Hmmmm, the fertilizer may be 10-10-10, or 10-20-5, etc.  But, what does that mean?

Think of it this way: Up - Down - All Around
The first number is the 'Up' number, this number will provide nutrients to the upper part of the plant, the foliage.  The second number is the 'Down' number, the number will provide nutrients for the root portion of the plant.  The third number is the 'All Around' number.

If you're fertilizing onions, or other root crops, you wouldn't want a 15-0-0, because the onion would have lovely green foliage at the expense of the root.  You'd want the middle (down) number to be the highest for root crops.

Country Blessings,
Jamie