Hoagland — 44 — Plant Nutrition 



disease of some apparently nutritional character. At 

 first, it was supposed that there was a deficiency of 

 iron in the ration. Actually, later experience showed 

 that primarily the trouble was a deficiency of cobalt, 

 small as the amount required was. The pasture plants 

 upon which the animals fed did not seem to give any 

 indication of deficiency of cobalt but they did not 

 contain enough cobalt for the needs of the animal. 

 We have not yet had reason to think that cobalt is 

 an essential element for higher plants. But under the 

 field conditions I have described there exists a question 

 of availability of cobalt in the soil and of the absorption 

 of cobalt by the plant, as far as the welfare of the 

 grazing animal is concerned. 



The problem of animal nutrition in relation to the 

 physiology of the plant and to soil conditions is not 

 restricted to deficiencies of chemical elements. Plants 

 may become toxic to the animal because of something 

 the plant has absorbed from the soil. Doubtless in 

 this connection you are familiar with the example of 

 selenium, since so much recent discussion of this ele- 

 ment has appeared in both scientific and popular 

 journals. It will be recalled that an explanation was 

 found of a previously obscure disease of cattle, horses 

 and other animals occurring in South Dakota, Wyom- 

 ing and elsewhere, which had been called sometimes 

 the "alkali" disease. Now it is clear that the animals 

 suffering from this disease are poisoned by selenium 

 derived from the vegetation growing on certain types 

 of soil. The soils supporting this vegetation are de- 

 rived from shales high in selenium. 



Of particular interest to this discussion is the 

 selective power of different plant species in the ab- 

 sorption of selenium from the same soil. Some plants 

 take up relatively little of this element, while others 

 accumulate large amounts. In the latter class are 

 several species of Astragalus. When the residues of 

 high selenium plants are returned to the soil and un- 

 dergo decomposition, selenium is left in a more avail- 



