66 SOILS ANE> FERTILIZERS 



and mineral acids, and represents the most soluble and 

 the most active and valuable forms of plant food. 

 There is only a very small amount in these forms. 

 In 100 pounds of soil, rarely more than 0.005 pound 

 of any one of the important elements is soluble in 

 the soil-water or more than 0.05 pound in dilute or- 

 ganic acids. 



Experiments have shown that the soluble plant 

 food from a fertile soil is not sufficient for plant 

 growth. 85 When oats, wheat and barley were seeded 

 in prepared sand and watered with the leach- 

 ings from a pot of fertile soil, they made only a lim- 

 ited growth. For comparisons with plants grown in 

 fertile soil, see Plate I. The oats grown in the pre- 

 pared sand and watered with soil leachings assimilated 

 only 25 per cent, as much phosphoric acid as the 

 plants grown in fertile soil. 



88. Acid Soluble Matter of Soils, The plant food 

 of the second class is in a somewhat more insoluble 

 form, and consists of all those compounds and zeolitic 

 silicates which are soluble in hydrochloric acid of 

 23 per cent, strength, sp. gr. 1.115. This represents 

 the limit of the solvent action of the roots of plants. 17 

 In this second class are also included all of the mineral 

 elements combined with the humus and soluble in 

 dilute alkalies. As a rule, not over 15 or 20 per cent, 

 of the total soil is in forms soluble in hydrochloric 

 acid, and of the more important elements only i to 6 

 per cent, form a part of this 15. In 200 samples of soil, 

 the potash, nitrogen, lime, magnesia, and phosphoric 

 and sulphuric acids, amounted to 3.5 per cent. In 



