262 PLANT PHYSIOLOGY 



An experimental field is usually divided into plots of a definite 

 size, and to each is applied either one fertilizer or a combination 

 of fertilizers. By determining separately the yield from each 

 plot, it is easy to find what the fertilizer requirements are. 

 This procedure of determining the needs of a soil is widely used 

 by experiment stations and in experimental field work. Expres- 

 sing it figuratively, Timiriazeff said that in applying this method 

 we ask the plant directly in what a given soil is deficient for its 

 welfare. 



A still more rapid answer to the question of how much sub- 

 stance and what substances the plant will receive from a par- 

 ticular soil is given by Neubauer's method. The procedure of 

 this method is as follows. A great number of seedlings (usually 

 100) are grown for 2 to 3 weeks in a flat glass tray on a small 

 amount of soil (100 g.). 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, shown by the analysis, 

 after the amount present in the seeds has been deducted, corre- 

 sponds to the amounts of these elements in the soil that are 

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

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

 over, the very small amounts of the substances that have to be 

 determined call for very delicate and very exact methods of 

 chemical analysis. Then, too, in making calculations from 

 100 gm. and applying them to acre plots, every error of the 

 analysis is multiplied many thousand times. At present, this 

 method is not very generally used. 



Very convenient and rapid is the method of determining the 

 nutritive value of soils by growing in them the mold Aspergillus. 

 This method was suggested by Butkevitch (1909) and has been 

 elaborated in detail by Nicolas. It is based on the fact that 

 molds react just as conspicuously as higher plants to the presence 

 of the indispensable mineral elements. The weight of the 

 mycelium of these fungi serves as a qualitative index of the 

 amount of elements, especially potassium and phosphorus, con- 

 tained in the soil. Some sucrose and citric acid are added to 

 the soil to satisfy the requirements of these organisms for organic 

 substances. 



