ORGANIC COMPOUNDS OF SOILS 95 



as plant food, of this product from partially decayed 

 animal and vegetable matter. Humus was formerly 

 regarded as composed only of carbon, hydrogen, and 

 oxygen, and inasmuch as plants obtain these elements 

 from water and from the carbon dioxide of the air, no 

 value was assigned to humus. Later investigators 

 added nitrogen to the list but stated that the nitrogen, 

 when combined with the humus and before under- 

 going fermentation, was of no value as plant food. 



Recent investigations have proved that the phos- 

 phoric acid and other mineral elements combined 

 with the organic matter of soils are of value as plant 

 food, 16 and it has been demonstrated that crops grown 

 on the black soils of Russia obtain a large part of 

 their mineral food from organic combinations. 83 Cul- 

 ture experiments have shown that under normal 

 conditions plants like oats and rye may obtain their 

 mineral food entirely from humate sources. Seeds 

 when planted in a mixture of pure sand and neu- 

 tral humates from fertile soils, produced normal 

 plants. In order to secure the best conditions 

 for growth, a little lime must be present to prevent 

 the formation of humic acid, and the usual organisms 

 found in fertile fields must also be introduced. The 

 following example is given of oats grown under such 

 conditions : 



