THE ABSORPTION OF MINERAL ELEMENTS 85 



from the soil itself, as for example, Chile saltpeter, rock phos- 

 phates and Stassfurt potash, or secured as byproducts of various 

 industries. Ammonium sulphate ; for instance, is obtained in the 

 process of purification of illuminating gas, and Thomas slag is 

 formed during the smelting of ore containing phosphorus. Some 

 artificial fertilizers are the products of direct synthesis, such as 

 Norwegian saltpeter or synthetic ammonia. Natural fertilizers 

 contain all the substances necessary for the plant, while artificial 

 fertilizers are unbalanced, usually furnishing only one or two of the 

 needed elements. It is clear, therefore, that the application of a 

 potash fertilizer to a soil unfertile as a consequence of phosphorus 

 deficiency, is a useless expense and will not lead to an increased 

 yield. Naturally, the transition from a primitive type of agricul- 

 ture, based principally on fertilization with manure, to more inten- 

 sive forms, in which the application of artificial fertilizers becomes 

 common, requires the development of a system of experiment 

 stations, where the requirements of the local soils can be studied, 

 as such experiments are beyond the capacity of individual farmers. 



The application of artificial fertilizers is subject to the law of the 

 minimum, which was established by Liebig at the time of the 

 introduction of artificial fertilizers into agricultural practice. 

 According to this law, as worked out by Hellriegel, the yield is 

 determined by the amount of that necessary element which is 

 present in a minimum quantity in proportion to the demands of the 

 plant. By gradually increasing the content of this element, the 

 yield will be increased in proportion to the amount applied until 

 another element will begin to act as a limiting factor. One of 

 Hellriegel's experiments showed that with the addition of 28 mg. 

 of nitrogen the yield was 2.99 g.; with 56 mg., 5.70 g.; with 112 

 mg., 10.80 g.; with 224 mg., 21.07 g.; or that by doubling the 

 amount of nitrogen used, the yield also was doubled. 



The law of the minimum has been applied not only to the influ- 

 ence of ash elements on yield, but to all other factors which 

 determine the development of plants, such as light, water, tem- 

 perature, etc. In this modified usage, this law has served as a 

 foundation for Blackman's law of limiting factors, which was 

 mentioned in Art. 14. 



Recently the law of the minimum has been seriously criticized 

 by Mitscherlich and his collaborators. He points out that a 

 proportionality between the amount of some nutrient element and 



