Question

Why do we don't walk in sandy and marshy soil

Answer 1

Sandy and marshy soil is soft and loosely held. Being loosely held cannot give the normal reaction equal and opposite to the weight of human body. This results in sinking of the feet down in to the sand or soil. This results in rising of the feet every time from the soil or sand. Rising the feet every time from soil needs an extra work to be done in the lifting the feet against the drag of the marshy soil/sand so walking becomes difficult.

Answer 2

due to the pressure exerted on the soil, our feet sinks into it as there is less friction in Sandy or marshy soil and the soil is loosely held Also it cannot give the upthrust equal to the weight of the body .