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[Clansgian AT wmconnect.com]

Phosphate depletion is a function of mechanized farming.  In the end the 
advice you proffer: 

>-If you own land, build your in-soil P reserves now while it is cheap 
and clean

is the only real solution.  But not for the reason that our (generally) 
mechanized mindset might suggest to us.  The image is of putting enough, 
prefereably slow release, sources of phoshpate on our land that we will be in 
good stead 
for years to come even when commercial sources are sarce and inexpensive.

But the tack to take on this, says I, is to keep in mind that just like there 
is a nitrogen cycle, there is also a phosphorous cycle.  In the nitrogen 
cycle organic nitrogen is consumed by bacteria which give of nitrites as 
waste, 
other bacteria consume the nitrites and give off nitrates as waste.  Plants 
can 
only use the nitrates directly.   Chemical fertilizers short circuit the 
process providing nitrates in the form of salts directly to the plants.  The 
overabundance of nitrates is toxic to the bacteria for whom it is a waste 
product so 
they die.  Then since they aren't there to take up the nitrite waste, those 
bacteria die as well.  The soil becomes sterile and the only way plants can 
grow is the continued application of inorganic nitrates.

When we put manure and compost on the garden, we are feeding the bacteria who 
"eat" organic nitrogen bearing compounds (protein, amino acids) who in turn 
promote the growth of nitrate fixing bacteria.

The phosphorous cycle is similar.  There is series of bacteria that ingest 
organic phosphorous compounds and give off other phosphorous compounds as 
waste, 
which other bacteria "eat" and give off others until it is converted into 
phosphate which the plants absorb.

So the idea is not to stockpile a reserve of phosphate compounds in the soil 
that will be slowly but certainly depleated over time, but rather promote the 
abundance of bacteria involved in the phosphorous cycle,