Martines initially told authorities that Peyton and her twin sister got into a bag of salt that had been left out, according to Fox Carolina.
But her four-year-old son later told detectives that he watched his mother feed the baby the salt and described what the bag of salt looked like. They later found the salt bag the boy described under a sofa cushion.
Martines eventually admitted to giving her daughter the salt to "get her husband back into her life" after they separated.
According to the station, Martines told investigators that she poisoned Peyton because the little girl was the closest in proximity to her at the time.
At the time, Peyton's family gave a statement, fondly remembering the toddler saying: "Peyton was the most beautiful, happiest, loving baby ever. She never cried and was a silly baby making everyone laugh.
"You could not help but smile when you was around her. She was a little diva who loved her nails done and her favorite thing ever was Minnie Mouse... Her presence will always be with us..."
Peyton died from hypernatremia, or salt poisoning, which occurs when too much salt is present in the bloodstream, causing cells to shrink as water rushes out of them.
This can lead to torn blood vessels as the cells are forced out of their normal location.
The imbalance of electrolytes can also cause fluid buildup in the lungs, as well as seizures and kidney damage.
Hypernatremia most often occurs when people don't drink enough water.
Infants are especially susceptible because their kidneys are too immature to process any added salt in their foods, according to the World Action on Salt and Health.
Multiple parenting sites also advise that salt never be added to any food cooked for babies and toddlers.
Martines' older child, as well as the baby girl's twin sister, were both placed in protective custody with the Department of Social Services.
On Wednesday, Martines was sentenced to 30 years in prison and will have to serve 85 per cent of her sentence before she is eligible for release.
