Sujiwan,
Plants can definitely be used as indicators of the type of soil and amount of moisture/flooding in a given location, but trees aren't always very helpful in this regard, at least in the Northeast. Certain trees do prefer certain types of habitats, but the trees may not precisely conform to the boundaries of the soils, and most tree species here in PA can grow in more than one type of soil and moisture regime. For example, Eastern Sycamore is typically found in moist soils, especially riparian areas (areas influenced by surface water). However, one can often find sycamores growing outside of riparian areas, particularly when there is a good stand of Sycamores nearby to supply seeds. Same goes for Black Walnut, which seems most common in deep, moist soils near rivers and streams, but can also occur in uplands. What's even worse, walnuts are often planted by humans and the nuts may be carried by squirrels. Trees are an indication of soil type, but not absolute proof.

In addition to soil and moisture, another major influence on the type of trees in a location is recent history. If a site was cleared of all trees, for instance cleared for pasture, then the first trees that might recolonize the site might be trees with seeds that are dispersed by the wind, or they might be the offspring of whatever trees were left standing nearby. If you have a stand of young maples, that might be an indication that a few maples were the only trees left in the pasture and therefore were in the right place to supply lots of seeds when the pasture was abandoned. In fact the pasture might have had very few maples when it was a forest.

Yet a third major influence on the type of trees growing in an area is predation - I'm talking about deer and perhaps cows. Lots of PA forests are growing black cherry and red maple after they are logged not because these are historically the dominant trees in the area, but because these trees are less palatable to deer and therefore are the only ones that survive heavy browsing by deer. Don't underestimate the impact deer may be having on the trees - they can be surprisingly hard to see even when very numerous. Cattle also can have an influence on trees - many former cow pastures are now stands of Red Cedar because cows avoid eating red cedar, allowing red cedar to repopulate the pasture even while the cows are still present. As soon as the farmer stops occasional mowing -viola, the baby red cedars can grow and you have a red cedar forest.

Finally, there are many types of hickories and they favor different types of soils. If you have a stand of walnut and hickory on former farmland, I might suspect that the previous owner of the land selectively saved walnuts and hickories because he liked nuts or liked hunting. Walnuts and hickories are both good for squirrels and generally recommended by game management literature.

I think if you want to use trees as indicators of the soil and moisture you should focus not on individual species, but on the communties of trees on your land. If you find red oak, white oak, chestnut oak, certain hickories, pitch pine and/or virginia pine dominating an area you might expect to find drier soils typical of uplands. If you have a stand of Eastern Sycamore, Silver maple, American Elm, and perhaps walnuts too, then you might expect to have moister soils and occasional to frequent flooding. If you see one set of tree species in the canopy and another in the seedlings, then you might either have a forest going through natural succession or some force (deer or the recent removal of a lot of the deer or cows) influencing the forest composition.

Hope this helps...

The National Resources Conservation Service (formerly Soil Conservation Service) publishes soil surveys of each county. This link indicates that the one for Adams county is out of print:
http://soils.usda.gov/survey/printed_surveys/pennsylvania.html

You should be able to locate one at the USDA Service Center in the link below. If they don't have one, your nearest library probably does. I don't know if they will be the same as the soil survey for my county, but if they are, they will have all the information you are looking for. They have detailed maps, plant association with soil types, detailed descriptions of each soil type, etc.

Here is a link that might be useful: USDA Service Center

Thanks so much for the information and for the detailed post. My neighbor tells me that our woods are alive with deer which makes me wonder how successful I would be attempting any (costly) planting. I dearly love the viburnum family for woods edge and was also thinking of elderberry and at least one willow near the water. Do you think everything mentioned would be eaten to nubs by deer?

you may find your property is mapped as an on-line resource. this site has been up for less than a year, i think. not the easiest to navigate but can be fun if you find your property in the database.
you can select your applications such as farming, livestock, timber, etc.
also, that would be "natural", not "national" resources conservation service if you go looking for more information. ;-)

Here is a link that might be useful: web soil survey

Deer are not big fans of viburnum as the whole genus is a little hairy in the leaf, try some and wait. Arrowood or highbush cranberry should even survive a browse or two, I suspect, especially in high density bottomland...

The USDA Plant database also has listings for whether a plant is a wetlands indicator that are used for official assessments; that might be definitive... I think Ladyslipper is right, perennials would be more telling...