David E. Ross wrote:

The purpose of compost is to improve the structure of the soil, not to
provide nutrients. Since compost is often applied only to the top
inches of the soil, you should want any nutrients -- as few as there
might be -- to leach down into the root zone. Inhibiting the leaching
of nutrients would thus be counter-productive.


Compost supplies (directly or indirectly) organic colloids eg humus, that
have a role in binding ionic nutrients It also improves drainage and tilth.
All that is part of improving structure, that is improve the way the soil
acts to allow nutrients, air and water to be available to plants. Also
compost does supply some nutrients for the plants and some for the
micro-organisms as well as directly supplying some useful microbes.

So isn't it fair to say compost supplies nutrients and improves structure?

Excessive leaching is a real PITA. I worked on a sand-based garden for 20
years and unless you were constantly replenishing organic matter and
nutrients it had very poor productivity except for the natives that were
adapted for poor soil. I will bet that most of the soluble nutrient ended
up in drains and then waterways soon after it was applied.

Now I have a clay-silt based garden and it is extremely productive after
only a couple of years. Having that clay colloid there (as well as organic
matter and compost) greatly reduces nutrient leaching so that I can grow
intensive crops with good productivity and only add manure once or twice a
year.

I think the way that it works is during rain events or deep watering the
colloids on top (near where the manure is applied) becomes saturated with
bound ions and so the excess is carried down to the root zone and then to
the subsoil which ultimately gives those regions a chance to bind ionic
nutrients too. Despite the difficulties with drainage sometimes, I will
take the low leaching silt before the high leaching sand every time.

David