Don’t Chase Impossible Plant Tissue Levels

Plant tissue testing is helpful for monitoring fertilizer inputs and diagnosing visual deficiency symptoms. However, tissue test reports should not be treated as a report card. It is not possible to get straight A’s or high ratings on every nutrient. Depending on the uptake mechanism and role of the nutrient in the plant, it is not possible for plants to accumulate excesses of most essential nutrients. The sufficiency ranges, or normal ranges, that the ratings are based on are determined from levels observed in average, healthy-looking plants.

In a high fertility soil under good growing conditions, the most likely nutrients to be rated high are nitrogen and potassium. These nutrients can be taken up through mass flow, which means the plant can accumulate the nutrients dissolved in the soil solution. A high rating for these nutrients is generally an indicator of adequate fertility and good growing conditions.

The primary role of phosphorus in a plant is energy transfer for converting primary photosynthates into structural components of the plant. Phosphorus exists in relatively low concentrations in the soil solution and is taken up through root interception and diffusion. These uptake mechanisms require more energy than mass flow, so there is no benefit for the plant to take up more phosphorus than necessary. A high or very high rating is not commonly seen for phosphorus.

The primary role of magnesium is building chlorophyl molecules and calcium is for building structural components of cell walls. Both nutrients are taken up through root interception. Plants generally have access to much more of these elements than they need, but do not take up more in normal growing conditions. The exception is during drought stressed conditions. Plants will accumulate excess magnesium to help fill the role of potassium when potassium uptake is hindered.

In general, micronutrients will not accumulate in excess in healthy plants. High level ratings of micronutrients often indicate a stunted or stressed plant. Most micronutrients such as iron, manganese, zinc, are immobile in plant tissue. So, as a stressed plant begins to cannibalize itself, the immobile nutrients are left in the necrotic tissues in higher concentrations.

High yielding environments can result in very unusual tissue test results. Often, most of the nutrients in these situations are rated as low. This is a result of the nutrients essentially being diluted throughout the massive amount of biomass being produced but does not necessarily indicate a deficiency.

A plant tissue test report on its own is of little value without additional observation and information. However, when used as part of a complete scouting and nutrient management program, it can help provide insight into factors that we cannot visually diagnose.