Good soil has benefits but plants don't get carbon from their roots. They make sugar in the leaves and make that into more complex molecules for growing.
The plants get carbon from the atmosphere of course, but retaining organic matter in the soil, which captures that carbon, has other value.
"Soil organic matter significantly improves the soil's capacity to store and supply essential nutrients (such as nitrogen, phosphorus, potassium, calcium and magnesium), and to retain toxic elements. It allows the soil to cope with changes in soil acidity, and helps soil minerals to decompose faster."
"Soil organic matter significantly improves the soil's capacity to store and supply essential nutrients (such as nitrogen, phosphorus, potassium, calcium and magnesium), and to retain toxic elements. It allows the soil to cope with changes in soil acidity, and helps soil minerals to decompose faster."
https://www.google.com/search?q=benefits+of+soil+organic+mat...