Soil tests from most soil test labs are based on the "extractable"nutrients, and the extracting solution is usually
a fairly weak one so that what is "extracted" mimics what is available toplant roots during the current growing
season. One can use stronger extracting solutions to get a largerproportion of the nutrient pool. In fact, with a
mass spectrometer, you can count pretty much every molecule and atom (fora pretty high price). The concern is that,
although those elements locked inside a grain of sand would eventuallybecome "available" as the sand is weathered to
silt and clay, that may require hundreds of years in the real world. Thusthat kind of laboratory measurement is not
a particularly useful number to tell us anything about what plants mightextract during our lifetime. So while the
results would be quite accurate, they would not be particularly usefulfrom a soil fertility standpoint.
stronger extracting solution, but then again,
The more rational approach might be to express the nutrients based on a
the interpretation of those results would depend on the specific chemistryof the extracting solutions and the soil.
Add in the variability from point to point in the field, nutrientinteractions that interfere with uptake, and
differences in nutrient requirements form one plant species to another,and the numbers you get might require a
five-year PH.D. dissertation in soil chemistry and biochemistry to fullyunderstand.
have all the nutrients required for plant
While it is true that soils with adequate organic matter generally will
growth (after all, the organic matter was originally plants), whetherthere is enough that becomes available in
adequate amounts, at the right time and in the right balance is adifferent question. Knowing the "total" nutrient
pool does not answer that question any better than knowing the "available"pool.
Jim Kotcon
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