Historical clue formulations

• Tarnishes — 2024-11-05
• Gets dirty — 2023-09-02
• Taints — 2021-04-18
• Muddies — 2017-02-26
• Their pH's are often measured — 2015-03-07
• Dirties — 2014-03-10
• Besmirches — 2011-11-20
• Agronomists' samples — 2009-02-01
• Makes a mess of — 2009-01-18
• Muddies — 2007-02-08
• Agronomists' study — 2005-11-11
• Dirties — 2005-06-12
• Taints — 2005-03-02
• Dirties — 2004-12-06
• Besmirches — 2001-02-27
• Agronomists' study — 1999-06-04