EXPERIMENTS WITH PHOSPHORIC ACID. 23 



same manner, and so on to the end. Thus it will be seen that 

 for each kind of plant there were three boxes which received the 

 same treatment. 



Such quantities of the phosphates were used that each box to 

 which they were applied received the same total amount of 

 phosphoric acid. To each box were also added all the ingre- 

 dients that a healthy plant takes from the soil. These, together 

 with the phosphates, were carefully mixed with the sand before 

 it was placed in the boxes. All the conditions were made as 

 uniform as possible in order that whatever differences were 

 observable might fairly be attributed to the differences in the 

 phosphates used. 



When the plants were harvested they were carefully dried, 

 weighed, and the total amount of dry matter determined for 

 each kind of plant grown. The experiments were continued 

 through three periods, the third period being made much 

 shorter than the others. 



In the diagram on the opposite page, the length of the black 

 lines shows the relative amounts of dry matter produced, while 

 the figures at the right show the actual weights, expressed in 

 grams., 



In these experiments the effect of the acid rock was very 

 marked with all the plants grown; those receiving it, in nearly 

 all cases, at once taking the lead and keeping it to the end. The 

 plants were darker green in color, and the tubercles, which were 

 developed on the roots of nearly all the leguminous plants, 

 were larger and much more numerous. It was noticeable, 

 however, that in some cases, especially with the clover, turnips 

 and ruta-bagas, the good effects of the acid rock were more 

 marked during the first few weeks of growth than at a later 

 stage, when the roots had become more fully developed and 

 had begun to forage for themselves. It would appear that the 

 young plants feed but little upon the insoluble phosphates; but 

 that the organic acids present in the sap of the roots exert a 

 solvent action upon the insoluble phosphates in the soil, grad- 

 ually converting them into available forms. 



It will be noticed that in this work only the immediate effect 

 of the phosphates has been taken into consideration, no mention 

 having been made of the unused phosphoric acid remaining 



