CHEMICAL COMPOSITION OF THE SOU, 



175 



o.i per cent, phosphoric acid, when accompanied by a fair amount 

 of lime, secures fair productiveness from eight to fifteen years; 

 with a deficiency of lime, twice the percentage will only serve for 

 a similar time. Soils containing between o.i and 0.05 per cent, 

 of phosphoric acid are considered as likely to respond to fertiliza- 

 tion with phosphates in a short time. A large supply of organic 

 matter appears, like a large supply of lime, to offset a deficiency in 

 phosphoric acid. Large quantities of hydrated ferric oxides may 

 render even large quantities of phosphoric acid inert and unavail- 

 able to plants. 



Lime. Lime is exceedingly important to the soil. Low 

 percentages of potash, phosphoric acid, and potash are 

 adequate when a large proportion of lime carbonate is present. 

 Many of our richest soils are calcareous soils, such as the blue- 

 grass soils of Kentucky, the black prairie soils of Mississippi and 

 Texas, the calcareous prairie soils of Illinois, Indiana, and Iowa. 

 These soils are productive and durable. 



Heavy clay soils with less than 0.5 per cent, of lime do not, 

 according to Hilgard's observation, carry the plants characteristic 

 of calcareous soils. The lightest sandy soil should not contain 

 less than o.io per cent, of lime; clay loams should contain 0.25 

 per cent. There is no advantage in more than 2.00 per cent. It 

 appears that an excess of potash may offset deficiency of lime. 



The following examples 1 show the effect of lime in overcoming 

 a deficiency in phosphoric acid : 



Soils A and B were highly productive, falling off suddenly at 

 the end of 15 or 20 years. Soils C and D scarcely produced 500 

 pounds seed cotton per acre when fresh, and then only for three 

 or four years. The difference appears to be due to an abundance 

 of lime in the first two, a deficiency in the second two. 

 1 Tenth U. S. Census. 



