22 IRRIGATION FARMING. 



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 re- 

 quires more cultivation both before and after plants are 

 put in them 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 of vir- 

 gin soils is largely due to the nitrogenous humus which 

 they contain. Of all soil constituents, humus has the 

 greatest power to absorb and retain moisture, and to draw 

 moisture from the subsoil by capillary attraction, and 

 it is in this power that is manifested its valuable util- 

 ity immediately on the application of irrigating waters. 

 It also possesses in a high degree the power to absorb 

 ammonia from the air, and by its dark color it adds 

 warmth to the soil during the day, while by cooling 

 quickly at night it assists in causing dew to be deposited 

 upon the soil which contains it. Humus also improves 

 the texture of the soils, by making clay soil more friable 

 and sandy soil more" compact and retentive. The 

 amount of humus in fertile soils is quite variable, but 

 usually runs from three to seven or eight per cent. 



The Acids. In all soils we find two essential 

 acids, known scientifically as liumic and ulmic. The 

 first is the acid in the humus, or vegetable and animal 



