Healthy soil and sustainable growing

[l kelly <elkel@hotmail.com>]

I've read about this topic before on this list.  The idea that even the most carefully managed organic farm must import fertility to replace the produce exported.
I remember one list member reminding us that the equation also included the "free" input of solar radiation, and resulting photosynthesis, regardless of soil condition.  I also remember reading Eliot Coleman's ponderings of how many acres of "green manure" does it take to support the fertility of an acre of produce for export.  
 
I own and operate a small market farm with only an acre (at most) in cultivation, including the 12-tree orchard, and I have about 20 acres of woods to provide leafy mulch fertility.  My husband and I practice sawdust composting, or "humanure" as described at the humanurehandbook.com website.
Weekly trips to the "city" always bring a truck-load of curbside compost, and the local sawmill dumps 10-ton piles of sawdust whenever I ask for it.  
 
I understand that phosphate and potassium deficiencies would still result if I did not supplement. 
Does anyone know of a more closed-loop method for those two minerals?
 
Laura
Windberry Farm, Arkansas