614 SOILS: PBOPJSBTIJES AND MANAGEMENT 



capacity of the soil. In clays it promotes granulation, 

 while in sands it acts as a binding agent. Under all con- 

 ditions it promotes granulation and tilth. The capacity 

 of a soil to resist drought is raised; its aeration is in- 

 creased and drainage is promoted. All these changes 

 tend to benefit plant growth and to produce those indirect 

 fertilizing effects that are characteristic of farm manure. 



From the chemical standpoint, the presence of manure 

 in the soil tends to increase organic acids, notably car- 

 bonic acid. The soil minerals are thus rendered more 

 easily soluble. The case of the influence of manure on 

 the action of raw rock in the soil has already been cited. 

 The humus, also, may combine with certain of the mineral 

 elements and hold them in a form more easily available 

 to crops. Nor is the chemical influence of farm manure 

 the final efl^ect. The modification of the soil flora can 

 by no means be passed by. Not only are millions of 

 organisms added by an application of manure, but those 

 already present in the soil are vastly stimulated by this 

 fresh acquisition of humic materials. Nitrification, am- 

 monification, and nitrogen fixation are all increased to a 

 remarkable degree. 



519. Residual effect of manure. — No other fertilizing 

 material exerts such a marked residual effect as does 

 manure. This is partly because of its indirect physical 

 and biological influences, and partly because of the stimu- 

 lated root development of the crops grown. The greatest 

 residual influence, however, is brought about by the slowly 

 decomposable nature of the manure, only a small per- 

 centage being recovered in the first crop grown after the 

 manure is applied. Hall ^ presents the following data 



^ Hall, A. D. Fertilizers and Manure, p. 210. New York, 

 1910. 



