Every person who, as owner or lessee of any threshing machine, clover huller, corn sheller or hay baler, threshes grain or seed, hulls clover, shells corn or presses hay or straw at the request of the owner, reputed owner, authorized agent of the owner or lawful possessor of such crops shall have a lien upon such crops, beginning at the date of the commencement of such threshing, hulling, shelling or baling, for the agreed contract price of the job, or, in the absence of a contract price, for the reasonable value of the services or labor furnished. Such lien shall run for a period of (8) eight months after the completion of such services or labor notwithstanding the fact that the possession of the crops has been surrendered to its owner or lawful possessor, provided that such lien shall not be valid and enforceable against a purchaser of said crops from the owner or lawful possessor thereof unless the lien holder shall, previous to or at the time of making final settlement for such crops by such purchaser, serve upon such purchaser a notice in writing of the existence of such lien.