The grass strips I have worked on (including RAF Conningsby’s present operational grass strip) were just grass but usually on firm drained existing soil.
Advanced airfields were initially grass during WW2, if they were going to be used for any length of time drainage and an all weather surface (Pierced Steel Plank ‘PSP’, Sommerfield Trackway, etc) would be used.
Using purely grass strips limits the all weather capability of aircraft, even those with hard surfaces and drainage put in were still subject to periodic flooding as they were purely temporary.