[stentman]

Depending on your stand point it maybe either light hearted or a serious issue but leaching? If you put "leaching" into the search function a few things crop up from the potential for wood preservatives to leach from railway sleepers, using green manures to stop it or even dyes leaching from old carpets (A seperate debate perhaps) but the most common is leaching nutrients from your plot. My question how does that work? I mean how does a molecule of good stuff get from say the centre of my plot to the pathway between me and next door? It may seem a daft question but where do the nutrients actually go?

[JayG]

Clay soils drain very poorly, which means that the nutrients contained within them are not easily flushed out or through them.

Sandy soils on the other hand drain very (if not excessively!) quickly, so the nutrients are leached downwards into the subsoil (which is also likely to be sandy)
Ultimately the nutrients will find their way into the underground water table; where that goes depends on your local geology!

(Of course if you have a combination of impervious soil and a low-lying plot the nutrients (and the water) will probably finish up in your front lounge!) 

[Trillium]

It may seem a daft question but where do the nutrients actually go?

They're not molecules attached to the soil, but are separate. So, a good rain will literally start washing them out wherever the water can leak away. From raised beds, they're likely to pool into paths or adjacent areas. In sandy soils, the easiest way is down, way down where they're hard to pull back with specific plant roots. As stated, clay soils tend to trap nutrients better so the leaching is slower, but it's inevitable.

[solway cropper]

I think this is one of the reasons for applying as much organic matter to your plot as you possibly can. It helps reduce the amount of useful stuff that get leached out. It also differs with what sort of nutrients you are talking about. Soluble ammonium nitrate will disappear very quickly but some of the less soluble phosphates will stay around for much longer. Like everything else in gardening.....it all depends.