ii.] THE NATURE OF HUMUS 45 



solution that is formed is akin to the dark liquid draining 

 from a dung heap, which contains humus dissolved 

 by the alkaline carbonates of the fermented urine. 



Occasionally soils are found which naturally pos- 

 sess an acid reaction, and in which the whole or part 

 of the soluble humus is uncombined with calcium, 

 so that it goes into solution in ammonia without the 

 preliminary treatment with acid. The portion of the 

 natural humus of soils that is soluble in acids contains 

 nitrogen, and seems to be of the nature of an amide. 



Although dark brown humic substances can be 

 prepared from carbohydrates, and therefore contain 

 only carbon, hydrogen, and oxygen, yet the soluble 

 humus of the soil, even when dissolved and reprecipi- 

 tated, always contains some nitrogen, nor can it be 

 obtained entirely free from phosphorus and mineral 

 matter. The original vegetable matter is made up not 

 only of carbohydrates, but of other carbon compounds 

 containing nitrogen, and in some cases both nitrogen 

 and phosphorus ; these all break down under bacterial 

 action into dark-coloured substances richer in carbon, 

 and roughly classed as humus. The splitting-up process 

 continues in the soil, so that humus becomes one of the 

 great sources of nitrogen for the food of plants, and a 

 soil well supplied with humus is generally regarded as 

 fertile. v 



During the formation and continued decomposition 

 of humus the carbohydrates appear to be first attacked, 

 and the nitrogen-containing bodies, tg. % the nucleins in 

 particular, resist the action of bacteria. For this reason, 

 where we find the proportion of humus in a soil is low, 

 the proportion of nitrogen in the humus itself will be 

 high, the decay of the humus falls more heavily on the 

 purely carbonaceous part of the material. 



This is seen in the figures obtained by Lawes and 



