Research has actually shown for a long time that amending planting holes is extremely counterproductive, especially for larger plants (as opposed to small annuals), but that knowledge has been slow to infiltrate the public consciousness.

Why is it so bad?
1) Roots are already very "reluctant" to venture out into soil that is inhospitable compared to the potting mix it was grown in. Amending a planting hole essentially creates a container in the ground, causing the roots to circle, become bound, or even strangle the stem.

2) Most plants have significant ability to adapt to the native soil, but will not do so unless "forced" to. Hence planting holes should be back filled with native soil to jumpstart this process.

3) Planting holes that are amended with large amounts of organic or coarse matter create a "sink effect", especially when the surrounding soil is finer and less permeable. When water is plentiful, this can create anoxic conditions at the barrier between the soil types. When water is scarce, highly retentive native soil will not readily release water to coarser organic materials, and can make the planting hole hydrophobic.

When amending native soil, more and more research is suggesting that attempting to plow in amendments more than very superficially is counterproductive and very harmful to the "soil food web". In fact, the much more convenient method of simply topdressing, minimally incorporating, or simply mulching with organic matter (or making it the main component of a raised bed mix) is much more effective in increasing percolation and thus improving drainage, along with a host of other benefits. Some aspects of this concept are referred to as "no till" by proponents.

For example, if you tried to use gritty mix directly in a planting hole, you'd essentially create a dry well... a situation very few plants could survive.

PS, you can find a lot more information about this on other forums, this being one for container gardening :P

Wow, thanks for all this Greentiger!

Being a landscaper for years, you have made good points and took the time to explain. Thank you:-0)

I would consider fertilizing quite often too if your soil is poor.

Mike

Thanks, greentiger - that's information I hadn't run across.
I know this wasn't the 'right' forum to post the question in, but since I was primarily asking about the gritty mix ingredients and I wasn't sure which forum would be the correct one to ask about amending holes (since I have no particular plant in mind), I figured I might get an answer on this forum. Which I did, and a good informative one. :D

... now I wonder how many plants I've doomed or made 'life' hard for by amending the holes. Poor babies, now I feel guilty! I'm glad I haven't yet moved my plumbago & blue daze into the ground.