II 



at the rate of 6 pounds of N per 1,000 square feet caused 

 no injury when applied on dry grass and only very slight 

 injury when applied on wet grass. The 10-10-10 formula- 

 tion caused no injury when applied on dry grass at rates 

 of 9 pounds or less of N per 1,000 square feet; at even 

 3 pounds of N on wet grass it caused some injury. Both 

 ammonium nitrate and ammonium sulface were safely 

 applied in the spring at 6 pounds of N per 1,000 square 

 feet. Ammonium nitrate applied in July, when the soil 

 was dry, caused injury at all application rates. However, 

 all rhe fertilizer burn was temporary. No burn from any 

 treatment was evident on October 1. 



The probable explanation for lack of injury from the 

 first ( April 1 5 ) fertilizer applications may be found in 

 precipitation data for the area. Rain fell on 5 of the 7 

 days following the April 15 applications; precipitation 

 recorded by the Urbana weather station for these days 

 totaled 5.92 inches. Rain fell on 3 of the 7 days following 

 the May 15 application; precipitation totaled 0.10 inch. 

 No precipitation occurred during the 7 days following the 

 July 31 application. Rainfall following heavy applications 

 of fertilizer may reduce the phytotoxic effect of the fertilizer. 

 Under normal lawn conditions, the fertilized area should 

 be heavily watered with a lawn sprinkler. 



Under the conditions of this lawn ferrilization test, it 

 appeared that most nitrogen-containing fertilizers can be 

 safely broadcast on dry Kentucky bluegrass at the rate of 

 6 pounds of N per 1,000 square feet. Moderate burn of 

 grass blades may occur when fertilizer is applied ar this rate 

 on wet grass. Higher rates of application are possible if 

 fertilization is followed within a few hours by precipitation 

 sufficient to wash the fertilizer off the leaf blades. Supple- 

 mental watering would undoubtedly lessen the amount of 

 injury that might occur following heavy applications of N. 

 Even in plots receiving rhe heaviest applications of fer- 

 tilizers, all signs of burning disappeared a month after the 

 fertilizer had been applied. 



DISCUSSION 



Recommendations for fertilization of shade trees should 

 specify time of fertilization, type and amount of fertilizer 

 to be used, and method or methods of application. Time 

 of fertilization and amount of fertilizer are variables not 

 investigated in our tree srudy reported here. All of our 

 fertilizer treatments were applied in the spring. All of 

 our experimental trees received fertilizer at the rate of 

 6 pounds of N, PO-, or K.,0 per 1,000 square feet of 

 area. This rate was selected since it is approximately 

 equivalent to the amount of fertilizer commonly recom- 

 mended for established shade trees and is a rate known to 

 be nontoxic to grass and trees when the fertilizer is ap- 

 plied dry in holes. 



Formulas that have been used for determining the 

 amount of fertilizer to be applied are varied, and some of 

 them are difficult for the average arborist to follow. One 

 of these formulas employs the sum of the following: the 

 height of the tree in feet, the branch spread in feet, and 

 the trunk circumference in inches 1 foot above the soil 



line. The total of these figures is the number of pounds 

 of 10-8-6 fertilizer to appiy in holes. The most common 

 formula now used is based on the diameter of the tree 

 trunk at breast height. Trees of 6 inches dbh or larger 

 receive one-half pound of nitrogen per inch of trunk diame- 

 ter; smaller trees receive one-fourth pound of nitrogen per 

 inch of trunk diameter. Another formula specifies 3 pounds 

 of a balanced fertilizer per inch of trunk diameter. 



Wikle (1963), in a discussion of shade tree fertiliza- 

 tion, emphasized the large differences between various 

 application rates that are based on trunk diameter. He 

 recommended that the rates be based on the area of soil 

 to be treated, not on trunk diameter. We agree with his 

 recommendation. The basic reason for application of fer- 

 tilizer is to supplement the nutrients naturally available 

 in the soil occupied by the tree roots, which, for most 

 species, extend at least as far as branch spread. Such a 

 recommendation greatly simplifies the problem of deter- 

 mining the amount of fertilizer to be applied. 



The answer ro the question, "Should 1 or shouldn't I 

 fertilize this tree?" depends on the answers to a number 

 of other questions. What is the soil type? Has the char- 

 acter of the soil type been changed by previous manage- 

 ment practices? Will the use of nitrogen in a fertilizer 

 give a measurable growth response? Will the addition 

 of other soil elements to the fertilizer give additional 

 growth response? Is the tree of such an age and species 

 that it will respond to the use of fertilizer? Will increased 

 soil fertility resulting from use of fertilizer prevent or at 

 least retard tree decline or dieback from certain physiologi- 

 cal or pathological diseases? 



Much research must be done before these questions can 

 have adequate answers. To some of the questions, research 

 already reported has given only partial answers, and, to 

 other questions, conflicting answers. 



Our use of nitrogen-containing fertilizers on pin oak, 

 white ash, and honey locust in northern Illinois resulred 

 in an increase in tree circumference that was 39% to 73% 

 greater than the increase for untreated trees. To wli.it 

 extent certain other tree species will respond to fertilizers 

 containing nitrogen or other nutrient elements can be 

 determined only through additional controlled experimenta- 

 tion. In our northern Illinois tests, significant growth 

 response was obtained from nitrogen and from no other 

 nutrient element. In Ohio and New York tests, growth 

 response was reported from use of nitrogen and additional 

 growth response when phosphorus was added to nitrogen. 



The method used in applying fertilizers is especially 

 important because it determines to a great extent the cost 

 of the operation and the ease with which it is accomplishe I 

 The drilled hole method and the solution injection method 

 are relatively expensive, for they require considerable time 

 and expensive equipment. The punch-bar method requires 

 a great amount of labor and time. The surface broadcast 

 method is fast and requires only that equipment already 

 owned by most gardeners or arborists. In our tests, the 

 tree response to nitrogen applied to the soil surface was 

 comparable to the response to nitrogen placed in the soil 



