86 SOILS AND FERTILIZERS 



soil. A fifth-normal solution of hydrochloric, or of 

 nitric acid, has also been proposed 24 for determining 

 the available plant food of soils ; a soil that yields 

 less than 25 parts of phosphoric acid per million of 

 soil, as soluble in fifth-normal nitric acid, is deficient in 

 available phosphates. 



The use of dilute organic acids renders it possible 

 to detect small amounts of readily soluble phosphoric 

 acid and potash. It has been stated that when a soil 

 has been manured a few years with a phosphate fer- 

 tilizer and brought into good condition as to available 

 phosphoric acid, a chemical analysis will fail to detect 

 any difference in the soil before and after the treat- 

 ment with fertilizer. In the case of hydrochloric acid 

 as a solvent, this is true, as an acre of soil to the depth 

 of one foot weighs about 3,500,000 pounds and 500 

 pounds of a phosphate fertilizer would increase the 

 total amount of phosphoric acid about 0.0002 per cent, 

 which is less than can be accurately determined by 

 analysis. When, however, a dilute organic acid is used, 

 only the more easily soluble phosphoric acid is dissolved, 

 and this readily allows fertilized and unfertilized soils to 

 be distinguished. By the use of dilute organic and min- 

 eral acids decided differences have been shown between 

 fertilized and unfertilized soils. 



101. Sampling Soils. A composite sample of the 

 soil of a field is obtained by taking several small samples 

 to a depth of 6 to 12 inches, from different places, and 



