SOIL FERTILITY 



pan), and the weight of it and the soil together. 

 Then, over a low flame so as to avoid scorching 

 or burning the earth, drive off the moisture it 

 contains, weighing from time to time until a 

 constant weight is obtained. Thus you will find 

 the weight of water and by ratio its percentage in 

 the soil. (2) Find the percentage of humus by 

 heating for twenty minutes, sufficiently to burn 

 out all organic matter as proved by again ob- 

 taining a constant weight. The difference be- 

 tween the constant weight of (i) and of (2) will 

 give the weight of humus and its percentage. 

 Then (3) test for the gravel and sands by sifting. 

 The United States government uses a set of brass 

 sieves. Homemade ones will answer, such as 

 boxes with their bottoms replaced by fine wire 

 gauze and by bolting cloth. Gravel will not pass 

 through a wire mesh of less than two millimeters 

 diameter. Coarse sand will not go through one 

 less than one millimeter; medium sand, through 

 one less than one-half millimeter; and fine sand, 

 through one less than one-fourth, while very fine 

 sand will not pass through a sieve of one-twentieth 

 of a millimeter mesh or less. For separating the 

 last two, bolting cloth of a mesh known as No. 

 5 and 13 may be used. The silt and clay are 

 mingled and will be separated by the fourth step. 

 (4) The silt and clay are carefully weighed, then 

 shaken with a quantity of water and boiled for ten 

 minutes or longer. Then they are allowed to 

 stand until the heavy silt sinks to the bottom and 



93 



