Learning from Plant Tissue Testing

Plant tissue testing can be an incredibly valuable tool in soil fertility management. However, plant tissue test results by themselves often lead to more questions than answers. While the results indicate which nutrients are normal, low, or high, it does not indicate why they are that way. To get the most information from plant tissue test results, they must be used in conjunction with crop scouting and knowledge of previous fertility practices and recent weather conditions.

Here is an example of utilizing tissue testing to evaluate a fertility trial graciously shared with the ALGL agronomy staff from a client in Northwest Indiana. The samples were collected to two areas where one had received ammonium thiosulfate in the starter program (Sample A) and one did not (Sample B). Upon visual inspection of the plants, the sample that received the sulfur treatment is obviously darker green. So, the conclusion here would be that the sulfur application was beneficial. However, comparison of the tissue test results shows that there may be more going on here than a simple sulfur response.

The first observation of the test results that seems unusual is that both samples fall in the normal range for sulfur levels. The sample not receiving no sulfur is obviously lighter in color, so a sulfur deficiency would be expected. Upon closer inspection of the normal ranges provided on the report though, you can see that the sulfur level in the untreated sample is only 0.01% above the lower limit of the normal range, so the plants may be experiencing a sulfur deficiency. It is important to look at the actual values of the test results and not just the bar graph ratings.

The unexpected result of this comparison is that the healthier looking plants are lower in both phosphorus and potassium. The darker colored plants are also showing some purpling in the older leaves which is common in the early growing season due to cooler nights or genetic differences, but the tissue test results show that the phosphorus is at the lower end of the normal range and the plants maybe experiencing a phosphorus deficiency even though they visually appear healthier. A closer look at the test results also shows that the untreated plants are marginally low in manganese which may be the cause of the lighter coloring.

What has likely happened in this scenario is that the sulfur application likely improved the formation of chlorophyl resulting in a darker colored plant that may actually be covering up the visual symptoms of other potential deficiencies that maybe occurring in the plants.

The takeaway from this is that visual comparison and tissue test results on their own do not always paint a complete picture of what may be going on in the field, but when used in conjunction with each other, a more complete conclusion can be made and future fertility plans can be adjusted accordingly.

Sample A

Sample B