Under Watering & Over Watering

I live in Southwestern Wyoming. The temperatures here vary all the time. Two weeks ago we were having snow. My biggest problem right now is I can not tell if I am over watering my plants or not watering enough. I have some plants in containers and some I have planted in several small gardens. Where I live its just like being in the desert mountains of WY, most of my soil is clay. When I started my gardens I dug out the soil in them and then filled them back in with bagged garden soil or potting soil. Some of my gardens and pots are doing very well, then some of my gardens and pots have some of the flowers growing and some have died, then some of my pots and gardens were growing well and then they just all died. Some of the soil seems to be really packed and some seems to be like it should and I put the soil in the gardens and planters the same way. Also I have no shade so all plants get full sun.
I have mostly planted pansies, marigolds, snap dragons, and petunias. I tried to plant impatiens, but every time I tried I would have the plants for a few days and it would snow. I made sure to take my plants in the containers and put them inside, but I didn't know what to do with the gardens, so they were left alone. It really didn't seem to matter though, because I ended up with some plants dying and some not in both situations.
When I bought my plants they were transplants. I tried to make sure that all of them read the same - sun, medium water, etc. The plants that seem to be doing the best are my petunias in the planters, but I have some snap dragons mixed in with them and some of them are dead and some of them look like sticks with green dying leaves on them. Same with my pansies, some look really great and are blooming and some look like they are wilting, (leaves all yellow brown or starting to turn that way), and dying. I have taken some of the dead pansies out of the containers and gardens and when I pulled them up they came off at ground level with no roots attached.
I have also fed my plants with Miracle Grow. I thought maybe they weren't getting enough nutrients.
I look at all my planters and gardens and think Wow they need water - what do I really need to look for so I know they need water? Any help is very much appreciated.

Hi Jeana, Clay soil is really hard to work, but you seem to have got off to a good start by adding good soil, you dont say what kind of soil you added, and if you dug it into the clay you had at the bottom of your beds after you removed some of your clay, clay holds moisture and cold much longer than any other soil and then when it gets baked with hot sun, it forms a crust on it which prevents water penitrating down to the roots so you should try, next time you big your beds, really mix the good soil with the clay bottom to break this up to allow any water to drain away freely rather than sitting in the bottom of the beds. it is also a good idea, as you plant, give the soil a good feed of fertilisers, blood/fish and bone meal that you can buy ready mixed from the garden centers etc( dont give more than the directions on the packets say as you can burn plan roots with too much food) this gives the plants all the feed they will need for the season, I always feed summers end and again in spring as the new growth starts to come into view,, then the rain/watering gets it down to the roots.
You said that when you pulled out some of your plants, they came away without the roots, if the ends were rotted, then the soil around them has either been too wet or too cold, or a mix of both, the watering in the garden beds will differ from what the pots will need because in the pots, you have a lot of roots competeing for little soil, so they usually need more water and fed maybe once a week with a very mild feed, maybe half dose of house plant food would do just to enrich the soil around the roots, how to tell if plants need water is by sticking your finger into the soil, if it feels dry, it needs water, if wet and cold, dont water, I think also your plants may have suffered the same as a lot of us gardeners this past year or so, by too many extermes of temp and as you said, late snow and then sun, your pots sound too dry and the beds too wet and cold, wish I could help further but maybe someone from your region will come in and help further. good luck, dont give up, it takes us all a while to get it all right. WeeNel.

I would like to know too, what kind of soil you used. How big and deep a hole you dug for each plant, etc. Pansies are a cold weather plant. The will live and bloom even in the snow. I plant them here in the fall to bloom all winter. When the weather starts to get hot I pull them up and plant my spring plants. Would love to see pictures.