Arable farming, particularly where the plough is the main means of cultivating, has profoundly altered the soil by both adding and removing plant nutrients, reducing acidity with lime, draining excess soil moisture with underground pipes, removing stones, and changing the soil structure. More fundamental changes have been brought about by the removal of surface water. Long and painstaking drainage schemes, notably in the English fenlands, the Dutch polders and inland lakes, and in the Po valley of northern