44 BULLETIN" 355, U. S. DEPARTMENT OF AGRICULTURE. 
abnormal decomposition takes place, through bacterial action, result- 
ing in a loss of free nitrogen from the soil. Such an abnormal de- 
composition is called denitrification. An acid condition of the soil 
is unfavorable, also, to nitrification. The farmer should recognize 
that suitable conditions must be provided for normal bacterial growth 
in the soil if good crops are to be expected. 
Commercial materials containing nitrogen (Ref. No. 1, pp. 190-196; 
or No. 7, pp. 244-260). — Besides the natural methods discussed 
above of keeping up the nitrogen supply of the soil, there are many 
commercial products on the market which are used to a considerable 
extent for certain soils and crops. These include mineral salts, 
together with waste products of both animal and vegetable origin. 
The principal mineral salts of nitrogen used on soils are sodium 
nitrate and ammonium sulphate, together with calcium cyanamid 
and calcium nitrate, which have recently been manufactured from 
atmospheric nitrogen. 
Cottonseed meal is the principal organic source of nitrogen used as a 
fertilizer in the United States. In fact, it is used to a larger extent 
in this country than any other kind of nitrogenous fertilizer, notwith- 
standing the fact that it is also a valuable stock food for which the 
demand is steadily increasing. 
The commercial animal products used as fertilizer include slaughter- 
house refuse, especially dried blood and tankage; fish not valuable for 
human food, which has been prepared for use as fertilizer by cooking 
and extraction of oil; bird guanos; and stockyard manure. Peruvian 
guano formed from the excrement of birds deposited in large quan- 
tities on islands off the coast of Peru is rich in nitrogen and was once 
extensively used in this country, but the original deposits are now so 
nearly exhausted that there is little or none of the material avail- 
able for export. 
Of the mineral nitrogen salts, ammonium sulphate is used to a con- 
siderable extent in this country. Its long-continued use has been 
found to produce unfavorable soil conditions, which, however, are easily 
corrected by applications of lime. A mineral material largely used 
to supply nitrogen to soils is sodium nitrate or Chile saltpeter, so- 
called because it is obtained mainly from the nitrate deposits of Chile. 
The great advantage of sodium nitrate as a plant food is that it is 
readily soluble in water and quickly becomes available to growing 
crops. When applied to a poor soil its effect can usually be quickly 
seen in the rapid growth and the rich green color which the plants 
take on. A disadvantage in the use of this material is its tendency 
to leach from the soil, as noted in a previous lesson. Sodium nitrate, 
and other products rich in nitrogen as well, must be applied to the 
soil with much knowledge and skill if they are to prove profitable. 
Usually from 100 to 200 pounds per acre of the nitrate is used, and 
