the pore spaces. Studies have shown that trees planted in highly amended planting holes often take off quickly for the first season or so, but growth slows radically and can frequently become stunted or otherwise unfavorably affected once the roots hit the different soils outside of the amended hole. In severe cases, it can cause girdling of the roots or a bathtub effect from overly moisture retentive backfill.

Granted, amending poor or nutrient weak soils is often advised, but it is best to do so over a very wide expanse - ideally, over the entire potential spread of the root system - not just for an individual planting hole. In fact, some evidence exists that all purchased trees should be bare rooted before planting to minimize soil interface issues.

Garden Gal,
Thanks for that explanation. I have heard the "new" thought on planting, but never the reasoning behind it.
Kay