Simple diffusion is a kind of diffusion by which molecules are transported along the concentration gradient but too slowly. Also, not all kinds of molecules can pass through it. Only those which are non-polar and small can pass. But plants also require mineral nutrients and charged ions like potassium, sodium, etc. which cannot be transported via simple diffusion because the cell membrane which is selectively permeable does not allow large, polar and charged particles through. So processes like facilitated diffusion that used membrane proteins like carrier proteins and channel proteins aid in the exchange of required substances. These are specific and occurs along the concentration gradient. Even active transport is required to transport molecules against the concentration gradient. So to suffice the needs of the mineral requirements of the plant, only simple diffusion will not be enough.