Kimberly Lightwine, 42, was discovered by police officers face down with a broken leg wearing just her underwear in Polk County, Missouri, on August 29.
Her disabled 19-year-old son Austin Anderson, who had the mental age of a child, was found dead nearby, dressed only in a nappy.
According to a police report, Lightwine repeatedly admitted to killing her son and told officers they had been lying in the field for a number of days.
A post mortem showed Austin had a swollen brain from dehydration and could have gone into shock after not taking a vital medication, court documents showed.
Investigators believe the teenager, who was blind in both eyes and had autism, may have died as early as August 26.
According to the probable cause statement, Lightwine told authorities: "I'm a terrible mother. I got high, and I got depressed, and I think I am going to throw up."
She reportedly told detectives that she was high on methamphetamine at the time and had used the drug for years. The mother told officers she remembered driving to the field and ordering her son from the car.
A probable cause statement shows she told the boy: "Austin, get out of the car and go reach out for help.
"Put your hands in front of your for help, and God is going to take care of you." She also said she threw him through a barbed wire fence to get him to safety, according to the statement.
Lightwine said Austin fell over and called out for his mother but she told him: "No, you don't want to love me, please let God take you."
Asked by officers if she went to her son's aid, Lightwine reportedly answered: "I just blocked it out.
"I just kept thinking that God or somebody came and got him and took him where he need to be. I don't know. I don't know what I was thinking."
She could not remember when the boy stopped calling out to her but recalled him shouting: "Mommy, I want to go home."
Austin's father confirmed the boy was blind, autistic, had limited vocabulary and was unable to walk far. He also told officers the boy needed several different types of medication, including hydrocortisone to stop him from slipping into a coma.
Lightwine has been charged with second-degree murder and second-degree elder abuse, which relates to disabled adults over 18 who are unable to protect themselves.
She faces between 10 and 30 years in jail for murder and up to 20 years for elder abuse.
Her bond was set at $250,000 but she is not currently behind bars as she is still being treated in hospital.