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Before 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.

A theoretical calibration curve looks like this:

Calibrating soil analysis methods

Each 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.

Between 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 deficient range.

At values above 3 ug/cm3, the soil can provide enough zinc for optimum crop growth, this is therefore our “sufficient” range.

In our calibration, we did not encounter any soils with zinc levels high enough to reduce growth, therefore our “excessive” range is not clear.

The 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 doses.

In 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” range.

In 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.

In 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.

The following is an actual calibration curve for zinc analysis using our test methods.