248 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



planted should be of uniform size and vary in moderate limits in 

 weight, such as 41 to 47 mg. each for wheat grains. The seed 

 are often germinated, and the seedlings planted. During the 

 period of growth, the pots are usually weighed at suitable inter- 

 vals, and sufficient water added to keep the moisture content uni- 

 form. The pots are kept in glass houses, in canvas houses, in 

 wire houses, or in the open air on trucks which can be run into 

 a glass house for protection against storms. The pots are also 

 buried in the ground sometimes, but the water content of the soil 

 is much less easily regulated under such conditions. Examples 

 of pot experiments have been given in the text. 



The following is the method of procedure used at the Texas 

 Experiment Station : l 



Washed gravel is added in sufficient amounts to an 8-inch 

 Wagner pot to make the total weight 2 kilograms. Five kilograms 

 of soil are then added. The soil is previously pulverized in a 

 wooden box with a wooden mallet until it will pass a 3 mm. sieve, 

 the gravel being removed. 



The addition of fertilizer consists of 2^ grams of acid phos- 

 phate, and 10 cc. of solution containing I gram nitrate of soda, 

 and T gram sulphate of potash. In later experiments I gram of 

 ammonium nitrate was used in place of nitrate of soda. If the 

 size of the crop appears to render it necessary, more nitrate of 

 soda or sulphate of potash is added to the pot, the solution being 

 diluted with about 200 cc. of water. 



The seed are weighed out so that each pot receives the same 

 amount of seed within o.i of a gram. Water is added to one- 

 half the saturation capacity of the soil. If this quantity is found 

 to be too great, it is afterwards reduced, but this is the case in 

 only a few instances. The pots are weighed, placed on scales 

 three times a week, and water added to restore the loss in weight. 

 If the plants need water between weighings, such quantity is 

 added as appears necessary. The object of the weighing is to 

 maintain as closely as possible a constant amount of water in the 

 soil. These pots are kept in a house with glass roof, and canvas 

 1 Bulletin 126. 



