TABLE 4–7 Soil Mineral Nutrients Essential for Plant Growth and Development Macronutrients Micronutrients (Major, Primary) Secondary Nutrients (Minor, Trace) Nitrogen (N) Calcium (Ca) Iron (Fe) Phosphorus (P) Magnesium (Mg) Manganese (Mn) Potassium (K) Sulfur (S) Molybdenum (Mo) Copper (Cu) Boron (B) Zinc (Zn) Chlorine (Cl) micronutrients (minor elements or trace elements). The macronutrients may be subdivided into primary nutrients and secondary nutrients (Table 4–7). Primary nutrients are utilized in large amounts by plants and are often prone to deficiency in the soil. Secondary elements are used by plants in much smaller amounts than the primary elements. Micronutrients are needed in only trace or minute amounts by plants and are not frequently deficient in soils. Trace elements are especially critical in greenhouse cultivation, where artificial mixes are often used. Sandy soils and soils that experience prolonged heavy precipitation or prolonged intensive cultivation provide conditions under which micronutrient deficiency is likely. Primary Nutrients (Macronutrients) The three primary macronutrients are nitrogen, phosphorus, and potassium. NITROGEN (N) Nitrogen is one of the most widely used elements in plant nutrition. Plants absorb this element in its inorganic form as nitrate ions (NO 3 ) and occasionally as ammonium (NH 4 +). A natural cycle (nitrogen cycle) exists for recycling nitrogen. When plants absorb nitrate ions, they become immobilized (mineral form is changed into organic form) by becoming part of the plant tissue. When plants die, their tissue is decomposed to release the organic form of nitrogen into inorganic ions by the process of mineralization. Microbes decompose dead tissue to release nitrates, and some are capable of fixating atmospheric nitrogen by the process of nitrogen fixation. This process, part of a symbiotic relationship between bacteria (Rhizobia) and legume roots, involves two chemical reactions: ammonification and nitrification. Nitrogen is used in the synthesis of amino acids and proteins and is a component of chlorophyll and enzymes. It promotes vegetative growth and as a result may delay maturity. Its deficiency causes stunted growth, especially lateral shoots with leaves turning light green then yellow (chlorosis), the most visible deficiency symptom. Entire leaves are chlorotic, starting with lower foliage. Older leaves later defoliate; stems are thin. Nitrogen is readily lost from the soil through leaching and soil erosion and is also readily removed by plants. Deficiency can be corrected by applying organic or inorganic fertilizers. Nitrogen is mobile in the plant. Thus, if the element is in short supply in the soil, protein nitrogen in older leaves is converted into a soluble form and translocated to younger leaves, where it is most needed. The older leaves then lose color while younger leaves remain green. PHOSPHORUS (P) Phosphorus is absorbed primarily as orthophosphate ions (mainly H 2 PO 4 and also HPO4 2 ). Phosphorus is found in proteins and nucleic acids (DNA and RNA) and is critical in the energy transfer process (adenosine triphosphate [ATP] and adenosine diphosphate [ADP]). Phosphorus is found to induce root proliferation and early crop maturity. When phosphorus is deficient in the soil, leaves become dark bluish or greenish and plants become stunted. This deepening of color is caused by an increase in nitrates in the leaves. Yield is subsequently reduced. Purplish color, especially of older leaves, or reddish-purple color on some grasses indicates phosphorus deficiency. Phosphorus is Macronutrient A chemical element that is required in large amounts (usually greater than 1 ppm) for the growth and development of plants. Chlorosis A condition in which a plant or a part of a plant turns greenish-yellow due to poor chlorophyll development or the destruction of the chlorophyll resulting from a pathogen or mineral deficiency. 4.3 Belowground (Soil) Environment 111
