FEEDING THE PLANT 95 



that they are more essential than the others in manures, 

 which are usually present in great abundance in most 

 soils, and their addition would not contribute to actual 

 fertility. 



In the second place, certain substances containing 

 these constituents have both a direct and indirect ferti- 

 lizing effect, that is, they add to the constituents in the 

 soil, as well as change and make available the otherwise 

 unavailable constituents already in the soil, while others 

 are only direct in their effect, and still others indirect. 

 The natural products, as farm manures and wastes of 

 various sources, belong to the first class, namely, those 

 which possess a direct and an indirect value. 



This indirect effect is often of great service, chiefly 

 in changing the physical character of the soil in such a 

 way as to enable the natural agencies to act more 

 effectively. As, for example, on a heavy, compact soil, 

 which does not freely permit the entrance of air and 

 moisture, the addition of the vegetable matter con- 

 tained in farm manure would have the effect of opening 

 up and separating the particles, thus making the soil 

 more porous and permitting the free access Of air and 

 water, and consequently greater and more rapid 

 changes, which contribute indirectly to an increase in 

 crop. On a sandy soil, on the other hand, the addition 

 of manures of this character would also improve be- 

 cause making the soil less open and porous, drawing 

 the particles of soil together and making it more com- 

 pact, thus preventing the rapid drying out of the soil, 

 and the rapid leaching from it of the soluble con- 

 stituents. 



Direct Manures — To the second class, or to the 

 direct manures, belong the artificial products, which 

 are valuable chiefly because they contain the constituent 

 elements, nitrogen, phosphoric acid and potash, which 

 contribute directly to the potential fertility of the soil, 



