Kiara Nirghin beat students from around the world for a $50,000 (£38,000) scholarship with her “fighting drought with fruit” submission.
Her work was in response to the recent drought that has hit South Africa .
The drought, the worst since 1982, led to crop failures and animals dying.
Ms Nirghin, a student at the Anglican Church-founded St Martin’s High School in the main city Johannesburg, said three experiments over 45 days resulted in her coming up with the “orange peel mixture” as an alternative to expensive and non-biodegradable super-absorbent polymers (SAPs).
It was made out of waste products from the juice-manufacturing industry, she said.
These included molecules found in orange peels and naturally occurring oils in avocado skins.
“The product is fully biodegradable, low-cost and has better water retaining properties than commercial SAPs. The only resources involved in the creation of the ‘orange peel mixture’ were electricity and time, no special equipment nor materials were required,” Ms Nirghin added in her online submission.
The student, who was awarded the prize at the annual fair in California, said she hoped it would help farmers save both money and their crops.
The competition was open to children from the ages of 13 to 18.