Quantitative Analysis

Quantitative plant analysis has several functions in assessing the nutrient status of plants (29). Among these functions, plant analysis can be used to confirm a visual diagnosis. Plant analysis also can help in identifying hidden hunger or incipient deficiencies. In confirming diagnoses or in identifying incipient deficiencies, comparisons are made between laboratory results and critical values or ranges that assess the nutritional status as deficient, low, sufficient, or high, or in other applicable terms. The critical concentration of a nutrient is defined as the concentration of the nutrient below which yields are suppressed (26,30). In the determination of critical concentration, analysis of a specific tissue of a specific organ at a designated state of development is required. Because of the amount of work involved, critical concentrations are rarely determined; consequently, ranges of sufficiency are most commonly used in assessment of plant nutrition (27). For each nutrient or beneficial element mentioned in this handbook, ranges of sufficiency are reported.

For any plant, it could be that only one nutrient is deficient or in excess, but it is also possible that more than one nutrient may be out of its range of sufficiency. Furthermore, the actual requirement for an individual nutrient may be different if other nutrients are not present in the plant above their own critical concentrations. For this reason, it is becoming common to consider concentrations of nutrients in relation to the concentrations of other nutrients within the plant. Forms of multivariate analysis such as principal component analysis and canonical discriminant analysis have been used to investigate relationships between the internal concentrations of many nutrients together and plant growth (31). Currently, a commonly used application of plant analysis is the Diagnosis and Recommendation Integrated System (DRIS), which compares ratios of concentrations of all the possible pairs of elements analyzed to establish values that help to identify nutrients that are most likely to be deficient (32,33).

Plant analysis is also used to determine if an element entered a plant. Fertilization is employed to correct deficiencies, often in response to a visual diagnosis. It is important to know that nutrients actually entered plants after the application of the nutrients to the soil or foliage. No response to the application of a nutrient may be understood as meaning that the element was not lacking, when in fact, it might not have been absorbed by the plant being treated. Plant analysis can also indicate the effects of application of plant nutrients on plant composition with regard to elements other than the one being studied. Interactions may occur to enhance or to suppress the absorption of other nutrients. In some cases, growth may be stimulated by a nutrient to the point that other nutrients become deficient, and further growth cannot occur. Plant analysis can help to detect changes in plant composition or growth that are synergistic or antagonistic with crop fertilization.

Collecting samples of plant organs or tissues is important in assessing nutrition by plant analysis. Comparable leaves or other organs or tissues from the same plant or from similar plants should be collected as samples that show symptoms and samples that do not. Samples of abnormal and normal material from the same plant or similar plants allow for development of standards of comparison for deficient, optimum, or excessive nutrition. The composition of plants varies with time (diurnal and stage of growth) and with parts of plants as well as with nutrition (34). It is wise to take samples from plant parts that have been studied widely and for which published standards of comparisons for deficient, sufficient, and optimum concentrations of nutrients are available. Jones and Steyn (35) discuss methods of sampling and sample preparation prior to analysis, along with methods of extracting nutrients for analysis and methods of analysis of plant tissues. A handbook edited by Kalra (36) also addresses sampling and analysis of plant tissues.