Hoagland —36— Plant Nutrition 



have a role in determining zinc availability to crops. 

 In our greenhouse experiments with a soil producing 

 zinc deficiency disease in corn (and in the field the 

 "little-leaf" disease of peach trees) we have found 

 that frequently this failure of the soil to supply zinc 

 can be overcome by sterilizing the soil and that the 

 condition of deficiency in supplying power for zinc 

 can be reestablished by reinoculating the sterilized 

 soil with a very small percentage of unsterilized soil. 

 One assumption is that soil microorganisms growing 

 close to or in contact with root surfaces offer com- 

 petition for minute amounts of zinc present in dis- 

 solved or available form*. (See plate 13). 



The interest of a biologist will be attracted by the 

 great differences among different species of plants in 

 their ability to absorb zinc from a soil with low zinc 

 supplying power. In the greenhouse alfalfa has been 

 observed to obtain sufficient zinc from a soil in which 

 corn plants suffer severely from lack of zinc. Some- 

 times the continued growth of alfalfa in an orchard 

 will prevent the development of little-leaf disease in 

 trees. By solution culture technique, however, it is 

 possible to show that alfalfa itself has a zinc require- 

 ment not very different in magnitude from that of 

 other plants that fail for lack of zinc in the soil in 

 which alfalfa grows satisfactorily. Whether alfalfa 

 roots have a special capacity for absorbing zinc ions 

 or whether the nature of the soil flora undergoes 

 change as a result of the crop growth with an accom- 

 panying influence on the available zinc status of the 

 soil remains unsettled. 



While the applied agricultural aspects of zinc de- 

 ficiency have a high degree of importance for a 

 member of an Agricultural Experiment Station, I 

 appreciate that this audience would be far more in- 



*These experiments have been frequently repeated and in 

 different years. However, failures have occurred. These are 

 perhaps to be expected, since under greenhouse conditions soils 

 after sterilization may readily become accidentally contaminated 

 with various microorganisms. 



