simy1 said:

OK, so P and K are both "structural". That much I knew because I knew
that wood chips typically have the same (about 200 ppm) P and K
content. But on my own I had also computed that the K taken out of a
given bed every year was close to the K supply in that bed. Here goes:

assume a bed, 1ftX4ftX20ft=80 cft (some 7 Tons of soil, assuming good
organic content). at 100ppm (fertile soil), that is 700 g of K. One
Big Boy tomato is 0.7 g K. Harvesting 1000 tomatoes from that bed will
completely exhaust the supply (that could be 3 or 4 summers).
Ultimately I figured that N (for greens) and K (for fruits) need to be
resupplied continuously (I do it every two years). Not so for P, since
the P content of crops is much lower (or rather, my underground crops
are a lot less than my aerial crops).

Be sure to make allowance for K lost to leaching in sandy soils. It isn't
'bankable' like P, and it may not leave only in your crops. That's one reason
that greensand is worth using source of K; it releases so very slowly that it's
not easily available to crops but is almost like having K in the bank.
--
Pat in Plymouth MI

Any technology distinguishable from magic is insufficiently advanced.
(attributed to Don Marti)