232 SCIENCE REMAKING THE WORLD 



derived from selection methods may be due entirely to 

 the elimination from the seed stock of tubers produced 

 by degenerate or diseased plants. From this viewpoint, 

 hill selection must be an annual task. Bud sports 

 or "mutants," do occasionally appear in the tubers. 

 New varieties have arisen in this way, most of them 

 differing from the parent variety merely in a modifica- 

 tion of the colour of the skin or tissue and in the period 

 of ripening. Bud sports are not usually considered of 

 much importance in selection work on account of their 

 infrequent occurrence. 



At the present time great emphasis is being placed 

 upon the importance of discovering strains of our 

 standard commercial varieties that are more vigorous 

 and productive than others. 



SALT NUTRITION. Different crops have different re- 

 quirements regarding the nutrients supplied to the 

 roots. All of them demand a group of essential ele- 

 ments, but these are utilized in different proportions by 

 different crops. If the soil does not already contain 

 these necessary elements in the proper amounts, they 

 must be supplied in fertilizers. The importance of 

 fertilizing the crop rather than the soil has led to ex- 

 tensive and carefully controlled experiments to dis- 

 cover the salt requirements peculiar to different crops. 

 This applies not only to the proper proportions of the 

 salts, but also to the kind of salts carrying the fertilizer 

 elements. Modern methods of salt nutrition studies 

 applied to the potato crop are now supplementing the 

 older empirical fertilizer tests and the outlook indicates 

 a more rational and economic fertilizer practice. 



