Why is it difficult to walk on sandy or marshy soil ?

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.