178 § 100. ANALYSIS OF SOILS AND ROCKS. 



the sediment at the bottom of the bottles. If a clear 

 filtrate is not obtained in this way, it must be evaporated 

 down to 400 or 500 c.c., just barely supersaturated with 

 hydrochloric acid while still hot, and then filtered again. 



Evaporate the solution to dryness, with the addition 

 of a few drops of nitric acid towards the close of the 

 evaporation, to peroxidize the iron and destroy organic 

 matter, and eliminate silicic acid. (§ 58, a, 1.) 



Treat the filtrate from the silica as in Scheme I., § 94. 

 Alumina, ferric oxide, and phosphoric acid, are usually 

 present, however, in such small quantities in this solution, 

 that it is hardly Avorth while to determine at least the 

 first two. 



Interesting results may be obtained by the successive 

 treatment of the same portion of soil with carbonated 

 w^ater, and a chemical examination of each solution; the 

 proportion may thus be learned in which the more im- 

 portant elements of plant-food are taken up by the suc- 

 cessive aqueous extracts, and data are obtained for esti- 

 mating, not only the general richness of the soil in 

 valuable elements of plant-food, such as phosphoric acid 

 and potassa, but also the relation between the immediate 

 fertility of the soil and the durability of its fruitfulness. 



A very great decrease in the amount of the elements 

 of plant-food in the second and third extracts, as com- 

 pared with the first, would indicate that the fertility of 

 the soil would be very much lessened in a single season. 

 If, on the contrary, there is but little diminution observ- 

 ed even in the fifth extract, the power of the soil to 

 produce crops will probably remain about the same, year 

 after year, for a long time. 



To obtain these successive extracts, replace the 4000 

 c.c. that wore poured oft' for the first extract, by an equal 

 quantity of fresh water ^ saturated witli carbonic acid as 

 before, let stand three days with frequent shaking, pour 



