THE ABSORPTION OF MINERAL ELEMENTS 



81 



as an indicator of soil fertility. The method of pot cultures is 

 valuable in this respect. It has been worked out by investigators 

 in agricultural experiment stations (Hellriegel and Wagner in Ger- 

 many; Kossowitch, Prianishnikov and Doiarenko in Russia). 



The principle of this method is the cultivation of plants on 

 natural soils in special clay or metal pots holding from 5 to 100 kg., 

 or even more. The environmental conditions are kept under con- 

 trol as far as possible. The soil moisture is held constant by means 



Fig. 33. — Greenhouse for pot experiments at the Institute of Applied Botany in 



Detskoje Selo, near Leningrad. 



of daily watering of the pots to a constant weight. The pots are 

 rilled with equal amounts of carefully mixed soil and are placed on 

 rolling platforms, which may be pushed outdoors during good 

 weather and back into a special greenhouse during wind and rain 

 (Fig. 33). 



Soils are commonly tested in the following way: A certain 

 number of containers, usually two to four, are fertilized with all the 

 necessary ingredients that may be deficient in a soil, usually N, K, 

 and P, since Ca, Mg, and S are present in almost every soil in 

 sufficient amounts. An equal number of containers remain 



