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  Section: Plant Nutrition
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Diagnostic Criteria

Plant Nutrient
Diagnostic Criteria
  Visual Diagnosis
  Plant Analysis
  Quantitative Analysis
  Tissue Testing
  Biochemical Tests
  Soil Tests
Approaches in Research

Visual Diagnosis
Careful observations of the growth of plants can furnish direct evidence of their nutritional conditions. Metabolic disruptions resulting from nutrient deficiencies provide links between the function of an element and the appearance of a specific visible abnormality. Symptoms of disorders, therefore, provide a guide to identify nutritional deficiencies in plants. Careful experimental work and observations are needed to characterize symptoms. For example, nitrogen is needed for protein synthesis and for chlorophyll synthesis, and symptoms appear as a result of the disruption of these processes. Symptoms of nitrogen deficiency appear as pale-green or yellow leaves starting from the bottom and extending upward or sometimes covering the entire plant. Magnesium deficiency also affects protein synthesis and chlorophyll synthesis, but the symptoms may not resemble those of nitrogen deficiency, which affects the same processes. Experience is necessary to distinguish the symptoms of nitrogen deficiency from symptoms of magnesium deficiency or in the identification of the deficiency of any nutrient.

Symptoms on foliage have been classified into five types (12): (a) chlorosis, which may be uniform or interveinal (Figure 1.1); (b) necrosis, which may be at leaf tips or margins, or be interveinal (Figure 1.2); (c) lack of new growth, which may result in death of terminal or axillary buds and leaves, dieback, or rosetting (Figure 1.3); (d) accumulation of anthocyanin, which results in an overall red color (Figure 1.4); and (e) stunting with normal green color or an off-green or yellow color (Figure 1.5). Symptoms of deficiency can be quite specific according to nutrient, especially if the diagnosis is made early in the development of the symptoms. Symptoms may become similar among deficiencies as the intensities of the symptoms progress.

Generalities of development of deficiency symptoms can be made among species. Many references are available with descriptions, plates, or keys that enable identification of nutrient deficiencies (12–20).

As mentioned above, for example, nitrogen deficiency appears across plant species as chlorosis of lower or of all leaves on plants. Advanced stages of nitrogen deficiency can lead to leaf death and leaf drop. Nitrogen-deficient plants generally are stunted and spindly in addition to showing the discoloration that is imparted by chlorosis. Potassium-deficient plants have marginal and tip necrosis of lower leaves. On the other hand, for elements that are immobile (not transported in phloem) or slowly mobile in plants, the deficiency symptoms will appear on the young leaves first. The symptoms might appear as chlorosis, as with sulfur, iron, manganese, zinc, or copper deficiency, or the symptoms might be necrosis of entire plant tips, as occurs with boron or calcium deficiency. Brooms or rosetting may occur in cases where deficiencies (e.g., copper or zinc) have caused death of the terminal bud and lateral buds have grown or where internode elongation has been restricted by nutrient (e.g., zinc) deficiencies. Accumulation of anthocyanin, exhibited by reddening of leaves, may indicate phosphorus deficiency, although nitrogen deficiency can lead to a similar development.

Interveinal chlorosis of iron-deficient borage (Borago officinalis L.).   Deficiency symptoms showing necrosis of leaf margins, as in this case of potassium deficiency
FIGURE 1.1 Interveinal chlorosis of iron-deficient borage (Borago officinalis L.).   FIGURE 1.2 Deficiency symptoms showing necrosis of leaf margins, as in this case of potassium deficiency
on cucumber (Cucumis sativus L.) leaf.

Deficiency symptoms showing necrosis on young leaves of (a) calcium-deficient lettuce (Lactuca   Stunting and development of red color and loss of green color of phosphorus-deficient tomato
FIGURE 1.3 Deficiency symptoms showing necrosis on young leaves of (a) calcium-deficient lettuce (Lactuca
L.) and necrosis on young and old leaves of (b) calcium-deficient cucumber (Cucumis sativus L.). With
cucumber the necrosis has extended to all leaves that have not expanded to the potential size of full maturity.
  FIGURE 1.4 Stunting and development of red color and loss of green color of phosphorus-deficient tomato
(Lycopersicon esculentum Mill.)

Some people try to distinguish the two deficiencies by noting whether the symptoms of reddening develop between the veins (phosphorus deficiency) or along the veins (nitrogen deficiency). Stunting is a good indication of nutrient deficiency, but often stunting cannot be recognized unless a wellnourished plant is available as a standard of comparison. A stunted plant may have normal color and not be recognized as being deficient until abnormal coloration develops with advanced stages of deficiency. In some cases, symptoms may not develop during the growth cycle of crops, but yields may be suppressed relative to plants that have optimum nutrition. Hidden hunger is a term applied to cases where yield suppression occurred but symptoms did not develop.

Cabbage (Brassica oleracea var. capitata L.) plants showing symptoms of stunting
FIGURE 1.5 Cabbage (Brassica oleracea var. capitata L.) plants showing symptoms of stunting. Left: stunting
and dark green color diagnosed as being caused by salinity in nutrient solution. Middle: stunting and mottling
of foliage due to condition diagnosed as magnesium deficiency. Right: stunting and discoloration of
foliage due to condition diagnosed as phosphorus deficiency.

Deficiency symptoms can occur at any stage of growth of a plant. The most typical symptoms are those that appear early in the cycle of deficiency. Early diagnosis of deficiencies may also allow time for remedial action to take place. Generally, however, if symptoms have appeared, irreparable damage has occurred, with quantity or quality of yields being suppressed or diminished with annual crops or with slowing or damaging of growth and development of perennial crops. Also, symptoms that resemble nutrient deficiency can develop on plants as a result of conditions that are not related to nutrient deficiencies, for example, drought, wet soils, cold soils, insect or disease infestations, herbicide damage, wind, mechanical damage, salinity, or elemental toxicities. Deficiency symptoms are only one of several diagnostic criteria that can be used to assess the nutritional status of plants. Plant analysis, biological tests, soil analysis, and application of fertilizers containing the nutrient in question are additional tools used in diagnosis of the status of plant nutrition.

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