276 PLANT PHYSIOLOGY 



has diminished. Furthermore, the main mass of roots descends 

 to the deeper soil horizons, where the fertihzers do not penetrate. 

 The phosphates, especially, remain concentrated in the upper 

 soil layers. Based on these facts, attempts are being made at 

 present to modify the customary practice of fertilization in such 

 a way as to make the fertilizers available to the plants at later 

 stages of growth. For this purpose, the fertilizers are applied 

 either between the crop rows, or else they are introduced deeper 

 into the soil so that the roots of plants reach them at a later 

 stage. In some cases, it is desirable to apply phosphates in 

 holes that have been driven around trees, so as to get deeper 

 penetration. Where artificial irrigation is applied, the fertilizers 

 are sometimes dissolved in the irrigation water. A more detailed 

 examination of the different practices of fertilization belongs 

 not to physiology but to soil science. 



General References 



Breazeale, J. F. Vitamin-like substances in plant nutrition. Ariz. Agr. 



Exp. Sta. Tech. Bull. 16, 1927. 

 Brenchley, W. E. The essential nature of certain minor elements in 



plant nutrition. Botan. Rev., 2: 173-196, 1936. 

 Briggs, G. E. The absorption of salts by plant tissues, considered as an 



ionic interchange. Ann. Botany, 46: 301-322, 1932. 

 BuRD, J. S. Rate of absorption of soil constituents at successive stages of 



plant growth. Jour. Agr. Research, 18: 51-72, 1919. 

 Chandler, W. H. Zinc as a nutrient for plants. Botan. Gaz., 98 : 625-646, 



1937. 

 Eaton, S. V. Sulphur content of soils and its relation to plant nutrition. 



Botan. Gaz., 74: 32-59, 1922. 

 FiNDLAY, A. "Osmotic Pressure." Longmans, Green & Company, New 



York. 1913. 

 Gardner, V. R., F. S. Bradford, and H. D. Hooker. "Fundamentals of 



Fruit Production," Sec. II. McGraw-Hill Book Company, Inc., New 



York. 1922. 

 Hartt, C. E. Some effects of potassium upon the growth of sugar cane 



and upon the absorption and migration of ash constituents. Plant 



Physiol., 9 : 399-452, 1934. 

 Harvey, R. B. "Plant Physiological Chemistry," Part I. D. Appleton- 



Century Company, Inc., New York. . 1930. 

 HiBBARD, R. P., and S. Gershberg. The salt requirements of Marquis 



wheat in water culture for the vegetative phase of development. 



Mich. Agr. Exp. Sta. Tech. Bull. 64, 1924. 

 Hoagland, D. R. The absorption of ions by plants. Soil Science, 16: 



225-246, 1923. 



