MINERAL COMPOSITION 0»F CROPS 43 



The addition of 250 pounds of superphosphate resulted in a further 

 increase in phosphorus. 



Austin (37) found that the phosphorus content of the soybean plant 

 grown in the Brookston clay loam was actually reduced when phos- 

 phates alone were applied to the soil. In general, however, the effects 

 of fertilizer treatments upon the composition of the plants were small, 

 except possiblv that of plants grown in the Coloma sand. 



Lagatu and Maume {324, 325 , 326, 327, 328, 329, 330) have made a 

 number of fundamental studies in recent years of the requirements of 

 the plant for the mineral nutrients. They describe a method of 

 sampling which consists of taking the two leaves just below the fruiting 

 branch of the plant. They claim that these samples give a true index 

 of the plant's fertilizer requirements, and they made the following 

 observations from their results: 



1. The lack of one of the three fertilizer constituents [nitrogen, phosphorus, or 

 potassium] increases the absorption of the other two above the amounts ordi- 

 narily absorbed from a medium containing a balanced supply of plant nutrients. 



2. In the presence of an incomplete fertilizer the plant fails to absorb from the 

 soil the missing constituent in amounts as great as that from an unfertilized plot. 



The work of Lagatu and Maume has been supported by that of 

 Thomas {557 , 558, 559, 560), using the potato vine grown in a Penn- 

 sylvania soil. He showed that the two observations of Lagatu and 

 Maume were "contrary to Liebig's 'law of the minimum,'" and that 

 whatever effect the absence of an element from a fertilizer has on the 

 yield, "it is not due to a depression of absorption of the other elements, 

 but, on the contrary, to a nutritional lack of balance due to increased 

 absorption of the remaining elements." 



The results obtained by Thomas on leaves of potato vines are pre- 

 sented graphically in figure 2, and his summary of the indications 

 given by the graphs follows: 



Nitrogen graphs. — The content of nitrogen in the leaves of plants which re- 

 ceived mineral nitrogen applications is higher throughout the whole period than 

 in those which received none. 



Phosphoric acid graphs. — The content of phosphoric acid in the foliage of plants 

 which received phosphate applications is higher throughout the whole period 

 than in those which received none. 



Potash graphs. — The content of potash in the foliage of plants which received 

 mineral potash applications is very much higher than in those which did not. 



In his concept of the nutrition of the plant, Thomas makes use of 

 two terms, viz, quantity and quality of the mineral constituents. 

 "The concept of quantity or intensity of nutrition of the selected 

 leaves consists of the sum (N-f P 2 5 -hK 2 0) of each element at the 

 moment of sampling, expressed as a percentage of the dried material. 

 The quality of nutrition is the ratio of these entities to each other at 

 the moment of sampling." The quantity of nutrition may be ex- 

 pressed as follows: 



S=X+Y+Z, 



where X is the percentage of N, Y the percentage of P 2 5 , and Z the 

 percentage of K 2 0. The quality of nutrition may then be expressed as 



This expression, according to Thomas, gives "the physiological ratios 

 between N, P 2 5 , and K 2 0." When this proportion of elements is 



