A high test result indicates that yield loss due to the nutrient in question is unlikely. However, when the test result is medium or low, it does not automatically mean that applying the nutrient will cause a crop response.

Fernandez notes that tissue nutrient analysis is usually more reliable than soil testing for secondary macronutrients and micronutrients, but some of the same issues mentioned for soil analysis apply.

Sufficiency ranges shown in reference tables are quite large for most nutrients. This is due in part to the large variability inherent in these measurements. As with the soil test, having a tissue test value below the sufficiency range does not necessarily mean that applying that particular nutrient will cause a yield response.

Fernandez said it is important to remember that, while the tables in the Agronomy Handbook or other such tables that may be used to obtain the standard value, tissue test levels are specific to a certain growth stage and plant part.

Both plant part and growth stage have to match the table; otherwise, the numbers are meaningless.

Because micronutrient and secondary macronutrient deficiencies normally occur only in parts of the field, tissue testing can be a useful diagnostic tool when it is used to compare healthy plants to those showing a problem.

However, Fernandez recommends caution and suggests using this information while taking into account the issues mentioned for soil testing.

"If, after considering all the evidence, one determines there is indeed a nutrient problem and an application of that nutrient can solve it, one should make the application only in the problem area and not on the entire field," he said.

Finally, he said, "If it ain't broke, don't fix it."