Hoes and spades are both cutting tools, and should be kept sharp. Shovels are not normally sharpened except to remove a bad knick. These tools get rough usage in the soil, and dull quickly. If you are doing a job where sharpness is important, carry a file with you and sharpen the tool every couple of hours.
Sharpen hoes and spades with a 10- to 14-inch bastard mill file. Both hoes and spades are sharpened on only one bevel, each at an angle of about 45 degrees. You can get them sharper by sharpening at a greater bevel, but the edge becomes correspondingly more delicate and quickly dulled. You can also sharpen spades and hoes with a grindstone or belt sander. Be careful not to overheat the metal with a grindstone.
Files work best if held with both hands to give a firm grip. It also helps to have the tool held firmly so it doesn’t move or chatter as you cut. Lock the tool in a vise or place it on a solid edge if in the field. Sharpen with long diagonal strokes that go all the way down the length of the blade. Files cut on the push stroke only. Lift the file for the return stroke. Maintain even and constant pressure on each stroke; filing one spot excessively will cause that area to be cupped.
Finish off the sharpening by turning the tool over and removing the bur with a couple of strokes flat against the back of the blade.