AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Summer 2001, No. 2
A plant tissue test can be considered a measure of a crop’s nutritional health. Not only is the nutrient supply part of this health assessment, it is also a reflection of all those factors influencing the crop, such as climate, plant disease, and mechanical or chemical stress, just to mention a few. A soil test is another important tool that can build on the interpretation of a tissue test, improving the confidence with respect to crop health, nutrient availability, and nutrient deficiency.
The growth stage at which a tissue sample is collected can have a major impact on the determination of plant health status. In cereals and oilseed crops, almost all tissue nutrient diagnostic criteria are based on samples collected during early flowering. This is usually just prior to heading in cereals and at the initiation of first flower on canola. All laboratories provide instructions on when samples should be collected to ensure that their interpretation is correct. Remember, if samples are taken at times other than recommended, a note describing the crop stage should be included. The laboratory may not have nutrient criteria that are suitable for interpretation at that stage of growth.
Understanding the difference between good and bad is important to tissue sampling. When collecting samples from a poor looking crop in a field, it is important to also collect a sample from the adjacent healthy looking crop whenever possible. This comparison provides some insight into whether a nutrient is limiting crop growth. It is also critical to make some assessment, either by estimation or sampling, of the crop production at time of sampling. For example, tissue samples collected from poor and good growth areas in a crop may have the same N content; however, the poor area may only have 30 percent of the crop growth as the good area. In this instance, the growth of the crop was limited to the supply of nutrient that prevented full expression of yield potential.
Cleanliness is critical to getting a clear picture of the plants nutrient status. When collecting plant samples, every effort must be made to reduce contamination from nutrient sources, such as soil or rust on the sampling scissors or knife. Removing soil particles from plant samples in the field is critical, as they may not be detected in the laboratory. Sampling cereals, oilseed, and forage crops in the early morning in the absence of heat stress is recommended. And avoid sampling immediately after rain or herbicide application. Drying samples at room temperature in paper bags will minimize any deterioration of the sample.
Keep tissue testing as one of the tools in your toolbox of nutrient management. However, remember that a tissue test on its own is not sufficient information to make a nutrient application recommendation. A soil sample, plant growth assessment, and consideration of prior environmental conditions are also important tools that will ensure that a tissue test provides a useful diagnosis for future nutrient management decisions.