CHAP. vii.J PROCESS OF DIGESTION 179 



that the composition of farmyard manure must be far 

 from constant, varying with the nature of the animal 

 making the dung, the kind and amount of food it 

 receives, the proportion between excreta and litter, 

 the nature of the litter, and the extent and character of 

 the decomposition which has taken place in the manure 

 itself. The composition of the excreta being the largest 

 of these factors, it will be necessary first of all to trace 

 the effect of the process of digestion on the various 

 manurial substances in the food — compounds of nitrogen, 

 phosphoric acid, and potash. Animals that are not 

 increasing in weight, such as working horses or full- 

 grown cattle simply being maintained in store condition, 

 excrete the whole of the nitrogen, phosphoric acid, 

 and potash they receive in a liquid or solid form, 

 the carbohydrates and fat of the food being mostly 

 got rid of as gases. But the fate of the manurial 

 constituents varies according as they are present in the 

 food as digestible or indigestible compounds ; for 

 example, part of the proteins of the food withstand 

 the action of the digestive ferments, and are excreted 

 unchanged in the fceces, but to a much greater extent 

 they are broken down into soluble compounds which 

 pass into the blood and eventually are excreted as 

 urea, uric acid, etc., in the urine. Similarly, for the 

 phosphoric acid and the potash in the food, whatever 

 is digestible is excreted in the urine in some simpler 

 combination, whatever resists digestion passes out 

 unchanged in the solid excreta. Hence a great 

 difference in the manurial value of the two portions of 

 the excreta ; the compounds in the urine — urea, uric 

 acid, soluble phosphates, and potash salts — are either 

 ready for the nutrition of plants or require but slight 

 further changes to become so ; whereas in the solid 

 dung the materials have several stages of decomposition 



