THE ANALYSIS OF VIRGIN SOILS. 345 



thousands of crop years. For, one-tenth of one per cent in 

 the case of the clayey soils of the preceding table would amount 

 to about 3500 pounds per acre-foot, and to 4000 in the case of 

 the sandy ones. Hence the amount of phosphoric acid in e. g., 

 the Mississippi delta soil from Houma would suffice for the 

 production of about 440 crops of wheat grain (at 20 bushels 

 per acre) if only one foot depth were drawn upon; but as the 

 roots of grain easily penetrate to twice and half and three 

 times that depth even in the humid region, the number might 

 be tripled. As a matter of fact, however, that soil has pro- 

 duced full crops for from forty to fifty years only ; yet this is 

 considered an exceptionally long duration of profitable pro- 

 duction without fertilization. 



The first and last soils in the above list represent probably the highest 

 types of productiveness known. The Yazoo bottom soil has produced 

 up to one thousand pounds of cotton lint per acre when fresh, and is 

 still producing from four to five hundred pounds after thirty years' 

 culture. The Arroyo Grande soil of California with its extraordinary 

 percentages of phosphoric acid and nitrogen, as well as exceptionally 

 high proportion of available phosphoric acid and potash, has made such 

 a record of productiveness, and high quality of the seeds produced, 

 that it has for a number of years been excluded from competition for 

 prizes offered by seed-producers elsewhere, in order to give other sections 

 a chance. Both these soils are rather heavy clays, but readily tillable 

 in consequence of their abundant lime-content. The remarkably high 

 content of acid-soluble silica, indicating the presence of much easily 

 available zeolitic matter, is doubtless connected with the exceptional 

 productiveness. 



Experience, then, proves that lands showing such high 

 plant-food percentages will yield profitable harvests for a long 

 time without fertilization, or with only such partial returns 

 as are afforded by the offal of crops. Also that when fertiliza- 

 tion comes to be required, instead of supplying all the ingre- 

 dients usually constituting fertilizers, only one or two of these 

 will as a rule be actually needed, and even these in smaller 



1 The Rio Grande and Colorado bottom soils contain amounts of lime carbonate 

 largely in excess of reqnirements, 2 to 3 of that compound being all that is 

 needed to insure all the advantageous effects of lime in any soil (see this chapter, 

 page 367). 



