148 AGRICULTURAL ANALYSIS 



ammonium citrate. The products soluble in water can be deter- 

 mined either separately or at the same time as the products solu- 

 ble in the ammonium citrate, without the necessity of mod- 

 ifying very greatly the method of operation 



The determination of the total soluble or available phosphoric 

 acid comprises, first, the solution of the soluble constituents in dis- 

 tilled water; second, the solution of the reverted phosphates in 

 ammonium citrate ; third, the determination of the phosphoric acid 

 dissolved in the two preceding operations or the determination of 

 the part soluble in ammonium nitrate by difference. 



135. Preparation of the Sample for Analysis. The sample sent 

 to the chemical expert is prepared as has been indicated ; that is 

 to say, it is poured on a sieve of which the meshes have a diameter 

 of one millimeter, and sifted upon a sheet of white paper. The 

 parts which do not pass the sieve are broken up either by the 

 hand or in a mortar and added, through the sieve, to the first 

 portions. The product is well mixed and, in this state, the mass 

 presents all the homogeneity desirable for analysis. 



Some fertilizers are received in a pasty state, which does not 

 permit of their being sifted. It is necessary in such a case to 

 mix them with their own weight either of precipitated calcium 

 sulfate dried at 160 or with fine sand washed with hydrochloric 

 acid and dried, which divides the particles perfectly and permits 

 of their being passed through the meshes of the sieve. 



136. Extraction of the Products Soluble in Distilled Water. 

 The substance having been prepared as has just been indicated, 

 one and a half grams are placed in a glass mortar. Twenty cubic 

 centimeters of distilled water are added, and the substance gently 

 suspended therein. After standing for one minute, the super- 

 natant part is decanted into a small funnel provided with a filter- 

 paper and placed in a flask marked at 150 cubic centimeters. This 

 operation is repeated five times and is terminated by an intimate 

 breaking up of the matter with distilled water. When the volume 

 of 100 cubic centimeters of the filtrate has been obtained, the 

 residue in the mortar is placed on the filter and the washing is 

 continued until the total volume reaches 150 cubic centimeters. 

 The filtrate is shaken in order to render the liquor homogeneous, 



