AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Summer 1996, No. 5
SCOUTING FOR NUTRIENT DEFICIENCIES
Plants provide clues to nutrient problems in the field if we pay attention to the symptoms they show. Unfortunately, by the time visual symptoms are observed, the potential yield of the crop has probably been severely reduced. This “hidden hunger” is one of the biggest yield robbers.
Know the growth stages and expected development rate of a healthy crop. This will be helpful in identifying deviations from normal growth—often the first sign of nutrient deficiencies. If growth is unusually slow or if plants appear stunted, take leaf samples for laboratory analysis. Know the common nutrient deficiency symptoms for the crops you are growing. For many crops, nitrogen deficiency causes yellowing of the leaf tissue (along the midrib of grassy plants), usually visible on lower leaves first. Supplemental nitrogen application may help recover some lost yield potential if the crop is not yet fully developed.
Potassium deficiency causes leaf margins to turn yellow and eventually die. Lower leaves exhibit symptoms first, because part of the potassium is moved from them to the more actively growing parts of the plant when insufficient amounts of potassium are available for normal growth. Response to supplemental potassium applied after symptoms develop is not very likely, but increasing potassium on that area next year may help prevent recurrence of the problem.
Phosphorus deficiency shows up as a purple coloration of newly developed leaves. Since phosphorus is critical to sugar utilization in plants, its deficiency leads to a buildup of sugar in the leaves and reduced chlorophyll content, allowing the purple pigments to be more visible. Phosphorus deficiency may be induced by restricted root growth or cool temperatures. Nutrient applications can be adjusted to prevent recurrence of the deficiency in successive years. The symptoms may disappear as the root system expands and growing conditions improve.
Nutrient deficiency symptoms are often actually caused by other problems such as insect feeding, compaction, poor root development, disease injury, competition from weeds, poor drainage or mechanical injury to the plant. These problems inhibit the uptake or utilization of available nutrients. The real cause may be masked by other symptoms, so don’t be too quick to diagnose the cause-effect relationships. On the other hand, maintaining adequate nutrient availability will often reduce the impact of these other yield limiting factors.
Make use of old tools as well as new technology to help detect, identify and correct nutrient problems. Soil testing is one source of clues. With the availability of more intensive sampling data, areas of low nutrient levels may be easier to locate. Make a special effort to check such areas. Plant analysis is another useful tool for identifying nutrient deficiencies. Take samples from suspected deficient plants and also from healthy plants for comparison. Standard sufficiency threshold values are available for most crops, but the comparison approach may be more useful in field diagnostics. Quick-test kits can be used as a first cut analytical tool. Such tissue tests should be confirmed with laboratory analysis before making major nutrient management decisions.
Ion-specific electrode tools are useful for in-field analysis. A sample of plant sap (or a diluted sample) is squeezed onto the electrode and the electronic display provides a reading calibrated for the relative nutrient content of the plant sap. Here again, confirmation with laboratory analysis is desirable, but research has shown these electrodes are reliable if properly calibrated.
If possible, use global positioning systems (GPS) to document the specific geographic location of deficiency symptoms and where soil and plant samples are taken for analysis. The GPS coordinates can link these observations to other data bases such as soil survey, soil test data and yield maps. These tools are all helpful in determining the true cause-effect relationships resulting in the nutrient deficiency. If GPS is not available, take detailed notes on where symptoms occur in the field. This will be valuable for future management decisions.
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