Bring the stock to simmer. The taste of the soup will be different depending on which stock you use. You can also use 2 quarts water + boullions.
Clean and slice the leek, white/light green part only. Clean and chop the celery. Peel the carrots and potatoes and dice them. Put everything in the simmering stock. Add salt, marjoram, oregano and basil. Simmer for 1-1/2 hour.
Take out a big bowl to hold the soup while you use the blender to puree/liquify it.
VERY IMPORTANT! Only fill the blender container half full and make sure the steam can escape on top. I usually hold a paper towel over the top to prevent splatter. You can also let the soup cool a bit before you puree/liquify it.
Put the pureed/liquified soup back in the pot and reheat. Taste and add salt and pepper if needed. If you have fresh basil around, chop some and sprinkle over the soup.