298 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



upon the rapidity and extent with which they become changed to 

 nitrates and ammonia in the soil. It is important to know the 

 relative values of these materials. The measure of their value is 

 the quantity of nitrogen which may be secured from them by 

 plants under favorable conditions. This is termed availability. 



Availability is based upon value to plants in pot experiments. 

 For the purpose of comparing nitrogenous materials, a soil 

 decidedly deficient in nitrogen is selected, mixed thoroughly, and 

 an equal quantity placed in a number of pots. Each pot then 

 receives an equal and abundant amount of phosphoric acid, 

 potash, and lime if necessary. One set of pots (two or more) re- 

 ceives no nitrogen. The others receive an equal quantity of nitro- 

 gen, say 0.3 gram for example, in the form of nitrate of soda, sul- 

 phate of ammonia, cottonseed meal, or other substances to be 

 tested. An equal number of seeds of equal weight are planted in 

 eacn pot, and the crops are grown under the same conditions as 

 regards moisture, air, light, etc. They are then harvested, and 

 the quantity of nitrogen secured from each pot determined. This 

 nitrogen comes from both soil and fertilizer. The quantity of 

 nitrogen secured from the pots to which no fertilizer nitrogen 

 has been added is subtracted from the others to ascertain how 

 much nitrogen was secured from the fertilizer. The amount of 

 nitrogen taken from one of the materials (usually sodium nitrate) 

 is adopted as a standard, (equal to 100) and the results expressed 

 in terms of this. For example, if 0.250 gram nitrogen was secured 

 from sodium nitrate by the plants and 0.180 gram from cottonseed 

 meal, the availability of the nitrogen of cottonseed meal would 

 be 0.250 : 180 : : 100 : x, or equal to 72. 



Considerable care is required in the conduct and planning of 

 experiments of this kind. Two or more pots must be used for 

 each material. Nitrogen must be the controlling factor in the 

 growth of the crop, and, in order to be certain such is the case, it 

 is best to have several sets of pots with different amounts of nitro- 

 gen, such as 0.3, 0.6, 1.2 grams per pot for example. If nitrogen is 

 the limiting condition, as it should be, the amount of nitrogen 

 taken up by the plants will be in proportion to the quantity 



