ENRICHING SOIL COMMERCIAL RESOURCES 89 



soluble in soil water; by its being of such com- 

 position that the plant will use it. 



132. One problem for the agriculturist is to 

 secure available plant- food, and to determine 

 whether it is better to unlock the plant- food 

 in the soil by means of tillage, or to supply 

 the elements in some manure or fertilizer. 



133. Barn manures are not always to be had, 

 and they are variable in composition. It is often 

 advisable, therefore, to substitute commercial or 

 concentrated fertilizers, in which the constituents 

 are of known amounts and often readily avail- 

 able. Barn manures are bulky. Even manure 

 of cattle from a covered yard contains as 

 high as 70 or 75 per cent of water, and usu- 

 ally less than 1 per cent of nitrogen, phos- 

 phoric acid or potash. If it were not for its 

 influence in improving the physical effects of the 

 soil, stable manure would have comparatively 

 little value. 



2. Nitrogen 



134. Nitrogen is the most important element 

 which the farmer adds to his soil. It comprises 

 part of all green and woody parts of plants. 

 It seems to be the element most intimately 

 associated with rapid growth in plants. Plants 

 that feed excessively on nitrogen tend to pro- 



