THE FERTILITY OF THE SOIL 227 



From my point of view fertility, seasonal variations 

 eliminated, is the measure of the ability of the soil to 

 feed a growing crop. This ability rests upon two fun- 

 damental conditions: First, porosity of the soil, by 

 means of which the roots can go out after their food; 

 and second, the amount of food available. 



It is well known that plants live chiefly on inorganic 

 materials, consisting mostly of compounds of potassium, 

 phosphorus and nitrogen. There are, indeed, other es- 

 sential food elements, such as lime, iron, magnesia, et 

 cetera, but as a rule these, with the exception of lime, 

 are always present in much greater quantity than is 

 needed in tillable soils. The compounds of phosphorus, 

 potash and nitrogen, may be locked up in such a way 

 as to be inaccessible for the food requirements of the 

 plant. The usual form of the phosphorus compound in 

 the soil is its union with lime, forming phosphate of 

 lime. Phosphate of lime in its natural state is quite 

 insoluble in the soil waters, and hence the mere pres- 

 ence of phosphoric acid in combination with lime is not 

 always a proper guarantee for a supply for the growing 

 plant. 



The same is true of potash, and to a less degree of 

 nitrogen. Many nitrogenous compounds exist in the 

 soil in an undecomposed, organic state, in which condi- 

 tion they are useless as plant foods. These nitroge- 

 nous compounds must be broken down under the influ- 

 ence of bacterial action, and the nitrogen converted into 

 nitric acid by a series of ferments which are capable of 

 producing nitric acid from the organic matters con- 

 taining the nitrogen. It is only in the form of nitric 

 acid that nitrogen reaches its full measure of ability 

 in the nourishing of plants. The potassium may exist 

 in the soil as one of the components of granite or 



