It can't do photosynthesis, so it can't feed itself, so it dies.
it dies --- end of story :) One of the plants that needs magnesium is the tomato plant. If tomatoes have insufficient magnesium they produce tomatoes with large black lesions known as blossom end...
The roots won't grow.
It will develop a poor root system
it will get yellow leaves
Planting Foundation Plants
How Compost Happens
Cucumber Plants Attacked by Spider Mites
The Problematic Planting Season