a method can be used for soil analysis, it must be calibrated. A
wide range of soils must be analyzed and a crop grown on them. A
calibration curve is then plotted to determine if there is a
relationship between the analytical results and crop growth.
calibration curve looks like this:
soil analysis methods
dot on the graph represents an actual case where crop growth was
compared with the soil zinc level determined by our method. Below a
value of 1.5 ug/cm3, crop growth is severely limited by a lack of
zinc, this is therefore our deficient range.
values of 1.5 and 3 ug/cm3, our “low” range, crop growth
is reduced by the lack of zinc, but not as severely as in the
values above 3 ug/cm3, the soil can provide enough zinc for optimum
crop growth, this is therefore our “sufficient” range.
our calibration, we did not encounter any soils with zinc levels
high enough to reduce growth, therefore our “excessive”
range is not clear.
area with test values below 18 is labeled as “deficient”.
In this range, the amount of the nutrient available is too low for
proper crop growth and we must therefore apply relatively high
the “low” range, between test values of 18 and 30 in
this case, the nutrient level is still too low for optimum growth,
but lower doses are needed than in the “deficient”
the “sufficient” range, between test values 30 and 70
for this case, the soil can provide sufficient nutrients for optimum
growth. A small maintenance dose of the nutrient is still
recommended for test values in the lower end of this range
otherwise crop removal will eventually reduce the nutrient level
into the “low” range.
the ”excessive” range, over test value of 70 in this
case, the nutrient level is high enough to actually reduce crop
growth. This range is often incorrectly referred to as the “toxic”
range. Excess amounts of calcium, for example, are not poisonous to
most plants, but reduce crop growth by affecting the uptake of other
nutrients. Obviously, we do not want to apply any of the excessive
nutrient under this condition.
following is an actual calibration curve for zinc analysis using
our test methods.