SOIL NUTRIENTS

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

TABLE 3-1.

The Essential Elements

Macronutrients

Micronutrients

Calcium (Ca)

Boron (B)

Carbon (C)

Chlorine (Cl)

Hydrogen (H)

Copper (Cu)

Magnesium (Mg)

Iron (Fe)

Nitrogen (N)

Manganese (Mn)

Oxygen (O)

Molybdenum (Mo)

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.

SOIL NUTRIENTS

‘Л – TABLE 3-2.

When

Essential Elements with Their Functions and Symptoms of Deficiency

Element

-unction in the Plant

Symptoms of Deficiency

Boron (Bo)

Role not clearly defined except in

• Dead shoot tips

translocation of sugar. It has a role in

• Leaves thicken, curl, and become brittle

flower and fruit development as well.

• Flowers fail to form

• Stunted roots

Calcium (Ca)

A component of the cell wall

• Stems, leaves, and roots die at tips, where

Needed for cell division and growth

growth is normally most active

• Chlorosis of young leaves, then necrosis

along margins

Chlorine (Cl)

Role not clearly defined, but important to

• Stunting

shoot and root development

• Chlorosis

Copper (Cu)

A catalyst for respiration

• Necrosis in young leaf tips and margins

Needed for photosynthesis

• Stunting

Iron (Fe)

Needed for chlorophyll synthesis

• Chlorosis of younger leaves only, usually

in an interveinal pattern

Magnesium

Essential for photosynthesis as a

• Interveinal chlorosis, appearing first in

(Mg)

component of chlorophyll molecule

older leaves, followed by red or purple

An important enzyme activator

color and necrotic spots

Needed for sugar and fat formation

Manganese

An enzyme activator in respiration

• Interveinal chlorosis and necrosis

(Mn)

Makes nitrogen available for plant use

Molybdenum

Makes nitrogen available for plant use

• Interveinal chlorosis in lower leaves

(Mo)

Needed for protein synthesis

• Marginal necrosis

• Flowers fail to form

Nickel (Ni)

Role not clearly defined, but believed to be

• None have been observed

instrumental in enzymatic functions

Nitrogen (N)

Important to synthesis and structure of

• Chlorosis, first noticeable in older leaves

protein molecules

• Stunting of growth

Encourages vegetative growth

Promotes rich green color

Phosphorus (P)

Essential to energy transfer

• Red or purple discoloration of older leaves

Stimulates cell division

• Premature leaf drop

Needed for flowering

• Stunting of growth

Promotes maturation

Promotes disease resistance

Promotes root development

Potassium (K)

Role not clearly defined

• Mottled chlorosis first noticeable on lower

Believed to activate important plant

leaves

systems and enzymes

• Necrosis at tips and margins of leaves

 

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TABLE 3-2.

] Continued

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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.