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They, actually, don't seem to mind being planted close such as this. If you feel that they are being constricted and affected by this, then you can pinch one of the tops off while leaving the roots in ground. This will avoid disturbing and killing the corn that you intend to keep.

I would leave it alone, though. They can usually do just fine like this.

This article will help: https://www.gardeningknowhow.com/edible/vegetables/corn/grow-sweet-corn.htm

These articles should help you with your corn

http://www.gardeningknowhow.com/edible/vegetables/corn/

I would start with soil testing. Contact your County Extension Office to help with this.
https://www.gardeningknowhow.com/extension-search

Possibilities include bird or rodent seed theft or complete failure to germinate.
Lack of moisture and or heat may be a reason for lack of germination.
Poor seed quality can also lead to poor germination.

These article will help you.

https://www.gardeningknowhow.com/edible/vegetables/peas/how-grow-peas.htm
https://www.gardeningknowhow.com/edible/vegetables/corn/grow-sweet-corn.htm
https://www.gardeningknowhow.com/garden-how-to/watering/watering-garden.htm
https://www.gardeningknowhow.com/garden-how-to/propagation/seeds/seed-coat-stuck.htm

Looks like Chickweed growing in there!

https://www.gardeningknowhow.com/plant-problems/weeds/how-to-kill-chickweed-best-way-to-kill-chickweed.htm

It is best to prepare the soil first, before planting, but yes you can add fertilizer or compost as a top dress. They can usually do fine with what is in the ground, but fertilizing can give them a boost. This article will give you more information on growing corn: https://www.gardeningknowhow.com/edible/vegetables/corn/how-to-grow-corn.htm