VI.] 



AVAILABLE PLANT FOOD 



163 



the soil. The unmanured plot and that which has 

 received dung do not show the same regular decrement, 

 indicating that the solvent is each time dealing with a 

 more complex mixture of phosphates successively going 

 into solution. This conclusion is strengthened when the 

 total amount of phosphoric acid dissolved in five extrac- 

 tions is compared with the amount known to have been 

 applied to the land during the period the plots have 

 been under experiment, after deduction has been made 

 of that which is also known to have been removed in the 



crop. 



Phosphoric Acid in Rothamsted Soils. 



* Approximate estimate, since the crop has rarely been analysed. 



It will be seen from the above table that the amount 

 of phosphoric acid dissolved by the five extractions 

 agrees closely with the surplus left by the manuring in 

 all the cases where the phosphoric acid has been put on 

 as soluble mineral superphosphate. This is not the case, 

 however, for the plot manured with dung, which contains 

 a considerable proportion of difficultly soluble phosphate. 



It should not be supposed that the whole of the 

 so-called " available " phosphoric acid or potash will be 



