10 Illinois Natural History Survey Circular 56 



conditions, ti ces often benefit f rotri fertilization, and annual 

 applications may be required for several years. An 

 agronomist, farm adviser, or extension agent should be 

 contacted for assistance with local soil problems. 



Two c hemical tests are used to determine soil deficiencies — 

 soil tests and plant tissue analyses. Although no method of 

 determining deficiencies in soil nutrients is applicable to all 

 plants under all conditions, soil tests reveal general soil 

 deficiencies and help to determine if the phosphorus or 

 potassium content is low in the soil around shade trees. 

 Diagnosing soil deficiencies by analyzing plant tissue is a 

 useful research tool but is impractical for determining the 

 fertilizer needs of trees. 



Disadvantages of fertilizing 



Although the advantages of fertilizing usually far outweigh 

 the disadvantages, certain outcomes should be kept in mind. 

 Fertilizing trees or shrubs in lawns also stimulates grass 

 growth, and frequent mowing may be necessary. Unless 

 regularly pruned, small ornamental shrubs that have been 

 fertilized may become too large for their locations in a few 

 years. Heavy nitrogen applications tend to increase twig 

 growth and to reduce flowering in some ornamental shrubs. 

 Prolonged fertilizing may cause some woody species to 

 become tall, spindly, or succulent and to develop a weeping 

 appearance. 



American beech, white oak, and some varieties of crab apple 

 are reported to have been injured by fertilizer formulations 

 containing nitrogen, phosphorus, and potassium. All 

 fertilized plants should be observed critically each year to 

 determine the effect of fertilization. 



Where should you place the fertilizer? 



Roots grow where the soil environment is favorable. They 

 do not grow where oxygen is unavailable or where the soil 

 is compact and difficult to penetrate. Since soil pores and 



