THE RELATION OF SOILS TO IRRIGATION. 25 



art. The term loam is a most indefinite chara(5leriza- 

 tion on account of the various constituents which it 

 contains. For instance, a heavy clay loam has but 

 from ten to twenty-five per cent, of sand; a clay loam 

 is twenty-five to forty per cent, of sand, and the sandy 

 loam is from sixty to seventy-five per cent, of sand, 

 while the light sandy contains from seventy-five to 

 ninety per cent. 



It has been demonstrated by practical experiments 

 that one hundred pounds of sand will absorb twenty- 

 five pounds of water ; one hundred pounds of loam, 

 forty pounds; one hundred pounds of clay loam, fifty 

 pounds; one hundred pounds of clay, seventy pounds. 

 This explains why some soils always appear drier than 

 others, why some soils will stand a drouth so much 

 longer than others, and why, after an irrigation, some 

 soils become like a thick paste while others are dry. 

 Sandy soils usually break up loose and mellow when 

 dug, forked, or worked in any way; black land is stiff, 

 breaks up in hard clods when worked either too wet 

 or too dry, and requires more cultivation both before 

 and after plants are put in it than does sandy soil. 



Humus. — The humus is the organic portion of 

 the soil, resulting from decayed vegetable matter. It 

 is of a dark brown or black color, the blacker the 

 better. A good example is well-rotted leaf mold. The 

 chief constituent of humus is carbon, but it contains 

 all the other compounds found in plants, and by its 

 gradual decay these all become available as plant food 

 in the most desirable form. Humus is the chief source 

 of nitrogen in the soil. A black soil rich in humus is 

 sure to be rich in nitrogen. The remarkable fertility 



