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Nutrient Concentration and Function in Plants
Plants require 13 mineral nutrient elements for growth. The elements that are required or necessary for plants to complete their life cycle are called essential plant nutrients. Each of these nutrients has a critical function in plants and are required in varying amounts in plant tissue (Table 1). Macronutrients (nitrogen, phosphorus, potassium, calcium, magnesium and sulfur) are plant nutrients required in the largest amount in plants. Micronutrients (iron, copper, manganese, zinc, boron, molybdenum and chlorine) are required in relatively smaller amounts. Additional mineral nutrient elements which are beneficial to plants but not necessarily essential include sodium, cobalt, vanadium, nickel, selenium, aluminum and silicon. The nutrient elements differ in the form they are absorbed by the plant, by their functions in the plant, by their mobility in the plant and by the plant deficiency or toxicity symptoms characteristic of the nutrient.
Nutrient Deficiency or Toxicity
Nutrient deficiency or toxicity symptoms often differ among species and varieties of plants. A nutrient deficiency occurs when the nutrient is not in sufficient quantity to meet the needs of the growing plant. Nutrient toxicity occurs when a plant nutrient is in excess and decreases plant growth or quality. One way to understand the differences in nutrient deficiency symptoms among the plants is knowing the function and the relative mobility of the nutrient within the plant. Table 2 describes the general symptoms of nutrient deficiency and excess often observed for those nutrients. Some nutrients, such as nitrogen, phosphorus, potassium, magnesium, chlorine and zinc, can be easily remobilized within the plant from old plant parts to actively growing plant parts such as young leaves. Other nutrients, such as sulfur, iron, copper, manganese, boron and calcium, are not easily remobilized within the plant. Therefore, the deficiency of the mobile elements usually initially occurs with older leaves while that of the immobile nutrients occurs with the young leaves or stem tips. Five types of deficiency or toxicity symptoms are observed:
Nutrient deficiencies may not be apparent as striking symptoms such as chlorosis on the plant, especially when mild deficiency is occurring. However, significant reductions in crop yields can occur with such deficiencies. This situation is termed "hidden hunger" and can only be detected with plant tissue analysis or yield decline. However, experience with growing a specific plant species or variety can greatly help in distinguishing poor crop performance and possible nutrient deficiency symptoms from normal plant growth.
Table 1. Essential plant nutrients: their relative amounts in plants, functions and classification
*Relative amounts of mineral elements compared to nitrogen in dry shoot tissue. May vary depending on plant species.
Table 2. Generalized Symptoms of Plant Nutrient Deficiency or Excess
1. Adapted from: W.F. Bennett (editor), 1993. Nutrient Deficiencies & Toxicities in Crop Plants, APS Press, St. Paul, Minnesota.