54 SOILS AND FERTILIZERS 



one, and as warmth is an important factor in plant growth, 

 especially in the spring, a dark soil usually has an advantage 

 over a light-colored one. 



55. Organic matter a carrier of plant-food material. — 

 In its relation to the supply of plant-food material, organic 

 matter is the storehouse in which nitrogen is held in a form 

 in which it cannot be leached from the soil in large amounts 

 and yet from which it gradually becomes available to plants. 

 Certain inorganic plant nutrients are likewise held in such 

 condition that they readily become useful to plants. In the 

 process of rotting, combinations are formed between organic 

 matter and certain inorganic plant nutrients, removing the 

 latter from the very insoluble minerals of the soil. On 

 further decomposition the inorganic substances are left in a 

 form readily usable by plants. 



56. Organic matter and nitrogen. — The relation of 

 organic matter to the nitrogen supply is of particular inter- 

 est because it is as organic matter that practically the entire 

 supply of nitrogen enters the soil. All soil nitrogen has been 

 secured from the air and the process is still going on. This 

 is done largely by the lower forms of plant life known as 

 bacteria, fungi and molds. These organisms living in the 

 soil, or in the roots of higher plants, feed on the non-nitrog- 

 enous organic matter of the soil and plants, and upon the 

 nitrogen of the atmosphere that passes into the pores of the 

 soil. The non-nitrogenous organic matter and the atmos- 

 pheric nitrogen are thus combined to form the tissues of 

 these lower plants, which soon die and finally add to the 

 soil the nitrogen they have accumulated. 



57. Organic matter and soil microorganisms. — We have 

 just seen how the nitrogen-fixing organisms use non-nitrog- 

 enous organic matter in their growth. They use it as a 

 source of energy, as do animals. Many other forms of lower 

 plant life use organic matter, both nitrogenous and non- 



