312 Manures, Composts, Green Manures 



nures offer a readily available supply of nitrogen, phosphoric acid, 

 and potash for growth of higher plants. (2) In the decomposition of 

 the carbon complexes in the manures by microorganisms, consider- 

 able carbon dioxide is liberated, which is essential for plant growth. 

 (3) The organic matter of the manures replenishes the supply of soil 

 humus. (4) Manures exert an important influence upon the micro- 

 biological activities in soil. (5) Because of their bacterial content, 

 stable manures are important for their inoculating properties. (6) 

 Manures influence the colloidal properties of soil and the state of 

 its aggregation. 



The phosphorus and potassium in stable manures are present 

 largely in forms readily available to higher plants, as are inorganic 

 salts. The nitrogen in the manure is only about one-third to one- 

 half as available as that of inorganic fertilizer; this has been brought 

 out in numerous field tests as well as by nitrification experiments. 

 There is little justification, therefore, for comparing the availability 

 of the nitrogen in organic manures and in inorganic fertilizers, with- 

 out considering the cumulative effects of the manure on the fertility 

 of the soil and especially on its physical condition; this error has been 

 frequently made in fertilizer trials, where only given crop yields for 

 one year have been measured. Stable manures were found to give 

 particularly significant results, as compared with inorganic fertiliz- 

 ers, in dry years and on light soils (Table 69). 



Table 69. Comparative Cumulative Effects of Manure and 

 Chemical Fertilizers (from Salter and Scliolleiiborger) 



Ohio Agricultural Experiment Station, Wooster, Ohio; .5-year rotation 

 fertility experiment 



Averages for limed and unlimed soils. 



