Plant Analysis
Plant analysis measures the nutrient content within the plant tissue. This can be a useful addition to soil testing. Plant analysis results are compared against established normal ranges for the crop, indicating whether a specific nutrient is deficient. For some nutrients, this can indicate whether the soil nutrient supply is adequate for optimum growth. If soil levels are known to be adequate, plant analysis may indicate other problems that are reducing nutrient uptake.
For specialty crops, it can be difficult to interpret a tissue analysis report because normal ranges have not been established. It is often not possible to compare plant analysis results of mainstream crops to those of closely related specialty crops, because nutrient levels in plant tissue vary greatly from one crop to the next. However, plant analysis is still useful for diagnosing patchy problems in the field, and for comparing the same crop at the same stage from one year to the next. Over time, a grower can establish their own estimated normal ranges by comparing crop observations and yields to plant analysis results.
Plant analysis can be used to evaluate phosphorus, potassium, magnesium and manganese fertility. It is very useful in assessing the status of boron, copper, iron or molybdenum as these nutrients do not have reliable soil tests. Plant analysis results for nitrogen and zinc are not always reliable.
Sampling
Time of sampling has a major effect on the results of plant analysis. Nutrient levels within a plant vary considerably with the plant age and physiological stage.
Recommended sampling stages have been developed for established crops, but not for many specialty crops. Results are difficult to interpret if samples are taken at times other than those recommended.
If you suspect a plant is nutrient deficient, sample it as soon as the problem appears. Take tissue samples from a problem area rather than from the entire field. Collect and submit a separate sample from an adjacent, non-affected part of the field for comparison purposes.
Take leaves from at least 20 plants distributed throughout the area chosen for sampling. Each sample should consist of at least 100 g (3.2 oz) of fresh material. For most crops, it is recommended to sample the most recently mature leaf from each plant. Very old and very young leaves often provide irregular test results.
Collect tissue samples into labeled paper bags. Plant tissues will rot if they are stored in plastic bags. Avoid contaminating the sample with soil. Even a small amount of soil will cause the results to be invalid, especially for micronutrients.
Collect and submit a soil sample from both affected and non-affected areas to accompany the tissue sample. This soil sample can help address whether the nutrient deficiency is a result of limited soil availability or due to another contributing factor.
Fresh plant samples should be delivered directly to the laboratory. If they are not delivered immediately, they should be dried at a low temperature to prevent spoilage.
Interpretation
It is often difficult to interpret plant analysis results for specialty crops because normal ranges have not been established. However, if a nutrient concentration is much lower than the same nutrient in plants in unaffected areas of the field, this nutrient is probably deficient. Over time, as long as samples are taken at the same growth stage, a grower can begin to establish normal ranges for specialty crops.
Plant analysis has limitations. Expert help in interpreting the results is often needed. Plant analysis does not usually indicate the cause of a deficiency or the amount of fertilizer required for correcting it. The timing of plant analysis on many specialty crops is difficult. Rapid growth and a relatively short growing season means that yield loss may have already occurred by the time sample results are available. However, it is a valuable tool for diagnosing nutrient-related problem areas for future corrective measures.
