284 Mineral Nutrition of Plants 



seem to have been related to the weather conditions at the harvest 

 period in the years of moderate crop. Cool bright weather at harvest 

 time was conducive to the best color development. In addition, the 

 very heavy crop in 1944 may have operated to reduce fruit color develop- 

 ment in that year. 



PRACTICES AFFECTING NITROGEN SUPPLY 



These data should serve to illustrate some of the complexities of the 

 problem of controlling nitrogen effects on apple trees with the purpose 

 of obtaining maximum production and fruit quality. 



The growth phenomena that are most important in determining 

 yield occur largely in the early part of the growing season while fruit 

 color and quality develop at the end of the growing season. Thus, a 

 relatively high nitrogen status of the tree, favoring high yield, is de- 

 sired in the early growing season, and a relatively low nitrogen status, 

 favoring fruit color and quality development, is desired in the latter 

 part of the growing season. 



In the northeastern United States, weather most often is a dominant 

 factor that limits crop size and quality. The grower cannot control the 

 weather; but he can manipulate certain cultural factors, the most 

 pertinent of which are 1) nitrogen application — material, rate, time, 

 and method; 2) ground cover; and 3) pruning and spraying practices.* 



Nitrogen application 



Readily available inorganic or simple organic forms of nitrogen 

 are used for ground application in Mcintosh apple orchards in prefer- 

 ence to forms which release available nitrogen over a long period of 

 time. When moderate applications of such materials as ammonium 

 nitrate, sodium nitrate, or ammonium sulfate are made in the fall or 

 early spring, most of the soluble nitrogen is usually absorbed by plant 

 roots before midsummer. At high rates of application, and with sparse 

 ground cover or under heavy mulch, considerable available nitrogen 

 may remain in the soil throughout the summer. Under such circum- 



*In the ensuing discussion it is assumed that nitrogen is the only limiting nu- 

 trient, that soil moisture and aeration are satisfactory, and that no unusual condi- 

 tion such as winter injury, trunk girdling, or insect or disease troubles have taken 

 on major proportions. 



