THE SOIL. 73 



tion necessary to fertility is capacity to be dissol/-' <I, 

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

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

 w'un'ral substance can enter into the roots of a plant 

 except it be 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 phosphoric. 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 phosphate of lime is but slightly 

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

 pound known as superphosphate 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 the section on 

 manures. Silicic acid exists in the soil usually in the 

 form of sand, 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 

 silicic acid in the soil, the application of an alkali, 

 such as potash, which will unite Avith 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. 



