Of ground: yielding to the feet, an animal's hooves or paws, a vehicle's wheels, etc.

Of ground: soft and yielding when trodden upon; boggy. Also in figurative context.

Having a soft elastic or porous texture resembling that of a sponge; deficient in solidity or… Of ground or soil, esp. through excess of moisture.

attributive or adj. Applied to ‘loose, spungy ground’ (Lisle).

as sense 3.