60 AGRICULTURE 



Fertilizers, such as farmyard manure, bones, 

 rape meal, etc., which supply organic nitrogen, 

 undergo rapid decomposition in calcareous 

 soils, the nitrogen being converted into 

 nitrates, which, unless appropriated by plants, 

 quickly sink down into the soil, and are 

 removed in the drainage water. Other things 

 being equal, such a soil is probably better 

 suited to the use of sulphate of ammonia 

 than of nitrate of soda. But, whatever form 

 of nitrogen be used, dressings for a calcareous 

 soil should be moderate and frequent, rather 

 than large and at long intervals. 



The fourth substance which is always 

 present in soil, and which may on occasion 

 form a large proportion of the whole, is 

 humus. This is a substance of indefinite 

 composition which has its origin in the 

 decay of vegetable matter. According to 

 the stage of decomposition, so does the com- 

 position of the humus vary, and its character 

 also depends to some extent upon the kind 

 of plant remains of which it is composed. 

 In all cases, however, it contains carbon, 

 hydrogen, oxygen, nitrogen, and the mineral 

 matter which the plants forming it took up 

 and stored in their tissues during growth. 

 Humus, therefore, is capable of supplying 

 crops with mineral plant food, such as 



