"Second, the soil itself changes over time, as particles break down, buffering ability is used up, and nutrients are leached out by the activity of roots or flushing action. Old soil in a pot will get to a point where pH changes (usually drops, since most commerical media are peat-based and peat haas a natural pH around 5.0-5.5). As larger pieces of soil material break down, the interstitial spaces between them also get smaller, and the soil holds water very differently. Drainage starts to slow, and the roots again end up sitting wet and stagnant."-StinkyAttic