Micronutrient Requirements

Crops require 16 essential elements to grow properly. The elements include carbon, hydrogen and oxygen, which are derived from air and water. All the remaining nutrients used by plants come from soil in the form of inorganic salts. Legumes are an exception because they can also fix nitrogen from the air.

The macronutrients obtained from the soil include nitrogen, phosphorus, potassium, calcium, magnesium and sulfur. The reaming essential elements needed by plants are known as micronutrients because plants use them in relatively small amounts. They include boron, chlorine, copper, iron, manganese, molybdenum and zinc. Carbon, hydrogen and oxygen comprise from 94.0 - 99.5% of fresh plant tissue. The remaining nutrients, which come from the soil, make up the balance of tissue.

The term micronutrient refers to the relative quantity of a nutrient that is required for plant growth. It does not mean that they are less important to plants than other nutrients. Plant growth and development may be retarded if any of these elements are lacking in the soil or are not adequately balanced with other nutrients. Table 1 lists amounts of micronutrients removed from the soil by good yields of various crops.

Table 1. Amounts of some micronutrients removed by good yields of various crops

Micronutrients removed (kg/ha)

Crops harvested and portion used for analysis  Yield Level (t/ha) Chlorine (Cl) Boron (B) Copper (Cu) Iron (Fe) Manganese (Mn) Zinc (Zn)
Alfalfa - Hay 1.3 6 0.10 < 0.1 0.20 0.7 0.7
Barley - Grain 4.0 8 0.10 < 0.1 0.30 0.1 0.1
Barley - Straw - 1 0.02 < 0.1 0.01 0.7 0.1
Corn - Grain 9.5 2 0.70 < 0.1 0.20 0.1 0.2
Corn - Stover - 1 0.06 < 0.1 1.00 1.7 0.3
Oats - Grain 4.0 1 - < 0.1 1.00 0.2 0.1
Oats - Straw - 1 - < 0.1 0.20 0.2 0.4
Peas - Vines & Pods - - 0.07 < 0.1 0.70 0.5 0.1
Potatoes - White, Tubers 40 27 0.07 < 0.1 0.90 0.2 0.1
Wheat - Grain 4.0 6 0.06 < 0.1 0.50 0.2 0.2
Wheat - Straw - 2 0.02 < 0.1 0.20 0.3 0.1

 Data compiled from several sources

Sources of Micronutrients in Soils

Inorganic micronutrients occur naturally in soil minerals. The parent material from which the soil developed and soil forming processes determine what the micronutrient content of the soil will be. As minerals break down during soil formation, micronutrients are gradually released in a form that is available to plants. Two sources of readily available micronutrients exist in soil: nutrients that are absorbed onto soil colloids (very small soil particles) and nutrients that are in the form of salts dissolved in the soil solution.

Organic matter is an important secondary source of some micronutrients. Most micronutrients are held tightly in complex organic compounds and may not be readily available to plants. However, they can be an important source of micronutrients when they are slowly released into a plant available form as organic matter decomposes.

Soil Factors That Affect Micronutrient Availability

Physical and chemical characteristics of soil affect the availability and uptake of micronutrients:

  • Soils low in organic matter (less than 2.0%) may have lower micronutrient availability.
  • Soils with higher amounts of clay (fine texture) are less likely to be low in plant available micronutrients.
  • Sandy soils (course texture) are more likely to be low in micronutrients.
  • Soils that have very high levels of organic matter (greater than 30% organic matter to a depth of 30 cm) often have low micronutrient availability.
  • Soil temperature and moisture are important factors. Cool, wet soils reduce the rate and amount of micronutrients that may be taken up by crops.
  • As soil pH increases the availability of micronutrients decreases, with the exception of molybdenum.

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