ABSORPTION OF MINERAL ELEMENTS BY PLANTS 263 



Before the application of artificial fertilizers, it is necessary 

 to learn which of the elements are needed for a certain soil. 

 In contrast to the natural fertilizers, such as manure or the 

 various organic products, by the term ''artificial fertilizers" is 

 understood the different mineral compounds obtained in part 

 from the soil itself, e.g., Chile saltpeter, rock phosphates, and 

 Stassfurt potash, or secured as products or by-products of 

 various industries. Ammonium sulphate, for instance, is 

 obtained in the process of the 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 synthetic nitrates or ammonia. Natural 

 fertilizers contain all the substances necessary for the plant, 

 while artificial fertilizers are unbalanced, usually furnishing only 

 one, two, or three of the needed elements. It is clear, therefore, 

 that the application of a potash fertilizer to a soil infertile 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 agriculture, based principally on fertil- 

 ization with manure, to more intensive forms, in which the 

 apphcation of artificial fertilizers becomes common, requires 

 the development of a system of experiment stations, where the 

 requirements of the local soils can be studied in connection with 

 the needs of different crops, such experiments being beyond the 

 capacity of individual farmers. 



The application of artificial fertilizers is subject to the law of 

 the minimum, first estabhshed by Liebig (1846) at the time of 

 the introduction of artificial fertiUzers 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 x)i 

 the plant. If the content of this element is gradually increased, 

 the yield will be increased in proportion to the amount appUed 

 until another element begins to act as a limiting factor. 



The law of the minimum has been apphed not only to th.^ 

 influence of mineral elements on yield, but to all factors tha'^ 

 determine the development of plants, such as light, water, and 

 temperature. In this modified usage, this law has served as a 

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

 mentioned in Art. 47. 



