There are 7 micro nutrients or trace elements that plants require for growth . These are only required in small amounts but a deficiency of any trace element in the soil can limit plant growth even when all other essential elements are present in adequate amounts.  

Total soil content does not necessarily indicate the level of nutrient available for plant growth.

 
Micro nutrients, range of soil contents and optimum pH range for availability

Micronutrient and 

chemical symbol

 Soil content 

range ppm

 Optimum pH

Boron (B)

 10 - 630

 5.0 - 7.0

Cobalt (Co)

 1 - 40

 5.0 - 5.5

Copper (Cu)

 1 - 960

 5.0 - 6.5

Iron (Fe)

 3000 - 100 000

 4.0 - 6.0

Manganese (Mn)

 30 - 5000

 5.0 - 6.5

Molybdenum (Mo)

 0.1 - 18

 6.0 - 8.5

Zinc (Zn)

 2 - 1600

 5.0 - 6.5

Source: Better Soils [Online]. 1997. Module 2: Soil Nutrition and Crops. Agricultural Bureau of South Australia. Adelaide, SA. Available at http://www.bettersoils.com.au/module2/2_2.htm (verified 19 August 2004).