ACTIVE PLANT FOOD, ETC. 



2O I 



Water Extract of Soils. In arid climates, water-soluble mate- 

 rial may accumulate and give rise to alkali. The solvent action 

 of water in a soil under natural conditions is increased by the 

 carbon dioxide formed from decaying organic material. 



On shaking a soil with water, a small amount of soil ingredients 

 enters into solution. The extract does not represent the solubility 

 of the soil constituents in water, but is the resultant of the solvent 

 and fixative forces, as in the case of the acid extract. The soil 

 has a much greater power of withdrawing material from water 

 than from acid solution, and hence the aqueous extract is a much 

 poorer measure of the solubility of soil materials. For example, 1 

 on shaking a certain soil with water, the resulting extract con- 

 tained 2.3 parts per million of phosphoric acid, and the same 

 results were secured on shaking it with a solution of potassium 

 phosphate containing 10 parts per million of phosphoric acid. 



Composition of the Water Extract. The soil extract varies 

 widely in composition, even from soils of the same type. The 

 following results are compiled (and recalculated) from Bulletin 22 

 of the Bureau of Soils : 



VARIATIONS IN COMPOSITION OF SOIL EXTRACT. 



The Bureau of Soils claims that the composition of the soil 

 extract is practically constant in all soils, but it is difficult to see 

 how the preceding analyses can be reconciled with such claim. 

 1 Texas Station Bulletin 82, p. 16. 



