THE SOIL AS RELATED TO PLANTS. 99 



gous growth) seems to develop more rapidly 

 when this crop is preceded by liming. 



One form of calcium the sulphate, called 

 land plaster or gypsum fixes ammonia, while 

 lime drives it off. Hence this is exceedingly 

 useful for sprinkling in the trenches of stables, 

 or upon the surface of compost heaps, to prevent 

 the escape of the ammonia. For use in connec- 

 tion with manure, no other form but the sulphate 

 (gypsum) should be used. It is also best for an 

 indirect fertilizer that is, for rendering the 

 present but unavailable plant-food available. 



For neutralizing acids, calcium oxide (CaO), 

 or quicklime, is the best form to use. It must 

 be slaked a short time before using. It may be 

 placed in heaps and water sprinkled over it, and 

 then covered with soil for a few days. It should 

 be free from lumps, spread or drilled evenly, 

 and harrowed in at once. This form is also a 

 cheap and very good indirect fertilizer. 



Another method of indirect fertilizing is by 

 the judicious use of cover crops (see " Legumin- 

 ous Plants" and " Rotation of Crops"). Plant 

 roots not only make mineral plant-foods more 

 easily available, but prevent them from being 

 leached out by the winter rains and snows. 



2. Stable Compost. 



As has been said, green manuring is expensive, 

 since the crop may be fed to stock, and if the 

 stable compost is properly cared for and returned 



