SUMMARY 271 



of commercial fertilizers lasts but a few years at most and 

 usually but one or two. 



208. Summary. — One of the most valuable materials 

 which can be used on the soil, and a material which is very 

 generally produced, is farm manure. This is defined as a 

 mixture of solid and liquid excrement of any domestic animal 

 with any kind of litter. The solid excrement is the undigested 

 portion of food eaten by the animal. The liquid excrement 

 consists of digested material which has broken down in the 

 process of metabolism and has been eliminated in solution. 

 Most of the nitrogen and potash occur in the liquid excrement 

 and practically all of the phosphorus in the solid excrement. 

 The composition of the solid and liquid excrement varies 

 with the kind and age of animal and kind of food eaten. 

 In general about 80 per cent, of the fertilizing constituents 

 in the food are recovered in the excrement. The composition 

 of the litter or bedding affects the composition of the manure. 

 The amount of the fertilizing constituents as a result of 

 these various factors, and in addition, on account of decom- 

 position and loss of material, varies a great deal, but for 

 purposes of discussion we may consider average manure 

 to contain 10 pounds of nitrogen, 5 pounds of phosphoric 

 acid, and 10 pounds of potash per ton. 



Due to the large number of bacteria voided in the solid 

 excrement, and multiplying so rapidly in the liquid excrement, 

 manure decomposes quickly. All compounds suffer change, 

 but the most important changes are those which affect 

 nitrogen. Under aerobic conditions proteins are changed to 

 ammonia and even to elemental nitrogen. Urea, a con- 

 stituent of urine, is changed to ammonium carbonate and 

 ammonia dissociates and disappears. Considerable heat is 

 produced. This fact is made use of in the case of horse 

 manure, which is drier than cattle manure, in making "hot 

 frames." Under anaerobic conditions, ammonia is produced 

 but there is no opportunity for it to escape readily, and much 

 of it is slowly absorbed by bacteria and changed to insoluble 

 compounds again. 



Under either condition phosphoric acid and potash are 

 made soluble and easily lost if the manure pile is subjected 



