| Soils, Crops and Fertilizer Use |
|Chapter 7: Evaluating a soil's fertility|
Severe nutrient deficiencies often produce telltale changes in plant appearance, particularly in color. Spotting these "hunger signs" can be useful in diagnosing fertilizer needs, but be aware of several drawbacks:
• Some hunger signs are readily confused with each other or with other problems such as insects, diseases, and nematodes. Even trained field technicians may be unable to make a definite diagnosis without lab tests.
• If more than one nutrient is dificient, the hunger signs may be too ambiguous for accurate diagnosis.
• "Hidden hunger": Hunger signs don't usually appear unless a nutrient deficiency is serious enough to cut yields by 30-60 percent or more.
• It may be too late to correct deficiencies by the time hunger signs appear.
Hunger sign diagnosis is likely to be most useful in areas where only one or two nutrients are commonly deficient on a crop that will manifest unusually clear symptoms. For example, maize shows the most clearly recognizable symptoms of zinc hunger of any crop. Nitrogen deficiency is relatively easy to spot, although a number of other factors can cause similar symptoms.
How to Spot Hunger Signs: Hunger signs for common crops are described in Appendix E. Refer also to Appendix H for useful references with color plates.