The “Dilution Effect” in Plant Tissue Tests

Tissue testing is a great tool to use in a grower’s toolbox. However, just like any analysis it comes with specific interpretations, limitations and variables. A tissue test will reveal nutrient levels at the current time of sampling. This can help or hinder certain scenarios. Each test must be conducted with an end goal. Whether the goal is to constantly spoon feed a crop for maximum yield potential, use the results for diagnostic testing or just get a general idea how the plant is utilizing what the soil has to offer.

Agronomists are constantly trying to make patterns and accrued data to justify what they are seeing in the field. This will lead to tissue samples being taken for a “good” and “bad” situation. One section of corn, for example, is greener or has more biomass than its neighboring rows. It is always good practice to have a soil sample in the same tissue sampling area to complete the story. This will reveal what the soil has to offer and compare what the plant is taking up.

In a particular situation, “good and bad” samples are sent in for analysis. The bad samples show sufficient levels of nutrients. The good samples show sufficient levels as well except for potassium. When reviewing the soil samples, soil potassium was adequate too. The first place to start is verifying that the sample was taken correctly and handled in the right manner. This refers to sampling the correct part of the plant at the right growth stage. Then correctly staging the plant for interpretations of the data. It is important to correctly handle plant matter when analyzing potassium since it can be shed from the leaf surface as soon as it starts to wilt.

The dilution effect occurs when the plant’s total biomass, or growth, exceeds crop uptake. In situations where one section of plants seems larger than others, this can be the case. For example, the amount of potassium in the soil is the same, and the rate of plant potassium uptake has not changed either, but the plant is distributing the same amount over a larger volume of biomass. This would lead to lower potassium concentration in the tissue sample leaf taken.

This does not indicate a lower yield potential. As mentioned before, a tissue sample is a glimpse of the “now” when the sample is taken. As the root system grows, and higher nutrient uptake occurs, concentrations in the plant tend to find an equilibrium. When diagnosing the “bad” tissue sample, taking a closer look at the roots of the plant and the soil are key. Ensure that compaction, water drainage, fertilizer application and soil type are uniform before developing a recommendation. All variables influence the crop’s nutrient uptake and the efficiency of nutrient utilization.