Phosphate-free detergents are better for the environment because they can encourage algae growth in fresh water.
The study found that many phosphate-free detergents don't do a very good job cleaning, but two of them do.
Consumer Reports challenged detergents beyond what they face in your kitchen, smearing a blend of peanut butter, pudding, macaroni and cheese and 14 other sticky, starchy foods. Then the goo was baked on, and eight of the dishes were loaded into each dishwasher the exact way each time, along with two pots.
They are then washed using the recommended amount of detergent.
"Five out of the seven detergents that lacked phosphates didn't do well at all in our cleaning tests. In fact, they left large amounts of food and goop all over the plates," said Tod Marks from Consumer Reports.
But other detergents were tough enough to make even the heavily soiled test dishes sparkle.
The clean winner was Cascade Complete All-in-1 packet. It did an excellent job cleaning, but it does have phosphates.
"The pros of phosphates in dishwashers is that they can boost the cleaning power of any product. However on the flip side, they're bad for fresh water," Marks said.
Two phosphate-free detergents did well in Consumer Reports tests: Method Smarty Dish tablets and Simplicity 2-in-1 packets.
While neither did a great job on pots, both did quite a good job getting plates and glasses sparkling clean.
In addition to the detergent, Consumer Reports says how you load your dishwasher makes a big difference in how clean your dishes get.
Put the dirtier side of the dish toward the center and load large items at the sides and back so they don't block water and detergent from other dishes.