26 IRRIGATION FARMING. 



of virgin 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 

 attradlion, and it is in this power that is manifested 

 its valuable utility 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 mak- 

 ing clay soil more friable and sandy soil more compadl 

 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 humic and ulmic. The 

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

 matter in the soil. As animal life is built by vegetable 

 matter, it must eventually turn back to vegetable mat- 

 ter. Ulmic acids are those that exude from the roots 

 of some plants. We should remember that nitrogen is 

 the costHest of all plant foods and the most difficult 

 to retain in the soil, and plants must have it, for it 

 corredls this humic acid in the plant as well as in 'the 

 soil. The ulmic acids are seldom in sufficient quan- 

 tity to do harm. But the humic acids when shut off 

 from the proportions of nitrogen or potash — both 

 alkalies— become too concentrated, or the dead microbes 

 become poisonous to plant life, as the great French 

 chemist Pasteur would have it. Now humic acid has 



