Stains from tomato juice, sauce or soup are some of the most challenging to remove. On light coloured carpets it seems to linger even longer than coffee, red wine or cola drinks. If you spill tomato soup on a carpet, act quickly. The longer the soup sits the more difficult it will be to remove.
Mix 1 cup of hydrogen peroxide with 2 tablespoons of non-sudsy ammonia. Pour a few tablespoons of the mixture on an inconspicuous part of the carpet, say in a far corner or under a sofa or chair. Most carpets are colour-fast and there won't be any change in colour, but some very dark carpets might lighten. If that's the case, don't use the mixture.
Remove as much of the tomato soup as possible. Use a spoon to get it off the carpet by scraping away the soup. Try not to press into the carpet, forcing the soup deeper. Finally, use paper towels to blot up as much as possible. Blotting is simply pressing down. Don't rub or scrub.
Flood the stain with cool water. Depending on the material of the carpet, hot water may just set the stain instead. Pour 1/2 cup of water on the stain then blot it up with a cloth towel. Repeat two or three times. Lay a fresh towel on the stain and weight with a couple of telephone books.
Check the test spot for any colour change. Since the spot is wet it will look darker than the rest of the carpet. That's OK. What you don't want to see is a lighter spot.
Pour the hydrogen peroxide mixture on the soup stain. Make a thick pad of paper towels a few inches larger than the size of the stain. Place the pad on the stain and press down. Place heavy books or weights on the pad for an hour. The hydrogen peroxide will wick up from the carpet into the pad taking the stain with it. Remove the pad and let air dry. Repeat as necessary to completely remove the stain.
If you have a steam carpet cleaner use that to flood the stain with cool water. The suction action of the cleaner will remove most of the dirty water. However, use a towel and the books as well. The water will make the stain a little bigger than it was originally so it's important to use a pad of paper towels larger than the stain. This process works for removing quite a few stains, not just tomato soup.
Don't try several treatments without letting the carpeting and padding dry in between treatments.