Plant analysis has been considered and primarily used as a diagnostic tool. Farmers and growers are urged to confirm suspected nutrient element deficiencies by a plant analysis before applying a corrective treatment. In order to effectively use a plant analysis in diagnosing growth disorders, employ specific sampling and evaluation techniques.
Sampling procedures were discussed in some detail earlier in this handbook. (See section, Purpose and Use: Procedures ). Collect plant samples from both affected as well as normal plants whenever possible. If plants have been under nutrient stress for a long period, a comparative analysis may be misleading, particularly when the stressed and nearby normal plants are at markedly different stages of growth. Therefore, sample the plants they are at the initial stages of a developing nutrient deficiency. This is essential when using a plant analysis in a diagnostic situation. Exercise great care to ensure that the proper sampling procedures are used. Plants selected for sampling and exhibiting symptoms of the suspected nutrient deficiency should be similar in appearance and all at a similar stage of development. Avoid dead or severely affected tissues, and do not include them in the sample. Confine the sampling area to plants in close proximity to each other. Use the same sampling procedures for those plants selected as the normal counterparts. Soil samples should be collected from both the affected and normal areas.
By comparing the analysis of both soil and plant tissue from the normal and affected areas, differences in test levels and concentration of particular elements can be evaluated. The comparison of analyses may be far more useful in the interpretation than using known interpretative values for this comparison and evaluation. Associations between a soil test value and the plant analysis value of similar or related elements should be examined. For example, differing soil pH's can result in changing levels of Mg and Mn in plant tissue. Variations in the soil test P and K levels are usually reflected in the P and K levels of the plant tissue. The presence or lack of such commonly occurring associations can be significant clues. With experience, you can become quite efficient in the evaluation of plant analyses when used for diagnostic purposes.
The most common error made when a plant analysis is used as a diagnostic tool relates to the failure to use sufficient care when collecting the plant tissue and soil samples. The effectiveness of a plant analysis to diagnose a particular nutrient problem is hampered by improper sampling and the failure to include both sets of tissues and the necessary soil samples. Therefore, follow the procedures prescribed with great care when collecting these samples. Failure to do so can significantly limit the effectiveness of the evaluation and may lead the interpreter into drawing incorrect conclusions.