This is not a new practice. Use of tissue testing (plant analysis) has been practiced in one way or another for over 50 years. Now, producers who have not used or are not using this management practice are asking if they have missed something. So, it’s probably appropriate to evaluate this management practice tool from my perspective.

Tissue testing is most frequently used for two reasons. These are crop diagnostics and nutrient monitoring.

When used to diagnose a problem that might be related to nutrient management, three plant samples (whole plant or specific plant parts) are needed. One should be collected from the portion of the field that is severely stunted.

A second sample should consist of plants or plant parts that appear to be normal. The third sample should consist of plants or plant parts on the margin of the affected area. These are the borderline plants; they are neither severely stunted nor normal and healthy.

Plant samples used for diagnosis of problems can be either whole plant or leaves. The diagnosis is possible after receiving the results of the analysis. If the concentration of any nutrient decreases from normal to stunted, this is a good indication that the nutrient may be the cause of the problem.

There are two major points to remember in using tissue testing for diagnosis of a problem. First, if the concentration of nitrogen decreases, the entire physiology and metabolism of the plant is affected. This disruption of metabolism usually has a measurable effect on the concentration of other nutrients in the plant. If nitrogen management appears to be the problem, you can disregard the other nutrients.

Then there is the problem of plant dilution. Let’s use Iron Deficiency Clorosis (IDC) in soybeans as an example. In many situations, the concentration of iron will be higher in the stunted, chlorotic plants than in the green, normal plants. This is caused by the fact that the chlorotic plants are severely stunted and, because of the stunting, the concentration of iron is higher.

A measure of nutrient uptake (usually expressed as grams or milligrams per plant) is the only way that I know of to get around the problem of plant dilution. For this measure, it is necessary to know the nutrient concentration in the plant and the dry weight of the plant. These two measurements are multiplied together to calculate nutrient uptake.

It may be difficult to get the dry weight of plants. This usually requires access to an oven that has air movement (forced air). Use of a microwave or a standard kitchen oven has not been found to be satisfactory for drying plant samples. The drying should be a rapid process.

I seriously doubt if problems with crop growth can be solved by using only tissue testing. Soil samples (0 to 6 inches) should be collected when plant samples are collected. Then the results of the analysis of soil can be compared to the the results of the analysis of the plant samples.

Use of tissue testing as a tool for monitoring the effectiveness of a fertilizer program is not as simplistic as it might apper. There are several factors that can affect the concentration of nutrients in plant tissue. For example, results of research have shown that concentration of nutrients in corn tissue can be affected by plant part sampled, hybrid, time of day when sample was collected, soil moisture conditions, etc.

When tissue testing is used for monitoring, it is necessary to compare the results to some set of standards that have been developed via research. In the past, these standards have been developed for whole plants or specific plant parts at a specific stage of development. The ear leaf of corn at silking is one example of a plant part for which standards have been developed.

The interpretation of tissue testing results has also been clouded by interchanging of the terms "critical value” and "sufficiency range.” From a historical perspective, “critical value” was used first. It was defined in various ways.

Essentially, a “critical value” of a given nutrient is that concentration below which yields decrease or deficiency symptoms appear. The “critical values” have limited use because they usually define the lower end of the “sufficiency range.” To add to the difficulty in interpretation “sufficiency ranges” vary depending on plant part and the research conducted.

For example, a “sufficiency range” for N concentration in corn ear leaves reported in scientific literature by one researcher was 2.76 percent to 3.50 percent. Yet, another researcher reported a sufficiency range for the same nutrient in the same plant part of 2.60 percent to 4.00 percent.

These values can be compared to a “sufficiency range” of 3.5 percent to 5.0 percent in young corn plants at 30 to 45 days after emergence. Similar variety in nutrient concentration has been reported for other nutrients.

Development of standards of nutrient concentrations in plant tissue has not been a high research priority in recent years. In addition, standards for the newer genetically modified hybrids may not be the same as those used for the older hybrids. I doubt if they are. But, we don’t know.

Considering everything, tissue testing cannot be used to predict rates of fertilizer needed for optimum production next year. Tissue testing can be a good tool only if combined with a good soil sampling program.  FG

—Source: University of Minnesota, agbuzz.com blog