Necrosis Death of tissue associated with discoloration and dehydration of all or parts of a plant organ. also mobile in the plant and is regularly recycled from older parts to younger growing parts. It has a tendency to be rendered readily unavailable (fixed) in the soil. It is most available at a pH of 5.5 to 7.0. POTASSIUM (K) Sandy soils may be deficient in potassium since the element is readily leached. Potassium is absorbed by plants in its ionic form (K ). It is a catalyst for enzyme reactions and is also important in protein synthesis, translocation, storage of starch, and growth of meristematic tissue. Whereas nitrogen and phosphorus are converted into compounds for plant growth, potassium occurs in the plant tissue as a soluble inorganic salt. It is very mobile. Luxury consumption of potassium is common in plants, even though the element is required in large amounts. When potassium is deficient in the soil, the root system and stems become weak and prone to lodging; yield is reduced. Readily visible deficiency symptoms vary among species. Some plants show marginal burning of leaves (marginal necrosis), speckled or mottled leaves, interveinal chlorosis, and leaf curling. These symptoms occur in older leaves and spread upward. Potassium is removed by plants, but it is also prone to fixation and leaching, especially in soils low in organic matter and from soilless growing media. The most abundant monovalent ion in plants is K , whose concentration may be equal to or more than that of nitrogen. Plants such as carnation may have as high as 9 percent potassium on a dry-weight basis. Roses have about 2 to 3 percent potassium on the average. Potassium appears to have a role in nitrogen metabolism; when deficient, plants show a high level of water-soluble nitrogen. When ammonium forms of nitrogen are used to fertilize plants in case of deficiency, the deficiency symptom is intensified and the plants become severely injured. This injury may be due to the accumulation of nitrogen (that has not been changed into protein) to excessive and toxic levels. Secondary Nutrients Calcium (Ca) Calcium is not only an essential plant nutrient but is also used in correcting soil acidity so that other soil nutrient elements can be made available to plants in appropriate amounts. It is absorbed as Ca 2 ions by plants. Calcium is important in cell growth and division, cell wall formation (calcium in the form of pectate), and nitrogen accumulation. The element also forms organic salts with organic acids in plants. For example, in dumbcane (Dieffenbachia) calcium forms calcium oxalate, which is irritating to the tongue and throat of humans when ingested. When deficient, plant tissue formation is incomplete. The terminal bud may cease to grow, leaving a blunt end. Deficiency symptoms for calcium are manifested frequently as defective terminal bud development. The margins of young leaves may not form, resulting in strap leaves. Slight chlorosis followed by brown or black scorching of new leaf tips and die-back of growing points are also characteristic of this deficiency. Roots grow poorly and are short and thickened. MAGNESIUM (MG) Magnesium is released when rock minerals such as dolomite, biotite, and serpentine decompose. Absorbed as Mg 2+ ions, magnesium is the central atom in the structure of a chlorophyll molecule. It is also essential in the formation of fats and sugars. Magnesium is mobile in plants, and thus deficiency appears first in older leaves. Large amounts of potassium ions may interfere with its uptake due to ion antagonism created by this situation. This antagonism is prevented when the potassium to magnesium ratio in the growing medium is about 3:1 to 4:1. There is interveinal chlorotic mottling or marbling of the older leaves, proceeding to younger leaves as the deficiency intensifies. SULFUR (S) Sulfur is obtained primarily from the decomposition of metal sulfides in igneous rocks. It occurs in the soil as sulfates and sulfides, as well as in humus. It is absorbed by plants as sulfate ions (SO 2 4 ). The unique flavors of certain vegetables, such, as onion and cabbage, as well as other cruciferous plants, are due to certain sulfur compounds. Sulfur is an ingredient in vitamins and amino acids. The dominant symptom of sulfur deficiency is chlorotic foliage. In addition, the stems of affected plants are weak, thin, hard, and woody. Sulfur is not usually added as a fertilizer element but is added indirectly when sulfate forms of other elements are applied. Sulfur is also available from air pollution. 112 Chapter 4 Plant Growth Environment
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