Essential Plant Nutrients

Three elements, carbon (C), hydrogen (H) and oxygen (O), are supplied by air (in the form of carbon dioxide) and water. When the chlorophyll (green pigments) of plants are exposed to light, these three elements are combined in a process called photosynthesis to make carbohydrates, with a subsequent release of oxygen. The water is brought into the plant by root absorption from the soil system. Carbon dioxide (CO2) enters the plant through small leaf openings called stomata. The rate at which photosynthesis occurs is directly influenced by the water and nutritional status of the plant. Maximum rates are determined ultimately by the genetics of the plant.

Calcium Stomata

Fifteen of the essential nutrients are supplied by the soil system. Of these, nitrogen (N), phosphorus (P) and potassium (K) are referred to as primary or macronutrients. This is because (1) they are required by the plant in large amounts relative to other nutrients (see Table 2) and (2) they are the nutrients most likely to be found limiting plant growth and development in soil systems.

Table 1. Essential plant nutrients and their elemental (chemical) symbol

Nutrients Supplied by Air and Water

Nutrients Supplied by the Soil System

Non-Mineral

Primary or Macronutrients

Secondary

Micronutrients

Carbon - C

Nitrogen - N

Calcium - Ca

Zinc - Zn

Hydrogen - H

Phosphorus - P

Magnesium - Mg

Chlorine - Cl

Oxygen - O

Potassium - K

Sulfur - S

Boron - B

Molybdenum - Mo

Copper - Cu

Iron - Fe

Manganese - Mn

Cobalt - Co

Nickel - Ni

Calcium (Ca), magnesium (Mg) and sulfur (S) are termed secondary nutrients because they are less likely to be growth-limiting factors in soil systems. Calcium and magnesium are added in liming materials when soil pH is adjusted and sulfur is added continually by rainfall and release from the soil organic matter. It is estimated that some 10 to 20 pounds of sulfur per acre may be deposited annually in precipitation.

Zinc (Zn), chlorine (Cl), boron (B), molybdenum (Mo), copper (Cu), iron (Fe), manganese (Mn), cobalt (Co) and nickel (Ni) are termed micronutrients because (1) they are found in only very small amounts (see Table 2) relative to other plant nutrients in the average plant and (2) they are least likely to be limiting plant growth and development in many soil systems. There is a much finer line between "enough" and "too much" for the micronutrients than for other plant nutrients. Use of micronutrient fertilizer materials should only be undertaken with very clear objectives (i.e., correction of clearly identified Zn deficiencies of corn grown on soils high in pH or P) in mind and with a knowledge of previously successful rates of application. Indiscriminate use of micronutrients is more likely to result in undesirable effects than similar use of other nutrients.

Table 2. Average concentrations of 13 soil-derived (mineral) nutrients in plant dry matter that are sufficient for adequate growth (Epstein, 1965)

Element

mg/kg (ppm)

%

Relative Number of Atoms

Molybdenum

0.1

--

1

Copper

6

--

100

Zinc

20

--

300

Manganese

50

--

1,000

Iron

100

--

2,000

Boron

20

--

2,000

Chlorine

100

--

3,000

SulfUr

--

0.1

30,000

Phosphorus

--

0.2

60,000

Magnesium

--

0.2

80,000

Calcium

--

0.5

125,000

Potassium

--

1.0

250,000

Nitrogen

--

1.5

1,000,000

0 0

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