ABSORPTION OF MINERAL ELEMENTS BY PLANTS 259 



gators in agricultural experiment stations — Hellriegel, Neubauer, 

 and Wagner in Germany; Kossowitch and Prianishnikov in 

 Russia. The principle of this method is the cultivation of plants 

 on natural soils in special glass, clay, or metal pots holding from 

 5 to 100 kg. or more. The environmental conditions are kept 

 under control as far as possible. The soil moisture is held con- 

 stant by means of daily watering of the pots to a constant weight. 

 The pots are filled with equal amounts of carefully mixed soil 



Fig. 77. — Greenhouse for pot experiments at the Institute of Apphed Botany in 



Detskoje Selo, near Leningrad. 



and are placed on rolhng platforms, which may be pushed out- 

 doors during good weather and back into a special greenhouse 

 to avoid wind, dust, and rain (Fig. 77). 



Soils are commonly tested in the following way. A certain 

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

 the necessary ingredients in which a soil may be deficient, 

 usually N, K, and P, since Ca, Mg, and S are present in sufficient 

 amounts in almost every soil. An equal number of containers 

 remain unfertilized. These two series serve as controls, the first 

 showing the maximum yield obtainable under the conditions of 

 the experiment, the second, the minimum jdeld. The rest of 



