Soil.

Someone told me onetime that if you put new topsoil or compost around a new planted tree it will make the tree roots not want to spread out into the native soil.

Is the tree already planted? Putting down compost over the surface of the soil when the tree's already planted won't have any effect at all on where the roots go.

What about if your planting it brand new.

I've always mixed lots of peat moss into the natural soil I remove from the planting hole for all of my trees. Hubby and I finished building our current home about 5 years ago and I began landscaping at that point. Many of my trees are now in the 15-20 foot range (depending on the type of tree and its growth rate) and are very healthy. I don't believe they could have achieved this amount of growth without having a full, strong, healthy root system. The only time I could see problems arising from the situation you describe is if you are planting in very hard clay soil. Adding amendments to the planting hole in that situation will cause a "bowl" problem. Water can easily travel through the amended soil but when it reaches the clay, the drainage will be very poor and it will fill the amended area with water that can't drain and will slowly drown the tree's roots.

I think I have read that if you dig a planting hole and fill it with a blend of some of the soil you removed, plus some amendments, it's more likely the roots will punch out into the surrounding unamended soil. More specifically, if you didn't put back any of the soil you removed, and filled the planting hole only with the amendments, you are encouraging the roots to stay in that hole.

I have also read about the problem NatureLover describes, that a planting hole with poor drainage could possibly collect water and hold it like a bowl, which could kill the roots.

Shallow watering is probably the worst thing you can do for root systems. The roots stay near the surface because they don't need to go deeper for moisture, and then along comes wind or drought and the plant is not prepared. Frequent shallow watering is not nearly as good for the plant as less frequent deeper watering.

Example: Roses, which need lots of water, should be watered deeply 2x per week, rather than a little every day. Of course they need more on really hot days.

If you're planting it new then I've heard the same thing as NatureLover and EllaTiarella. For trees and shrubs, I generally plant straight into my native soil without amending, I figure if they're not going to be happy with my native soil I'd rather find that out sooner. If you are going to amend, the best thing is to mix the amendments with your native soil and do it over a larger area vs just in the planting hole. With non-clay soils it doesn't matter as much but with clay soil you do need to be careful.

Thankyou.