I presume you have them planted so the root flare is visible. If not, remove the necessary soil to reveal it. The next thing is to run a soil test. Some trees grow better in acid soils while others will grow better in neutral or alkaline soils. Most conifers prefer acid soil while a Shumard Red Oak will tolerate either and a sweet gum prefers acid soil. Was the manure composted? Raw manure will burn tree roots and will smother the roots if it is too thick.

Tree roots spread out more in clay soil because of poor oxygen deep under and it contains much more moisture at the surface. While sandy soil is very porous and dryer so the roots grow long in about all directions because there is more oxygen lower down plus most of the constant moisture is located deeper in sand. Top surface of sand fluctuates between very dry when it evaporates quick to moist when it rains. Clay surface stays moist about always.

What determines the branch spread is how young it is and the amount of sun. If a tree is shaded by other trees in a forest it will grow tall and skinny as it tries to reach the sun and the bottom limbs will die off as they are shaded out. But if a tree is in a open field then it will keep most of it's lower limbs. It's light dependent.

Thanks for the responses.

Treeguy, I was basing my speculation not only on the illustrative root photos that I mentioned, but also on the fact that I have friends living on a nearby piece of land with a very clayish soil... and their trees and shrubs seem to grow huskier and fuller, even though the light conditions are pretty well identical to here on my own place.

So I've wondered about nutrient retention in the soil. Or even something at the microbiological or chemical/physical level. Something influencing the nutritive dimensions of root/soil relationship.

Okay, and if there IS a difference of that sort, the next question would be: can I compensate for the sort of soil I have, via some wise fertilization, etc.

J.

If you want to fertilize, first you have to do a soil test, to see what nutrients are already there or are lacking. Adding too much of what is there can set up an imbalance that can be as bad as not having enough of whatever. Adding compost, or composted manure, is usually not a problem, depending on the composition, as it's usually reasonably balanced in and of itself.

If your soil is really sandy, then the best thing you can add, along with/as well as a layer of composted manure, is a layer of natural, organic (organic in the sense of being able to break down, not as certifiable) mulch, no more than 4" total depth, as widely as possible, out to the drip line being a good minimum, beyond being better, and putting no mulch in the inch or 2 next to the trunk. Annually, add another inch or so, to keep the total layer at about 4", as it breaks down from underneath and improves the soil. It will also add to the insect, fungal and bacterial population.

Yes, compost and mulch about 4 inches deep (No deeper) but the wider the area the better for the tree. As it breaks down it will improve the soil, conserve moisture, and add many nutrients, etc. Do keep the mulch off the trunk to prevent bark rot. Sand can lack many things good for trees that clay has, so that's probably why. Less soil moisture may also keep them smaller also.

Infertility and drought are indeed two common reasons why sandy soils ain't so great and claylike ones not that bad. Of course, it depends on what you are trying to grow. Trees are not able to utilize high nutrient levels in the same manner as field crops so it is just as well to give most of the manure to the vegetables. And if a soil test showed a need for a nutrient such as nitrogen and it was known the manure product supplied that in the needed amount then it would be good to put that on the trees for that reason (and assuming there was not some drawback, like high salt content).

All trees have surface roots, those on deep sandy soils or other dry soils may also have deep roots. The main factor is drought driving these down in search of moisture, outside of dry climates it's usual for nearly all roots to be near the surface, where the air is. Even tap-rooted trees will be seen to have most of the fully developed, active roots near the surface, with the tap root and other anchoring roots being different-looking.

And Joel........I absolutely do not think there is a relationship between soil types and narrowness/bushiness of top growth. There are other factors at work.

+oM

Well, +oM, you wrote: "I absolutely do not think there is a relationship between soil types and narrowness/bushiness of top growth. There are other factors at work." Such as what?

Like tree species and age of the trees, for example. Many trees go through a gangly 'awkward' stage before putting some meat on their bones.

joel_bc,
Age is a big factor, genetics of the tree is a big factor, and of course light amount, and water/nutrients.

The soil type can contribute to the branching structure of a tree because of some the factors I posted above, but Not because of mimicking the roots.
The above ground parts of the tree do not mimic the roots.

Well, then... compost & mulch out to at least dripline seem to be the main recommendations?

J.