The Composition and Use of Fertilizers. 217 



in light sandy soils. The number is greater in summer than in 

 winter. The addition of organic matter like stable manure 

 greatly increases the number of germs. As a rule, an increase 

 in the amount of available plant food in the soil follows an in- 

 crease in the number of micro-organisms or bacteria. Such in- 

 crease can be secured by thorough cultivation, liberal manuring, 

 and the use of leguminous plants as green manures. 



Conditions Required for Nitrification. — The activity of the 

 micro-organisms which prepare unavailable forms of nitrogen for 

 the use of plants by converting them into nitrate nitrogen is 

 hastened by certain favorable conditions and retarded by 

 unfavorable ones. The principal conditions that have to be 

 considered in relation to the growth of these micro-organisms will 

 now be mentioned. 



(a) The Presence of Moisture. — In a dry soil nitrification 

 does not take place. Hence, in periods of drouth this important 

 process stops in the upper portions of the soil. In a wet soil, on 

 the other hand, where water is stagnant, this process does not take 

 place actively. 



(b) Food Requirements of Nitrifying Organisms. — The or- 

 ganic matter of the soil forms the main supply of organic food for 

 these organisms. It is possible to have so much organic matter 

 present that the organisms will stop growing, as in a manure pile. 

 It is also necessary that certain forms of mineral matter be pres- 

 ent, chief among which is a supply of phosphoric acid compounds. 



(c) Oxygen Required. — Nitrifying organisms cannot nourish 

 without a supply of air. Water-saturated soils prevent the pro- 

 cess by shutting off the air supply. The stirring and pulverizing 

 of the soil by cultivation greatly promotes the change into nitrate 

 nitrogen, because it brings the oxygen of the air into closer con- 

 tact with the nitrifying ferments. 



(d) Temperature of Soil. — The nitrifying organisms work 

 most rapidly between the temperatures of 54 degrees and 99 de- 

 grees Fahrenheit. The process may take place as low as 37 de- 

 grees Fahrenheit and as high as 120 degrees Fahrenheit. In this 

 latitude nitrification stops during the winter, while in warm cli- 

 mates it continues during the whole year. 



(e) Some Material Like Lime Necessary. — The presence of 

 some mineral compound in the soil that will prevent acid condition 

 of soil is necessary, since the action of these micro-organisms is 

 checked by acids. The lime compounds are the most available 



