I disagree with ecnerwal's answer. That split is called "a check". Yes, when lumber dries it will twist and often check. However, failure occurs when the check runs diagonally, which is happening to your post (column).
If, as you say, it supports 40-50 square feet of the kitchen floor, then that represents a minimum load of: 40-50 x 40 lbs. per square foot (minimum load per code) = 1600-2000 lbs. That's significant...especially if it also supports a refrigerator.
You're safe for now because the check has not extended completely through the post. When it dies, it will fail because it does not run perfectly vertical. I'd use 10d nails at 8-10" oc staggered across the grain (stay about 1 1/4" away from edges) and nailed from both sides or use screws #8 at 8-10" oc to fasten the two pieces together. (Do not align nails or screws with the grain or you could split the post.) Also, don't "over nail" (nails too close together or driven too deep) for fear of splitting the wood.
Pay attention as you nail or screw to see if it starts to split more...either direction. Btw, you're not going to pull the two pieces together. The purpose of nailing is to transfer the load to both pieces of the post.