Answer: Assuming they are already outdoors and acclimated to being in the weather, you can plant as soon as the soil is no longer frozen and not too sopping wet/muddy. Actually they probably could have been planted last fall rather than held in containers. I hope the roots did not freeze too badly.
The best soil would be evenly moist yet well drained and a bit humusy, however they are pretty adaptable. (I would not recommend a constantly wet location for them.) Both are also adaptable in terms of soil being acid or not. In your area the soil should be naturally somewhat acid, but that is not something you probably need to worry about.
They do need full sun all day long or for at least six hours a day including the hour of noon. In shadier sites they will grow sparse and thin.
Modern tree planting suggests using little or no amendments since the roots will have to become accustomed to the native soil as the tree grows. The planting hole should be several times wider than the root ball and about the same depth so that you set the plant into the ground at the same depth it is growing in the container, or a little higher. Loosening the soil thoroughly encourages the roots to grow through it more easily. You could also add a little bit of good quality compost if you wish, mix it throughout the backfill soil.
When you plant, make sure to cut or loosen any encircling roots so they will grow outward rather than continue in a circle. Set the plant in the hole, refill it with soil (put it all back, it will settle in time) and then water slowly and thoroughly to eliminate air pockets and settle the soil.
Mulching is important, use a natural mulch just several inches deep in a flat layer over the root area year round. Do not let it touch the trunk, however.
Watering is the most important thing you can do for a new tree. Water as needed to keep the soil slightly moist, not sopping wet. It is better to water less often and let it soak in deep than to water lightly every day. After you water, wait a few hours and then dig down to see how far it has soaked down, sometimes this can be surprising. To know if you need to water, pull aside the mulch and use your finger to dig into the soil and feel it.
As far as fertilizing, I prefer to topdress annually with a good quality compost and use an organic mulch which helps feed the soil as well. If soil tests indicate a need, then fertilize accordingly.
It is better to fertilize too little than too much, so you would need to run some soil tests and see if you need to supplement. Your county extension should be able to help you with the tests and interpreting the results. I prefer a granular slow release fertilizer that can be spread evenly over the entire root zone rather than the spikes which concentrate in one spot.
Spacing will depend on which specific trees you have. Generally you would set them as far apart as their expected width at maturity. This will allow them room to grow without crowding. Along a road you may also need to leave room for being able to see past them when you pull out of a driveway. There may also be setback restrictions or right of way and also salt spray during winter to be taken into account.
There are a number of cultivars of both spruce and arborvitae on the market so you would need to know specifically what you have in order to predict the size range they will reach. Colorado Spruce sometimes called Blue Spruce as a straight species (Picea pungens) usually spreads about 10 to 20 feet when planted in a home landscape, it is bigger when growing in the wild. Since this is an estimated range of mature size, with seedling variation and also growing conditions and weather affecting what happens over time, I would suggest about 15 feet apart. You might however have a named cultivar which could possibly have a different size range, so you should check what you really have if possible. There are so many arborvitaes with a wide range of widths and heights, unfortunately I couldn't give you any kind of estimate for them.
I hope this helps, enjoy your new trees!
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