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Luxury Uptake of Boron: How Much is Too Much?

By Dan Kaiser, Extension Soil Fertility Specialist

The ultimate goal of nutrient management is to ensure that the plant has enough nutrients to produce maximum potential yield. This involves monitoring soil nutrients and crop uptake, and often supplementing nutrients that the crop is lacking. But what happens when the plant takes up more than enough of a certain nutrient? That’s called luxury uptake. Though it isn’t usually a problem for crops, it can become an issue if a nutrient reaches toxic levels in a plant. In Minnesota, the main concern is with Boron in soybeans and other broadleaf plants.

A Quick Primer on Luxury Uptake

Plants vary in their ability to regulate the uptake of nutrients through the roots. Nutrients that move to the plant and are taken up by mass flow tend to accumulate in the plant regardless of whether it needs them. When a plant takes up nutrients beyond those that contribute to plant growth, the concentration shows up in the plant tissue and is considered luxury uptake.

One clear example of luxury uptake is when corn is over supplied with nitrogen. Nitrate-N can easily move with soil water and be taken up by the plant. Oversupply of N to corn typically does not negatively affect corn grain yield, but nitrate will accumulate at higher concentrations in the plant. The basal stalk nitrate test measures luxury uptake of N. Anything above a basal stalk concentration of 2000 ppm is considered luxury nutrients. 

What about Boron?

Boron is the main luxury uptake concern for Minnesota farmers, especially in soybeans and edible beans. Boron is not regulated well by the plant, which will take it up in high concentrations if it’s available in the soil. While Boron is not suggested for most crops in the state, tissue tests have been interpreted to indicate an insufficiency of Boron in corn, tissue testing has which has led to an increase in questions about Boron application. 

The question is: are current sufficiency ranges accurately predicting Boron sufficiency? Since most crops do not respond to micronutrients, establishing sufficiency levels is difficult. The challenge is finding the point where the concentration of Boron results in yield reduction. 

What does the research tell us?

University of Minnesota Extension began assessing the need for Boron in Minnesota corn production with an on-farm research project funded by AFREC. In the study, which began in 2016, three irrigated and three non-irrigated fields were selected, and replicated randomized strips with no Boron and 2 lbs of broadcast B were applied to the soil surface after planting.

Within each site, we took plant samples for 30 side by side comparisons with and without sulfur, sampling the uppermost fully developed leaf at V10, and measured yield at the end of the season. There was no evidence of a yield response to Boron at any site. The average yield was 225 bu/ac with Boron and 225 bu/ac without across locations, but there was a 50 percent increase in tissue B concentration from 8 ppm to 12 ppm when Boron was applied. 

Interpreting the Boron tissue results

Range (ppm) of middle 50% of samples
Without Boron
With Boron

One major focus of the research was to evaluate plant tissue Boron concentration and how it relates to grain yield. In the past, descriptive statistics of a given population have been used to develop pseudo sufficiency ranges for crops when a yield response does not occur. The table above shows a calculated sufficiency range using the values representing the middle 50 percent of data collected for Boron concentration from the 2016 studies. Since the ranges only represent the middle 50 percent of the population centered around the mean, the remaining 50 percent would be split with 25 percent of the values considered below optimum and 25 percent above. The problem with this interpretation is that there was no yield response over the entire range of values, thus we should consider any value above the minimum value (4 ppm) to be sufficient. 

When interpreting values, keep in mind that sufficiency ranges may be developed in situations where luxury uptake of a nutrient has occurred. For the Boron trial, we wanted to determine what a sufficiency range would be if we used descriptive statistics instead of corn response data. If we use the range for corn treated with Boron and consider anything below 9 ppm low, two-thirds of our samples collected in the no Boron strips would be considered insufficient in B. Using the mean Boron concentration for the strips with Boron (12 ppm), 90 percent of samples collected from the control strips would be considered below the sufficiency range. Since the plant took up luxury nutrients, the range in plant tissue concentration of the applied treatments should not be considered optimum nor should the middle 50 percent range of the no-Boron strips. In this situation, concentrations would be consistently reported as low. 

The Bottom Line

This example shows a major flaw in the interpretation of plant tissue results. Without backing yield response data, population means and ranges alone are not enough to interpret results from plant tissue reports. Current University of Minnesota publications show that 4 to 15 ppm concentration of B is sufficient. Our ongoing AFREC research suggests that this range may also work for leaf samples collected earlier in the growing season. Keep all these factors in mind when interpreting your tissue reports for nutrients that don’t commonly result in yield increases, like Boron. Remember that high concentration of plant nutrients in tissue are not correlated to high yield. Many factors influence the uptake of nutrients and can result in variations in nutrient concentration in plant tissue, which should be considered when interpreting your report.

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Support for this project was provided in part by the Agricultural Fertilizer Research & Education Council (AFREC).

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