The Value in Tissue Testing for P and K

When working with plant tissue data you can either react to the data with a nutrient application, or you can use the data to continue to develop your overall nutrient management strategy. Either way, routine monitoring of plant tissue test results is the starting point. While taking one tissue test at R2-R3 soybeans to identify a nutrient deficiency so that a foliar product can be added to the fungicide pass is common, it does not identify how long the deficiency endured. While on the other extreme, weekly tissue samples can be a pile of data to wade though. A good tissue testing monitoring program starts with a tissue test once the crop is well established, one or two while the crop is rapidly growing, one at the start of reproduction, and the final sample during pollination or grain fill.

When tracking tissue test results, weather and crop observation notes throughout the season, especially the week or two leading up to a sampling event, are critical. The main nutrient uptake routes for plants, diffusion and mass flow, require water. If water is limited in the root zone, short term nutrient deficiencies can appear until moisture improves. If the soil is saturated, nutrients that are taken up by mass flow will be diluted in the water and will have limited availability to the plants. 

Tissue testing provides an opportunity to identify and correct micronutrient deficiencies. If you can spray every week or two in a higher value crop, collecting tissue tests ahead of application will allow for management of low trending secondary and micronutrients. In commodity grains, weekly application of nutrients is not as practical, so being reactive to every change in a weekly collected tissue test is not practical. It is possible though with season long tissue testing to identify weak areas within the overall crop fertility plan and adjust the plan for future growing seasons.

For macronutrients like phosphorus and potassium, foliar applications in grain crops are often not enough to have a significant impact. The value to repeatedly collected tissue samples for management of phosphorus and potassium are the adjustments made to the timing and frequency of nutrient applications, target soil test levels, and rate of fertilizers in subsequent seasons. The goal for phosphorus and potassium to correct common issues that appear in tissue tests proactively in future years. To validate the effectiveness of the management changes, continue tissue testing after the implementation of the management changes.