316 Mineral Nutrition of Plants 



In another zinc-deficient soil, 1500 pounds of zinc sulfate applied to the 

 soil failed to cure zinc deficiency in apples. It would be easy to miss the 

 nature of the deficiency in these cases if treatment were confined only 

 to the soil. On the other hand, the essential status of zinc was demon- 

 strated by techniques of foliage spraying or tree injection. Though the 

 soil was by-passed completely, a correct diagnosis and treatment was 

 achieved. In California, these are indeed the practical methods of treat- 

 ment of zinc and other micronutrient deficiencies in the field. 



There seems to be a basis then for reaching the paradoxical conclu- 

 sion, that the natural growth medium of land plants, the soil, is least 

 adapted for the study of the indispensable nature of plant nutrients. It 

 is with artificial nutrient media, water and sand cultures, that the 

 essential status of the various elements found in plants and derived 

 from the soil was established. The recent history of plant nutrition 

 offers no case of a discovery of a new essential element through soil 

 treatments. For every one of the micronutrients for example, evidence 

 of their indispensability was available from nutrient solution experi- 

 ments, well in advance of any responses reported from the field. The 

 artificial culture technique continues to be a powerful and discrimi- 

 nating tool in evaluating the indispensability of inorganic nutrients in 

 plant nutrition. 



A question may be raised as to the soundness of applying conclusions 

 drawn from nutrient solution studies to the growing of plants in soil. 

 There seems little doubt that the availability and absorption of several 

 important plant nutrients is different in soils than in nutrient solutions. 

 There is likewise little doubt that the nutritional requirements of plants 

 are the same in soil as in nutrient culture. It is not implied, of course, 

 that the same elements will be absorbed in the same amounts from dif- 

 ferent media, be they a group of soils or a series of different nutrient 

 solutions. The capacity of the plant to absorb nonessential elements was 

 already mentioned. It is also well established that plants can grow in 

 soils or nutrient solutions of varying compositions. It is difficult to con- 

 ceive, however, that a different set of inorganic requirements would 

 govern the growth and successful completion of the life cycle of a 

 plant such as the tomato, depending on whether it was grown in soil 

 or in water culture. The relative inorganic composition of the various 



