SOILS AND FERTILIZERS 455 



that the commercial value, that is, the cost of any fertilizer constitu- 

 ent, need not necessarily bear a direct relation to its agricultural 

 value, that is, the power of producing a certain crop increase. 



The agricultural value of a fertilizer is measured by the bene- 

 fits received from its use, and depends upon its fertilizing effect, or 

 crop-producing power. As a broad, general rule, it is true that 

 ground bone, superphosphates, fish scraps, dried blood, potash salts, 

 etc., have a high agricultural value which is related to their trade- 

 value, and to a degree determines the latter value. But the rule has 

 many exceptions, and in particular instances the trade-value cannot 

 always be expected to fix or even to indicate the agricultural value. 

 Fertilizing effect depends largely upon soil, crop and weather, and as 

 these vary from place to place and from year to year, it cannot be 

 foretold or estimated, except by the results of past experience, and 

 then only in a general and probable manner. 



The different phosphates used in fertilizers differ widely in 

 availability, i. e., in the readiness with which they are taken up by 

 plants, and different plants have been found to vary greatly in their 

 capacity to assimilate the phosphoric acid of a given phosphate. 

 The insoluble phosphates are much cheaper than the soluble forms. 

 Phosphoric acid in soluble form costs in the market from 4 to 6 

 cents per pound (often more in mixed fertilizers), while that of the 

 crude mineral phosphates is worth less than 2 cents per pound. If, 

 therefore, it can be shown that certain plants can utilize some of the 

 insoluble phosphates to good advantage the outlay for phosphates 

 may be greatly reduced. 



The agricultural value and the commercial value of fertilizing 

 materials are not synonymous terms. The agricultural value of a 

 fertilizer is determined by the increase and quality produced by it in 

 the crop. The commercial value of a fertilizer is determined by the 

 cost of the materials which enter into its composition. A fertilizing 

 material of high commercial value may have a low agricultural 

 value on a particular soil or crop. On another soil or crop the same 

 material may have a high agricultural value. 



^VHAT ARE COMMERCIAL FERTILIZERS? 



In connection with the subject of fertilization the terms ma- 

 nures, fertilizers, commercial fertilizers, chemical fertilizers, indi- 

 rect fertilizers, natural fertilizers, artificial fertilizers, superphos- 

 phate, complete and incomplete manures, etc., are used, and there 

 is often a misunderstanding of the meaning of some of these. A fer- 

 tilizer is any substance which furnishes deficient plant food in an 

 available form. Fertilizers are either natural or artificial ; the former 

 including manures, or the solid and liquid excrement of animals and 

 green crops plowed in to increase fertility. 



The latter, (artificial fertilizers) ' including commercial fertil- 

 izers, sometimes called prepared fertilizers, and chemical fertilizers, 

 or those mixed from crude fertilizing chemicals. 



A fertilizer is complete, sometimes called general, when it 

 contains nitrogen, phosphoric acid and potash, and incomplete, 

 sometimes called special, when furnishing only one or two of these 



