Collecting plant tissue samples throughout the entire growing season to monitor nutrient levels has become a common practice over the last few years. As most of the crops in our region are now well into later grain-fill stages, plant tissue test results need to be evaluated with a cautious eye.
As plants transition from vegetative growth stages to reproductive stages, the nutrient content of the plant leaves will change, most noticeably nitrogen and potassium. These nutrients are mobile in plants, so as the plant starts transitioning to grain-fill, they may be translocated from the leaf to the grain resulting in low tissue test ratings that may not necessarily indicate a yield-reducing nutrient deficiency.
Another common trend in plant tissue test nutrient levels is an increase in micronutrient concentrations as the plants approach physiological maturity. This is a result of carbohydrates and other carbon-based molecules being translocated from the leaf tissues to the grain effectively reducing the biomass of the leaf. The micronutrients (iron, manganese, zinc, and copper) are immobile in the plant tissue, so they remain in the leaf that has a lower mass and are now present at a higher concentration. The micronutrients may be rated as high or very high, however, this may not be an indicator of excessive fertility or potential toxicity.
While plant tissue testing can be a very effective tool for fine-tuning a fertility program, be careful not to make drastic decisions based on late-season plant tissue test results alone.