CHAPTER XV. 



INFLUENCE OF MANURE ON THE BACTERIAL 

 ACTIVITIES OF THE SOIL. 



THE application of barnyard manure to a soil brings about a 

 far-reaching change within the soil. It has been found that, on the 

 average, one ton of barnyard manure contains 10 or 12 pounds each 

 of nitrogen and potassium and 2 or 3 pounds of phosphorus. It also 

 carries other substances of less importance which may be directly 

 utilized by the growing plant or which may react with substances 

 within the soil, changing their solubility. This direct and indirect 

 nutritive value of a manure is not its only function, for it greatly 

 changes the physical structure of the soil. It improves the tilth of 

 a clay soil by increasing the granulation within it, while in a sandy 

 soil it tends to bind the particles together, making it Jess porous. 

 Each of these changes react upon the water-holding capacity and 

 the capillarity of the soil, greatly altering the aeration of the soil 

 and with the aeration the temperature. 



The biological changes which the manure produces in the soil, 

 especially when small quantities are added, may be even more far- 

 reaching than either the chemical or physical changes which it 

 produces. Every pound of manure carries with it to the soil millions 

 of bacteria. Many of these will find the new conditions unsuited for 

 their growth, but some will continue to multiply, and in so doing 

 not only will decompose the constituents of the manure but also 

 will greatly alter other organic and inorganic substances of the soil. 

 The bacterial content of the soil is, therefore, changed both quanti- 

 tatively and qualitatively. There are added with the manure many 

 new species; the changed physical and chemical conditions of the 

 soil due to the manure will greatly modify those already present, 

 for the microflora and microfauna originally present in the soil were 

 due to specific soil properties. 



This changed flora and fauna will in turn change the chemical 

 and physical properties of the soil still more. Acids are generated, 

 which react with insoluble constituents, rendering them soluble. 

 Gases are formed, which change the air within the soil; in these 

 reactions heat is generated, thus changing the temperature of the 

 soil. The metabolism of the bacterial cell requires nutritive sub- 

 stances, among which are water and the elements essential to plant 

 growth. Some soluble constituents will be changed to insoluble and 



