This article is written by Dr. George Rehm, University of Minnesota, Soil Fertility Specialist (retired).
See more of Dr. Rehm’s blogs at http://www.agbuzz.com

During the past two or three years, there has been an increase in the promotion for the use of tissue testing/plant analysis as a management tool in development of fertilizer programs.  At times, if you  read all of the advertising literature, you  might get the idea that the practice of plant analysis/tissue testing is so important that you can’t make a profit without it.  So, is this really a new and exciting management tool to be used by every crop producer?  A close examination of the facts without all of the advertisement leads to the answer: not really.

There are problems with placing dependence on the use of this management practice.  Some of the problems and pitfalls have been identified by Dr. Dan Kaiser, Associate Professor and Extension Soil Scientist at the University of Minnesota.  These are briefly described in the paragraphs that follow.

STAGE OF GROWTH at sampling is a major consideration.  With corn, for example, it’s impossible to compare analysis of plants sampled at the V5 growth stage with analysis of plants sampled at some later growth stage.  As the corn plants grow, nutrient concentration is diluted and concentrations, therefore, decrease.  If all other factors are equal, a concentration of nitrogen, for example, may be higher and adequate at V5.  The concentration percentage will be lower at V10 and still be adequate.  This concept has been verified by substantial research conducted by faculty at Land Grant universities.

In order for tissue testing/plant analysis to be meaningful, the results of analysis of the plant tissue must be compared to some standard.  For corn and other crops, these standards have not been developed for every stage of growth.  This is usually true for stages early in the growing season.  That’s primarily because concentrations are rapidly changing at those times.  So, what’s the point of analyzing corn plants at the V5 growth stage if there are no standards for nutrient concentrations at that growth stage?  I don’t know.  I don’t believe that there is general agreement among researchers knowledgeable about plant analysis as to what the adequate concentrations are in whole plant corn tissue at the V5 growth  stage.  With corn, accurate interpretation of plant analysis information is possible if plant samples (leaves) are collected at the time of silking.

At silking, however, it’s too late to apply nutrients that might correct a deficient situation.  So, analysis of corn leaf tissue at silking cannot be used to predict rates of any nutrients needed during the growing season.

TIME OF DAY used for sampling can also affect concentration of nutrients in specific plant parts.

Research has shown that this is especially true for nitrogen.  Nutrients may be more concentrated in plant tissue in the morning; but, as plants grow, the concentration can be diluted by dry matter added during the day that is the result of the normal growth process.  This effect of time of sampling just adds to the variability that may be experienced with plant analysis.

HYBRID AND?OR VARIETY can also have a substantial influence on “critical levels” associated with plant analysis.  Researchers are finding that the rate of nutrient accumulation is different among modern hybrids or varieties.  Therefore, it’s reasonable to expect that nutrient concentration in any plant part at any stage of growth will vary with hybrid or variety.  This is yet another source of variability in plant analysis.

It is known that nutrient concentration in plant tissue is affected by stage of growth at time of sampling, time of day used for sampling, and hybrid or variety.  There are obviously other factors that contribute to variability in the results of plant analysis.

Many have used the results of plant analysis as an aid in the diagnosis of a problem in a field.  Plant analysis was originally developed as a diagnostic tool.  When combined with companion soil samples, this tool has helped to solve many problems.  It is, however, a stretch to use this practice as a tool to predict the rate of any nutrient that should be applied to any crop.  It is simply not a predictive tool that can be used with confidence.  There are too may opportunities for error.

While there are several factors that can produce variability in the concentration of nutrients in plant tissue, there is only minimal variability in the laboratory procedures used in the analysis.  The analytical procedures have been standardized among laboratories by using “standards” with known concentrations.  If there problems with the laboratory analysis, the routine use of these “standards” will identify those problems.

Plant analysis/tissue testing is not something new.  The concept has been around for many years.  When used appropriately, it has value.  However, the ability of this management practice to predict rates of nutrients needed for crop production is now and has been limited.  Don’t expect any more than what this practice can deliver.

Dr. George Rehm,
University of Minnesota
Nutrient Management Specialist (retired)
rehmx001@umn.edu