Subsistence farming is about energy.
Mainly carbohydrates.
Because you can easily get enough protein fibre and nutrients just by foraging wild plants.
You can easily get enough protein by hunting, trapping or fishing.

You grow potatoes, corn, wheat, sugerbeet, oats, peanuts, carrots etc. for subsistence.

When the system breaks down and supermarket shelves go empty you're not going to be able to feed people with lettuce, tomatoes, and zucchini.