SOILS AND FERTILIZERS 451 



in a well prepared seed bed and to give the crop thorough and intel- 

 ligent cultivation, yet even if these things are all properly done and 

 the soil is so poor that a small yield is obtained then there remains 

 little or no profit to repay the husbandman for his efforts. 



The fertility of the soil depends upon and is influenced by 

 many complex factors. The physical condition of the soil; its 

 water-holding capacity ; the amount and availability of the different 

 essential plant food materials; the presence or absence of injurious 

 substances or organisms, each and all have their influence in affect- 

 ing productivity and all are important. Plants cannot grow, how- 

 ever, unless they have food, and other things being equal the luxuri- 

 ance of growth depends, not upon the total amount of available 

 plant food present in the soil and atmosphere, but upon that partic- 

 ular food material which is present in the smallest amount relative 

 to the needs of the crop. For example, if only enough potash in an 

 acre of soil becomes available in one season for the production of one 

 thousand pounds of hay per acre, then only one thousand pounds of 

 hay per acre will be produced, even thougn there is present enough 

 available phosphoric acid, lime, nitrogen, iron, sulphur, magnesia 

 and silica for the production of five tons of hay per acre. 



When commercial fertilizers are relied upon entirely to main- 

 tain a high degree of crop production a fertilizer containing all 

 three constituents, nitrogen, potash, and phosphoric acid will give 

 better results than a fertilizer containing only one or two of these 

 constituents. Aside from the fact that a fertilizer should be es- 

 pecially rich in phosphoric acid experiments do not indicate the 

 exact proportion in which nitrogen, potash and phosphoric acid 

 should enter into a fertilizer for best results. 



If leguminous crops are raised and either plowed under or fed 

 on the farm and the resulting manure carefully saved and applied 

 to the soil it is very probable that in practice it will 'be necessary to 

 purchase only phosphoric acid in order to increase the productiveness 

 of such soils and to maintain them in a condition of high fertility. 



DO COMMERCIAL FERTILIZERS INJURE THE SOILS? 



There are two ways, and possibly three, in which the use of 

 the common commercial fertilizers injure the soil. First, they 

 actually contain practically the full measure of the acidity repre- 

 sented by the sulfuric acid used in their manufacture. They do not 

 contain sulfuric acid as such, nor any other free acid; but they do 

 contain acid salts, which require just as much lime for their cor- 

 rection or neutralization as would the sulfuric acid itself. On the 

 whole, however, this tendency to produce, or increase, acidity in the 

 soil is a minor matter, because it can be corrected from time to time 

 by the addition of ground limestone at small expense. 



The second way in which the use of commercial fertilizers in- 

 jures the soil is of much greater importance, although it is indirect. 

 It is due to the fact that they are too expensive to permit their use 

 in sufficient quantity to supply as much plant food as the crops re- 

 move ; and the result is that where they are used they serve in part 

 as soil stimulants. 



