THE SOIL. 73 



tioii necessary to fertility is capadti) to he dissok'ed, 

 it being (so far as we have been able to ascertain) a 

 fixed rule, as was stated in the first section, that no 

 mineral substance can enter into the roots of a plant 

 except it he dissolved in water. 



The alkalies potash, soda, lime, and magnesia, are 

 in nearly all of their combinations in the soil suffi- 

 ciently soluble for the purposes of growth. 



The acids are, as will be recollected, sulphuric, 

 silicic, and phosplioric. These exist in the soil in 

 combination with the alkalies, as sulphates, silicates, 

 and phosphates, which are more or less soluble under 

 natural circumstances. Phosphoric acid in combi- 

 nation with lime as pliosphate of lime is but slightly 

 soluble ; but, when it exists or has existed in the com- 

 pound known as s/^drphosphate of lime, it is much 

 more soluble, and consequently enters into the com- 

 position of plants with much greater facility. This 

 matter will be more fully explained in tlie section on 

 manures. Silicic acid exists in the soil usually in the 

 form of sandal in which it is, as is well known, per- 

 fectly insoluble ; and, before it can be used by plants, 

 which often require it in large quantities, it must be 

 made soluble, by combination with an alkali. 



For instance, if there is a deficiency of soluble 

 sih'cic acid in the soil, the application of an alkali, 

 such as potash, which will unite with the sand, and 

 form the silicate of potash, will give it the ability to 

 be dissolved and carried into the roots of plants. 



Chlorine in the soil is probably always in an 

 available condition. 



4 



