Horticulture Principles and Practices

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