As described in Chapter 1, green plants require nutrients that they obtain from the soil. A rich loam soil will provide a balanced supply of the elements essential to the growth of most plants. A soil that is predominantly sand or silt will be nutritionally poor.
The need by green plants for at least 17 separate chemical elements has been proven repeatedly through tests that demonstrate growth abnormalities when any one of these essential elements is lacking. As
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TABLE 3-1. |
The Essential Elements |
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Macronutrients |
Micronutrients |
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Calcium (Ca) |
Boron (B) |
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Carbon (C) |
Chlorine (Cl) |
|
Hydrogen (H) |
Copper (Cu) |
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Magnesium (Mg) |
Iron (Fe) |
|
Nitrogen (N) |
Manganese (Mn) |
|
Oxygen (O) |
Molybdenum (Mo) |
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Phosphorus (P) |
Nickel (Ni) |
|
Potassium (K) |
Zinc (Zn) |
|
Sulfur (S) |
previously mentioned, the amount of the chemical element required by a plant is not a measure of the element’s essentiality. Whether required in large amounts (macronutrients) or very small amounts (micronutrients), the element is essential if the plant cannot grow and develop normally without it.
The elements presently known to be vital to the survival of green plants are shown, along with their chemical symbols, in Table 3-1. One method of remembering the essential elements is to associate them with a catchy phrase. For example:
See MG men mob Cousin Hopkins nice clean cafe
C Mg Mn MoB CuZn HOPKNS Ni Cl CaFe
Of the essential elements, the plant obtains only carbon, hydrogen, and oxygen from sources other than the soil. The remainder are absorbed as minerals from the soil around the plant’s roots.
Knowing that the elements are necessary still does not address the question of what each element does for the plant that makes it so essential. The roles of several have not yet been clearly defined; they are believed to allow certain enzyme systems to function normally in the plant. At least some of the functions of other essential elements are known, as well as the symptoms shown by the plant when the element is lacking or in short supply. A brief summary of the functions of 14 essential mineral elements and the symptoms of their deficiency is given in Table 3-2. Both the functions and symptoms are discussed further in later chapters.
In addition to the 14 essential mineral elements and 3 essential nonmineral elements, there is another group known as the beneficial elements. They have been found to promote plant growth in many species, but have not been proven to be absolutely necessary for completion of the plants’ life cycle. Future research may yet prove them to be essential. Currently regarded as beneficial elements are silica, sodium, cobalt, and selenium. Other elements being considered for inclusion as beneficial are chromium, vanadium, and titanium.
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* ^ г ‘ |
TABLE 3-2. |
] Continued |
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E QJ Ш |
ent |
Function in the Plant |
Symptoms of Deficiency |
|
Sulfur (S) |
• Important to structure of protein molecules • Needed for enzyme activity to occur |
• Leaf chlorosis first noticeable in younger leaves • Weak stems |
|
|
Zinc (Zn) |
• Important to the synthesis of plant auxins • An important enzyme activator • Needed for protein synthesis |
• Interveinal chlorosis on younger leaves • White necrotic spots • Leaf dwarfing • Distortion of leaves |
Plants may exhibit symptoms of nutrient deficiency for several reasons:
• The element may be lacking totally or not be present in sufficient quantity.
• The element may be bound in a chemical form unavailable or too slowly available to the plant.
• There may be an overall imbalance of nutrients in the soil.
While micronutrients can be and often are deficient in soils, macronutrients are most often deficient. Nitrogen is foremost among the elements regularly lacking in sufficient quantities to produce strong, healthy plants. When nitrogen, in the nitrate form, is not absorbed by the colloidal particles of the soil, it passes quickly through the root region of the soil in an action called leaching.
