84 TEXTBOOK OF PLANT PHYSIOLOGY 



This is the reason why the requirements for fertilizers by plants 

 are always higher in pots than in the field. For exact determina- 

 tion of the amounts of fertilizer to be added, it is more expedient, 

 therefore, to use the field method. An experimental field is usually 

 divided into plots of some definite size and to each is applied 

 either one or a combination of fertilizers. By determining sepa- 

 rately the yield from each plot, it is easy to find what the fer- 

 tilizer requirements are. This procedure of determining the 

 needs of a soil is widely used by the experiment stations and in 

 experimental field work. Expressing it figuratively, Timiriazeff 

 says that in applying this method we ask the plant directly what is 

 deficient in a given soil for its welfare. 



A still more direct answer to the question of how much and 

 what substances the plant may receive from the particular soil is 

 given by Neubauer's method which has been widely applied 

 in Germany during recent years and is beginning to find its way 

 into other countries. The procedure of this method is as follows: 

 a great number (up to 100) of seedlings are grown for 2 to 3 weeks 

 in a flat glass cup on a small amount (100 g.) of soil. The plants 

 are then separated from the soil, dried, and carefully analyzed. 

 It is supposed that the plants have had time to absorb all of the 

 available mineral substances from the soil and that the quantity 

 of the mineral elements, particularly phosphorus and potassium, 

 revealed in the analysis, after deducting the amount present in the 

 seeds, corresponds to the amounts of these elements in the soil, 

 which are available to the plants. Neubauer's method is no doubt 

 very ingenious, but evidently it cannot be applied to all soils. 

 Moreover, the very small amounts of the substances that have to 

 be determined call for very sensitive and very exact methods of 

 chemical analysis. Then, too, in making calculations from 100 g. 

 and applying them to acre plots, every error of the analysis is mul- 

 tiplied many thousand times. At present, this method is not very 

 generally applied. 



29. Yields as Determined by the Amount of Nutritive Sub- 

 stances. Law of the Minimum. The Action of Growth Factors. 

 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" are 

 understood the different mineral compounds obtained in part 



