2 ILLINOIS NATURAL HISTORY SURVEY CIRCULAR 52 



orous trees tend to resist borers, while those growing under un- 

 favorable moisture or nutrient conditions are more susceptible 

 to attack by these insects. 



Established trees weakened by leaf diseases, insect defolia- 

 tion, mechanical injury, soil compaction, drought, or other causes 

 often show poor growth or dying of branch ends. Fertilization 

 may stimulate additional growth so that the plant can compensate 

 for the conditions that cause decline. 



WHAT IS A FERTILIZER? 



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

 posed of elements essential or beneficial to plant growth. The 

 essential elements present in plant tissue in relatively large 

 quantities are called macronutrients. They are nitrogen, potas- 

 sium, phosphorus, calcium, magnesium, sulfur, oxygen, carbon, 

 and hydrogen. Those essential elements present in plant tissue 

 in relatively small quantities are called micronutrients. The 

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

 molybdenum. 



Magnesium, sulfur, and the micronutrients are usually ade- 

 quate in most soils and rarely limit plant growth. The carbon, 

 hydrogen, and oxygen used by plants come from the atmosphere 

 or from soil water, and under normal conditions occur in suffi- 

 cient amounts. Nitrogen, phosphorus, and potassium are of pri- 

 mary concern as soil supplements. 



Nitrogen 



Plant growth is more often limited by deficiency of nitrogen 

 than of any other element. Although nitrogen comprises 78 per- 

 cent by volume of the earth's atmosphere, it is in a form not 

 available to plants. Nitrogen compounds are rare in the rocks 

 from which soil is formed. Certain bacteria in the soil use atmos- 

 pheric nitrogen and change it into a form that can be used by 

 plants. Also, some atmospheric nitrogen is added to the soil dur- 

 ing electrical rainstorms. However, most soil nitrogen available 

 to trees is derived from decomposed plant material returned to 

 the soil. Microorganisms in the soil must break down this com- 

 plex plant material into simple inorganic compounds before the 

 nutrients can be used by trees. 



Nitrogen in plants occurs in proteins. Proteins are primary 

 components of protoplasm, the living material in plant cells. 



