What's with "row spacing"?

Hello, I am going to plant my first garden this week. I want to plant onions, turnips, spinach, and corn in a 4x12 foot raised bed. All of the seed packets I bought were seem to have two sets of information for "plant spacing". they have "row" and "plant". I can understand why plants would need a certain amount of room to grow, but I'm not sure why they have two different values listed. Why would being in a row mean a plant needs more space?

Seed Packet

I'm going to guess that for larger operations, the 18" refers to the width between the rows.  The recommended plant spacing of 6" is the same no matter how you do it, but 18" spacing would be a pretty common distance between rows for a lot of mechanized planting equipment.

I have no problem using the smaller spacing both ways, but do it on a diagonal spacing.

If first row is 6 12 18 24

Next row is 3 9 15 21

I tend to plant in beds rather than rows to maximize space.  18 inches between rows seems like a tremendous waste of space.  However 6 inches between plants is tight.  So somewhere in the middle might be the Goldilocks just right.  

However, I always make the mistake of planting cabbages too close.  They need a good 2 feet of space or more.  Onions, not so much.

my guess would be the row spacing is based on how tall the plant is going to get, so - it allows for enough sunlight to get to the plant if they are planted in many large rows.  I think that is to get the maximum production from each seed possible, but for home gardens - I plant things closer to have less weeds to deal with.

The plants need to be just so far apart that their leaves touch when they are fully grown.  The seed packet tells you how far apart to have the rows because it assumes you are going to be walking between them to hoe, pick, etc.  If you grow on a raised bed system where you never tread on the soil but reach in from the sides, you can plant them the same distance in the rows as between the plants.

Phil, Welcome to gardening!  I agree with everything above. I personally don't space my plants nearly as far apart as most seek packets say.

For onions: a friend shared with me a few years ago that transformed my onion planting. He said that if I plant three in a spot 6" apart (not very deep) all three will grow nice round bulbs. So, I tried it and it works. I always start onion seeds in a small container or flat. You can just spread them out fairly close together and then cover them with a little growing medium of your choice (I use river loam that I get myself from the river). When the onions have two or three "leaves" I transplant them. Soaking the whole bunch in water allows for the roots to come apart easily. I take two or three in one hand, use my trowel in the other, stab the ground with the trowel, pull the soil apart a little and then insert the 2 or 3 little onion plants into the crevice, drop some water on the roots, remove the trowel, push the dirt down and together.

Planting two or three at a time in this manner goes very fast.  Last week I just transplanted a whole seed packet of Walla-Wallas. Good luck on your first garden!