6 CIRCULAR 436, U. S. DEPARTMENT OF AGRICULTURE 



by plants, some plants doing so to a much greater extent than others, 

 without first undergoing change to the nitrate form. This appears 

 to be true also of certain organic nitrogen compounds, although less 

 pronounced than with the ammoniacal form. 



PROPERTIES OF SODIUM NITRATE IN RELATION TO THE SOIL 



Sodium nitrate is readily soluble in water and is one of the most 

 quickly available sources of nitrogen for plants on account of the ease 

 with which it dissolves in the soil moisture and its direct utilization 

 by plants without having to be changed by the intervention of soil 

 bacteria. Owing to its ready solubility, sodium nitrate, in common 

 with other nitrates, possesses a high degree of diffusibility, which 

 enables it to move rapidily in the soil in accordance with either upward 

 or downward movements of soil moisture. 



The ready solubility and diffusibility of nitrates enable them to 

 pass easily through the soil after a rainfall. This is commonly 

 referred to as leaching. The leaching of nitrates is much more serious 

 on light sandy soils with pervious subsoils than on heavy, less open 

 soils; also it is much greater from bare land than from land occupied 

 by crops. The effect of the crop is to utilize the nitrates. The plants 

 decompose and the absorbed nitrogen is returned to the soil. Th's 

 method is a good way to avoid unnecessary losses of valuable nitrogen 

 compounds. Capillarity, unlike ordinary leaching, is instrumental 

 in concentrating the nitrates more heavily in the upper layer of soil, 

 until either removed by the growing crop or again leached downward 

 by rainfall. 



Sodium nitrate has been found to have a deflocculating effect on 

 heavy soils possessing a high content of fine material — principally clay 

 and fine silt particles — when applied to such soils continuously. 

 This is ascribed to the ultimate formation of sodium carbonate in the 

 soil, a compound which tends to break up the so-called "crumb 

 structure" of a soil and thus leads to poor mechanical condition, as 

 evidenced by increased stickiness and greater difficulty in plowing 

 and in cultivating. While this result may follow the use of sodium 

 nitrate under the indicated soil conditions, it rarely occurs in the 

 United States because sodium nitrate is not applied in large enough 

 amounts to seriously affect the average heavy soil, and when used in 

 complete fertilizer it is usually combined with materials which tend 

 to counteract such a reaction. 



Soils are either acid, neutral, or alkaline in reaction, conditions 

 brought about by various influences including applied fertilizer salts, 

 such as sodium nitrate, ammonium sulphate, and other inorganic 

 nitrogen compounds. The ultimate effect of sodium nitrate when 

 applied to the soil is to make it less acid if already possessing an acid 

 reaction, and more alkaline if the soil reaction is alkaline. If the soil 

 reaction is neutral the addition of sodium nitrate would tend to change 

 the reaction to alkaline. While the quantity of sodium nitrate 

 applied, usually not over 200 pounds to the acre, will decrease soil 

 acidity or even render soils alkaline, it would be uneconomical to 

 attempt to lower soil acidity with sodium nitrate. When the control 

 of soil acidity becomes necessary the most economical method of 

 accomplishing this would be to apply some form of lime. 



