66 SCIENTIFIC AGRICULTURE. 



147. The newly sprouted plant stands ready to 

 grow, it may be with all its organs perfect, and with 

 every article of food at hand, but, so long as there is . 

 only a limited supply of one important element, the 

 rapidity and vigor of its growth will be retarded. The 

 air contains an abundant supply of nitrogen, but not 

 one atom of it, as chemists believe, is contributed 

 directly to the growth of plants. This important 

 element, before it can be used as plant-food, must be 

 in union with some other element, and guano furnish- 

 es it in the best possible combination. The phos- 

 phates, too, which exist so largely in guano and other 

 salts, give additional value to this fertilizer. 



148. Guano, then, as we have shown, promotes 

 the growth of vegetation by furnishing a large supply 

 of nitrogen in an available form, and a few other 

 highly important constituents of our most valuable 

 crops. As it does not contain all the elements of 

 plant-food, there may be soils which it will not great- 

 ly benefit unless mixed with other substances. It is 

 well to keep in mind the fact that all the elements 

 of plants are necessary for growth. The reports of 

 failure in the use of guano and other fertilizers, and 

 statements to the effect that they " run out " and, in 

 the end, "make the soil poorer," may be explained 

 by the fact that such manures lack some elements 

 which every fertile soil must contain ; or they may 

 so stimulate a comparatively poor soil that in a few 

 years the supply of some important element which 

 the jnino or other manure lacks will be exhausted. 



