FORAGE CROPS. 363 



enormous quantity, in an acre of soil 6 inches deep, of a constitu- 

 ent forming the smaller proportions of the whole weight of an 

 acre of soil of that limited depth. It shows the existence of a 

 practically unlimited amount of the most important mineral con- 

 stituents of plants, and clearly points out the propriety of render- 

 ing available to plants the natural resources of the soil in plant- 

 food ; to draw, in fact, up the mineral wealth of the soil by 

 thoroughly working the land, and not leaving it unutilized as so 

 much dead capital. 



" The exact determination of phosphoric acid in a soil, it may 

 be observed in passing, is attended with no difficulty, if certain 

 precautions, which it is feared are sometimes neglected by chemists, 

 be taken. I will, therefore, give a brief outline of the plan — com- 

 monly known to chemists as the molybdic acid plan of determin- 

 ing phosphoric acid — which yields accurate results. 



" Not less than lOO grains, or better, 200 grains, of the dried 

 and finely-powdered soil are digested for an hour, or thereabouts, 

 with 3 or 4 ounces of moderately strong nitric acid. The acid 

 solution is then passed through a filter, and together with the wash- 

 ings from the insoluble portion of the soil left on the filter, is 

 evaporated to a small bulk ; thus getting rid of the greater part of 

 the acid employed for effecting the solution. During evaporation 

 a large excess of molybdate of ammonia is added to the solution, 

 care being taken to keep it strongly acid. 



" If there be much phosphoric acid in the soil, a bright yellow 

 precipitate, consisting of molybdic and phosphoric acid, makes its 

 appearance at once ; if traces only be present, the yellow precipi- 

 tate appears only on the concentration of the liquid, after the 

 great excess of nitric acid has been been expelled by evaporation. 

 The yellow precipitate containing the whole of the phosphoric 

 acid present in the soil, molybdic acid, together with a little silica, 

 and frequently some oxide of iron, is thrown on a filter and washed 

 with a solution of molybdate of ammonia rendered strongly acid 

 by nitric acid, until a drop of the washings passing through the filter 

 ceases to show a reaction of iron with yellow prussiate of potash 

 solution. It is then dissolved on the filter in an excess of ammo- 

 nia, and the ammoniacal liquid precipitated with an ammoniacal 

 solution of sulphate of magnesia, which throws down the phos- 

 phoric acid as phosphate of magnesia and ammonia. After stand- 

 ing at rest for about 12 hours, the magnesia precipitate is collected 

 on a small filter and washed clean with strong ammonia water. 

 Together with the phosphoric acid, traces of silica, and generally 

 also traces of oxide of iron, are thrown down with the magnesia pre- 



