Assessing Nutrient Variability in the Field and Within Plants

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Article by Dale Cowan
Agronomy Strategy Manager and Senior Agronomist
AGRIS Co-operative - CENTRAL OFFICE

I spend a lot of time on this article describing field variability and uncovering opportunities to practice 4R Nutrient Stewardship principals more effectively.  Keeping the soil test current is required so we know which nutrient management path to follow based on soil test levels. If tests are low, we are in a buildup approach by applying a build factor in addition to crop removal. If soil tests are on target, we maintain by applying to match crop removal rates and when tests are above target, we can effectively drawdown soil test levels without fear of yield loss by applying less than crop removal and pull down some of the soil reserves.

There is another test that we sometimes do, called a plant tissue test. Sampling specific plant parts at specific growth stages and submit to the lab for analysis. The resulting report offers an indication of plant nutrient content which indicates how well a plant is taking up nutrients from the soil. Both a soil tests and plant tissue test can help assess how well a plant is accessing a nutrient program. Lab reports offer a critical level for each test.  Results above the critical level indicate the plant is taking up sufficient nutrients to meet its nutritional   requirements at specific growth stage.  In the case of corn, the whole plant is submitted in seedling stage, later on the most recently mature leaf and prior to pollination collecting the ear leaf has been the standard practice.   The ear leaf content has been used to determine the overall nutritional status of corn plant. 

Recently that is being questioned, is the ear leaf the best leaf to assess nitrogen and Sulphur?  It is now  a consideration that the 2 leaves below the ear leaf maybe better for determination of the nitrogen status and the very top leaf or flag leaf for Sulphur.

We began investigating as a curiosity this year to see what the nutrient differences might be in various leaf positions.  A different kind of nutrient variability was uncovered, variability of nutrients within the plant by leaf position.   I have highlighted some results below.  Nitrogen by leaf position, Sulphur and Boron. 

EL – ear leaf, EL-1, leaf below ear leaf, EL-2, second leaf below ear leaf and FL flag leaf very top of the plant.

The leaf below the ear leaf has higher N content. Not sure what it means as we have not yet harvested the crop.

Not a lot of difference on Sulphur by leaf position. With Sulphur being relatively non mobile in the plant  it is thought the flag leaf might be a good indicator position for Sulphur status,. All are considered just under the critical level of 0.20%.

The Boron results were somewhat more surprising with the FL having the highest concentration

I say somewhat because we know that Boron is Xylem mobile in the plant but not very Phloem mobile.  Boron does not remobilize very well.  In a high yield corn environment, the ear leaf concentration ranges from 9 to 25 ppm.  This example is at 7 ppm on average from multiples sites in the same field. 

Foliar applications of boron are not very effective as mobilization after leaf absorption is very limited with current Boron formulations.  Soil applied boron is taken up and transported in the xylem system along with water and other nutrients, this has proven to be effective in increasing corn yields

Boron is required all season long, placing some boron in the corn starter and some in a sidedress application with 28% nitrogen seems to help meet the plant needs.  The next step to explore is an additional application foliar with the fungicides to extent the supply of Boron.

We can discuss that more during the Crop Planning for next year.