An ideal grain size for a planted aquarium is about 2 to 5 mm.
Conversely, small-grained substrate like sand is likely to compact too tightly for roots to penetrate deeply, and deep sand beds may also cause anaerobic zones where there is no oxygen and the flow of water and dissolved nutrients also cannot penetrate well. While some anaerobic zones can be beneficial for reduction of nitrate into nitrogen gas by anaerobic bacteria, these areas may halt proper root growth if you choose to plant near them. An ideal grain size for a planted aquarium is about 2 to 5 mm.