You need to cut a trench all the way around the perimeter of the basement, to the depth of the foundation footings. Do not carve the earth below the bottom surface of the footings, although if you dig the trench 12 inches in from the footing side (probably 16 inches in from the foundation wall) then you can dig four inches further down which may assist in sloping the perforated drain pipes better.
Do you get coverage you can count on? From the bag of fertilizer? From your lawn sprinklers?