SOLUBLE AND INSOLUBLE POTASH, ETC. 69 



Wheat Heavy clay Grass and 

 soil. soil. grain soil. 

 Solu- Insolu- Solu- Insolu- Solu- Insolu- 

 ble in ble ble in ble ble in ble 

 HC1 residue HC1 residue HC1 residue 



Insoluble matter... 63.07 84.77 84.08 



Potash 0.54 2.18 0.21 3.46 0.30 1.45 



Soda 0.45 3.55 0.22 2.95 0.25 0.25 



Lime 2.44 0.36 0.48 0.16 0.51 0.35 



Magnesia 1.85 0.25 0.34 0.47 0.26 0.46 



Iron 4.18 0.78 3.76 0.72 2.56 1.07 



Alumina 7.89 5.54 6.26 5.44 2.99 9.72 



Phosphoric acid.... 0.38 0.12 0.08 0.23 0.05 



Sulphuric acid o.n 0.24 0.09 0.25 0.08 0.02 



The insoluble matter, after digestion with hydro- 

 chloric acid, was submitted to fusion analysis, and the 

 figures given under insoluble residue represent the 

 amounts of potash, soda, etc., insoluble in acids. In 

 the clay soil, 94 per cent, of the total potash is in 

 forms insoluble in hydrochloric acid. 



90. Soluble and Insoluble Potash and Phosphoric 

 Acid. From the preceding table it is to be observed 

 that the larger portion of the potash in the soil is in- 

 soluble in hydrochloric acid. A soil may contain 

 from 2 to 3 per cent, of total potash, and 90 per cent, 

 or more may be in such firm chemical combination 

 with aluminum, silicon, and other elements, as to re- 

 sist the solvent action of plant roots. The larger por- 

 tion of the phosphoric acid of the soil is soluble in 

 hydrochloric acid. In some soils, however, from 20 

 to 40 per cent, is present as the third class of com- 

 pounds. When a soil is digested with hydrochloric 

 acid, the insoluble residue is usually a fine, gray 

 powder. Some clay soils retain their red color even 

 after treatment with acids showing that the iron is in 



