My zucchini and squash seedlings bottom leaves are turning yellow. They are in a greenhouse waiting to be put out in the garden.
Zucchini and Squash Seedling
Certified GKH Gardening Expert
If by the bottom leaves, you mean the cotyledon leaves - which are the first leaves the plant gets after sprouting, this is normal. Plants drop their cotyledon leaves after they have a few true leaves.
If the plant is dropping/yellowing its bottom true leaves (also referred to as their second leaves), then this indicates either a lack of water or a lack of nutrients. It will correct itself with some additional water or with some diluted fertilizer.