They do both. Usually though, soil organic matter which contains the most SOC ensures food for bacteria and fungi which make healthy soil. Plants, much like humans, require microbiota to survive and stay healthy. You are not just feeding plants, but also the bacteria and fungi that prevent disease and help produce nutrients.
In addition, plain sand and clay is very poor at retaining nutrients and moisture. Organic matter tends to soak up nutrients like a sponge. This is why media based hydroponics uses porous material like volcanic rock and pumice.
In addition, plain sand and clay is very poor at retaining nutrients and moisture. Organic matter tends to soak up nutrients like a sponge. This is why media based hydroponics uses porous material like volcanic rock and pumice.