36 



PRACTICE 17. 

 PREPARATION OF POT CULTURES. 



Use clean, white sifted sand in 5-liter heavy glass battery jars, having a 

 I cm. hole within I cm. of the bottom. Into the hole fit a drain tube made 

 of glass-tubing with a glass-wool filter at the inner end, so that it will 

 take liquid from the lowest place in the jar. Put up a series of ten of 

 these pots, eight to be used as indicated in the table below and two in an 

 experiment to be devised by the student. The previous treatment of the 

 sand in the latter two will depend upon the experiment to be made. 



To extract the sand, fill the jar within I cm. of the top with dry sifted 

 sand and add to this dilute sulfuric acid (made by adding 100 cc. of con- 

 centrated chemically pure sulfuric acid to 900 cc. of ammonia-free water) 

 until the sand is saturated. Let stand two hours and then add ammonia- 

 free water, allowing the drainage to flow into a second jar until it is sat- 

 urated. Allow this jar to stand two hours and then wash both with 

 ammonia-free water until free of acid. In this way any soluble plant 

 food is removed from the sand and one portion of acid extracts two jars. 

 The sand for two of the jars, in which experiments are to be made to 

 show the effect of nitrogen-gathering bacteria, is first heated to 120 to 

 130 for one-half hour and then extracted and washed as above. 



Before planting, mix with the sand of each pot 10 gm. of pure calcium 

 carbonate. 



In making applications of plant food as indicated in the following table 

 and in such amounts as are shown in Practice 16 the solutions to be ap- 

 plied to each pot are to be mixed together, and diluted to 1000 cc. Mix 

 thoroughly and apply the whole amount to the pot, allowing any water 

 present to be forced out through the drain. 



The first application of the plant food solutions is to be made at the 

 time of planting, the second three weeks later, the third two weeks later, 

 and subsequent applications at intervals of one week, each time making 

 the application as directed above. 



Each student in each group will prepare and care for a series of pots 

 as indicated in the following table . 



