I imagine it would be safe to plant sweet corn about the middle of May. I like to give it room..like 12 to 14 inches apart in the row and rows 26 inches apart. Plant them 3x as thick and thin out the less vigorous seedlings. If you like bi-color, Ambrosia is very good and not hard to find seed for. For yellow I like Incredible which also is not hard to find. I suggest you start with these se+ corns.

Potatoes are likely planted in your area about the end of April. It is best to lay the seed potatoes out in low/medium light about 2 or more weeks before planting so they will grow short sturdy sprouts instead of long white sprouts. Plant them about 3 inches deep and hill lightly when a foot tall....rows 28 inches apart.

Potatoes need good levels of organic nitrogen and potassium in particular, as does corn.

how about potatoes varieties? any suggestions?

I would check with the local feed store. Ours has seed potatoes for sale when it's time to plant. May 15 sounds early to me for you to plant corn. Google EDIS for a vegetable planting guide for your area. It has alot of helpful info. Also your local agricultural center should be able to tell you what varieties grow best in your area. Our local box stores don't always sell the varieties that grow best in our area. Maybe you have a good garden center near you that can help you?

oak leaves the size of squirrels ears for corn, they used to say.

There are a lot of varieties of potatoes because there are a lot of uses. It depends on how you want to cook them and when you want to cook them. You would be best off buying a mix and trying a few different varieties to learn what you like best and what grows best in your conditions.

Here is a link that might be useful: Choosing Potatoes

you might want to try googling "potato tower" for some great space saving/harvest maximizing tips

Here is a link that might be useful: potato tower video

I am in the middle of Minn. and I will plant potatoes this week.
Depending on the spring, here we start planting from April first to as late a June first for potatoes.

I will be putting some corn in next week also.
I have planted corn in early April before and in all the years, thirty plus, I have been planting corn, I can count on one hand and come up with change at the number of times I have even hybrid sweet corns rot in the ground so I had to replant.
At the same time I have also waited till the first of June to plant in cold wet springs.

All you can really do is experiment.

Go on the net and put seed potatoes in a search, many sellers describe what type x, y or z is best for. Plant the type/s that will fill your needs best.
The same for corn.

Over the years I have planted well over a dozen different types of potatoe and literally dozens of different types of corn. Both field and sweet.
I planted Maize Morado a Peruvian 160 day corn one year and by the grace of God, the weather stayed in the seventies all through Oct. I got several fully developed ears.

When I plant, I put potato rows two feet apart and the plants in the rows sixteen inches apart to give me room so as to not spade or spike potatoes when I dig them.

For corn on average I put the rows 28 to thirty inches apart and depending on corn type six to twelve inches apart between plants.

One year I wanted to experiment and planted seven varieties
three inches between plants and rows hip width apart. I planted both sides as I crawled along.
Each block was eight feet long, four or five rows wide.
I got a massive gardent of, for lack of a better word, grass. Tall stalks, eight to twelve feet tall, these were the tall types sold by Shumers, with few fully developed ears.

Two years ago, I had corn that was from four to eight years old.
I figured the germination would be spotty so this time I put in eight blocks, but spaced the corn rows thirty inches apart, and put two seeds in each hole nine inches apart.
I had at least a minimum of ninety percent germination and a garden full of sweet and field corn of differing maturity.
I/we ate sweet corn till it came out of our ears and the squirrels had so much corn during the winter, I think some had belly-rolls.

Last year, figuring how good my old corn had done; I put in only six types of fresh corn packed for 2011.
I got a total of six plants.
Planted same time of year, first week in June, and watered during a dry spell.

In a separate garden fifty miles to the north, of three types I planted, same week, one rotted in the ground and of a second, I had to transplant stalks to form three short rows.
The transplanted ones did well, but it just goes to show, that one can never expect the expected.

Experiment and if it goes well, enjoy; if it goes badly, write it off to experience.