For most nutrient deficiencies, growth may be limited considerably without any symptoms. Hence it is essential that reliable procedures for soils and plant tissues analyses should also be used to monitor nutrient status and diagnose nutrient deficiencies.
Soil analysis is not generally useful for either micronutrients or nitrogen. For micronutrients it has proved difficult to find an extractant that estimates accurately that small fraction of the total amount of the nutrient in the soil that is available for uptake by plants, particularly across a range of soils. For nitrogen, soil analysis is of limited value because of the strong influence of soil temperature and soil moisture on nitrogen transformations within the soil.
Guidelines for collecting samples
Where samples are taken for analysis of plant tissues, it is essential to record growth stage, agronomic practices used (including past fertilizer applications), and other site details (soil type, growing conditions). It is also important to avoid contamination (e.g. galvanized iron and brass in collection instruments could be sources of zinc and copper, respectively) and to take a representative sample.
To diagnose deficiencies using tissue analysis in plants showing symptoms, it is essential to clearly describe the symptoms and to collect plant samples from affected and unaffected areas within the crop. To avoid secondary effects, take plant samples when symptoms of the disorder are first observed.
When collecting plants from small, uniformly affected areas of a crop, choose the sample size by the amount of material required for chemical determinations. Usually 30-50 shoots or plant parts are collected systematically and combined. When sampling, exclude diseased or insect-damaged plants when possible. In addition, do not take samples when plants are under drought stress.
Avoid contamination by trace elements by using clean plastic shovels, stainless steel cutting implements, and distilled or deionized water. Wrap samples in tissue paper and place them in open paper bags before drying at approximately 65ºC.
Tissue analysis and growth stages
Tissue analysis is based upon the relationship between nutrient concentrations within the plant and its growth stage. The most important factor affecting this relationship is the extent to which nutrients are remobilized from old tissues to new growth and grain. For nutrients such as nitrogen, phosphorus, and potassium, which move readily from old leaves in all situations, analysis of whole shoots gives a reasonable indication of nutrient status with critical concentrations decreasing with plant age. For nutrients that are immobile or variably mobile (see nutrient mobility), concentrations in whole shoots will be poor indicators of nutrient status because they reflect the previous supply rather than the current status. Nutrient concentrations in young leaves have proven to be reliable indicators of current nutrient status. An additional advantage is that critical concentrations in tissues of the same physiological age (e.g. the youngest fully emerged leaf) tend to decreased less with plant age than do critical concentrations in whole shoots.
Tissue analysis can only be used to separate plants with an adequate supply from those with a deficiency supply. It cannot be used to indicate degree of deficiency or amount of nutrient required to eliminate the deficiency.