FORMS OF PLANT FOOD 79 



When extended series of soil analyses have been made, 

 much valuable information has been obtained. 



Suppose a soil contains 0.40 per cent, of acid-solnble 

 potash and field experiments indicate that there is a 

 deficiency of available potash. This may be due to 

 some abnormal condition of the soil, as an insufficient 

 amount of other alkaline compounds as calcium car- 

 bonate to take the place of the potash which has been 

 withdrawn by the crop, in which case the deficiency 

 of potash could be remedied without purchasing solu- 

 ble potash fertilizer, to become insoluble by fixation 

 processes. If a soil contains only 0.04 per cent, of 

 acid-soluble potash, the purchasing of potash fertili- 

 zers would be more necessary, but with 0.40 per cent, 

 of acid-soluble potash the way is open to render 

 this potash available for crops. The various ways of 

 rendering acid-insoluble potash and other compounds 

 available for crop production, as by rotation of crops, 

 use of farm manures, use of lime and green manures, 

 or by different methods of cultivation have not been 

 sufficiently studied as yet to offer a solution to all of 

 the problems of how to render inert plant food availa- 

 ble. 



90. Action of Organic Acids upon Soils. — Dilute 

 organic acids, as a one per cent, solution of citric acid, 

 have been proposed as solvents for the determination 

 of easily available plant food. It has been shown in 

 the case of the Rothamsted soils which have produced 



