88 THE PRINCIPLES OF AGRICULTURE 



be in condition to be used by the plant. These 

 are nitrogen, phosphorus, and potassium. A 

 fourth plant -food is also sometimes deficient, 

 calcium. These four substances, therefore, are 

 the ones which the farmer needs to consider 

 when fertilizing the land. 



129. Before the plant can use any of these 

 elements of plant- food in the soil, they must 

 become dissolved in the soil water, which is 

 absorbed by roots. 



130. While all plants need certain elements 

 for their growth, they cannot use the elements 

 in their elemental or uncombined forms. In 

 fact, the elements as such do not exist in the 

 soil. They are united with each other in com- 

 pounds, and it is by absorbing the compounds 

 that the plants obtain the necessary elements. 

 Phosphorus is essential to the life of plants, 

 but it is never used by them in the form of 

 elemental phosphorus. It is always in some 

 compound, as phosphoric acid or a phosphate. 



131. When the compounds exist in such 

 condition as to be readily absorbed by the 

 roots, the soil is said to contain available 

 plant -food. Often there is sufficient plant -food 

 present, but not in condition to be taken up 

 by the plants. It is then said to be unavail- 

 able, or to be locked up. Availability is deter- 

 mined by two factors : by the substance being 



