All assessments shall be liens against the property assessed from and after the first Monday in March of any year. The lien for the payments due the United States under any contract between the district and the United States, accompanying which bonds have not been deposited with the United States, shall be a preferred lien to that of any issue of bonds or any series of any issue, subsequent to the date of such contract, and such lien shall not be removed until the assessments are paid or the property sold for the payment thereof.