HOW NITROGEN IS HELD IN SOILS 33 



The original source of all nitrogen is the atmosphere. Nitrogen is 

 the important element in ammonia and in all nitrates. In the 

 soil it is found mainly as compounds in organic matter. 



Hydrogen in its pure form is a colorless, odorless and tasteless 

 gas. It is the lightest of all known substances. It is the most 

 widely distributed element in the universe, being found in the sun 

 and the stars. On the earth it is found mostly combined with 

 other elements. It is an essential constituent of water. When 

 hydrogen gas burns, the resulting compound is water hence the 

 name hydrogen (water generator). Hydrogen is an essential part 

 of all plants and animals. 



The Mineral Elements. Silicon, aluminum, iron, calcium, 

 magnesium, sodium, potassium, phosphorus and sulfur are fre- 

 quently called the mineral elements (see table). 



The Elements Crops Take From Soils. The elements that 

 crops take from the soil components and which are absolutely 

 necessary for plant growth are iron, calcium, magnesium, potas- 

 sium, phosphorus, sulfur and nitrogen. All of these except nitrogen 

 are the essential mineral elements required by all crops. 



The elements most commonly deficient in soils and which 

 affect crop yields the most are nitrogen (N), phosphorus (P), and 

 potassium (K). These three are, therefore, commonly regarded as 

 the fertilizing elements. With these might be included calcium (Ca). 

 Some regard sulfur fully as important as phosphorus in crop pro- 

 duction. Conclusive evidence, however, is lacking. Some tests 

 seem to show that certain soils require sulfur to aid in obtaining 

 larger crop yields. 



Supply of Important Elements not Large. The total amount 

 of these four elements in a sand is more often less than 2 per cent; 

 in a productive silt loam often less than 5 per cent. The soil 

 supply of nitrogen, phosphorus, potassium and calcium, therefore, 

 is not large. 



Of the essential elements, crops require the least iron, yet 

 soils are rich in iron. Generally speaking, more than 85 per cent 

 of soils consists of oxygen, silicon and aluminum. This oxygen 

 exists in the soil in the form of insoluble compounds and is not used 

 by plants. Both silicon and aluminum are non-essential to 

 plant growth. 



How Nitrogen is Held in Soils. Nitrogen in free form is a gas, 

 .a component of air. The nitrogen which makes up a part of the 

 composition of soils is held in soils largely as organic matter. 

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