Illinois Natural I listoiy Survey Circular 56 



inadequate. Healthy, vigorous trees tend to resist boiers, 

 whereas those growing luifler uniavorahle moisture or 

 nutrient conditions are more suscej)til)le to attack. 



Established trees weakened by leaf diseases, insect defolia- 

 tion, mechanical injury, soil compaction, or drought often 

 show poor growth or the dying of branch ends. Fertilization 

 may stimulate additional growth so that the plant can 

 compensate for the conditions that caused the decline. 



What is a fertilizer? 



A fertilizer is a supplement, usually added to the soil, 

 composed of elements beneficial to plant growth. The 

 essential elements present in plant tissue in relatively large 

 quantities are called macronutrients. They are nitrogen, 

 potassium, phosphorus, calcium, magnesium, sulfur, 

 oxygen, carbon, and hydrogen. The essential elements 

 present in plant tissue in relatively small quantities — the 

 micronutrients — are iron, manganese, copper, zinc, boron, 

 molybdenum, and chlorine. 



Magnesium, sulfur, and the micronutrients are adequate in 

 most soils and rarely limit plant growth. Ihe carbon, 

 hydrogen, and oxygen used by plants are components of the 

 atmosphere and of soil water; under normal conditions these 

 three nutrients are never deficient. Calcium in soils is a plant 

 nutrient that primarily serves to neutralize soil acidity and 

 to increase the availability of other nutrients. It can be 

 required as a fertilizer in rare instances when soil pH is 4.0 

 and below. Nitrogen, phosphorus, and potassium are of 

 primary concern as soil supplements. 



Nitrogen 



Plant growth is more often limited by a deficiency of nitrogen 

 than by a deficiency of any other element. Nitrogen 

 compounds are rare in the rocks from which soil is formed. 

 Although nitrogen comprises 78 percent by volume of the 

 earth's atmosphere, that is a form not available to plants. 

 Certain bacteria in the soil use atmospheric nitrogen and 



