94: ELEMENTS OF AGRICULTURE 



CHAPTER XVIIL— Nitrogen in Soils 



94. Importance of Nitrogen in Soils. — -We have 

 already mentioned the fact that nitrogen is one of the 

 most important of all plant foods. It is important 

 because no plant can grow without a supply of this 

 element, and, furthermore, because the supply in the 

 soil is easily exhausted through careless cultivation, 

 and, when once exhausted, is with difficulty replaced. 

 It seems strange that plants should ever want for 

 nitrogen when they are surrounded by air which is four- 

 fifths nitrogen. But there are only a few kinds of 

 plants that can make any use of the nitrogen of the 

 air, and even for them the nitrogen must first enter the 

 soil and there be changed into the proper compounds. 

 All of the plant's nitrogen supply is taken in through 

 its roots. 



95. Sources of the Soil's Supply of Nitrogen. — The 

 soil's supply of nitrogen comes from three sources. 

 First, and most important, is the nitrogen combined 

 in the organic matter in the soil; second, the nitro- 

 gen contained in the air which fills many of the spaces 

 between the soil particles; third, the compounds of 

 nitrogen, ammonia and nitrates, which are washed 

 from the air by rain water. The amount of nitrogen 

 brought to the soil by rain water is insignificant com- 

 pared with the amount supplied by organic matter. 

 The nitrogen held by the air in the soil is useful to 

 only certain kinds of plants, and by far the most im- 



