12 The Philippine Journal of Science lyu 



(2) It is desirable that the greater portion should be the 

 "reserve" food material; that is, in the form available to plants 

 only by the solvent action of acids in the soil and the action of 

 the roots of the plants.-' 



King 2* found that in some soils enough plant nutriment for 

 a season's crop may be dissolved by distilled water alone, if the 

 soil be subjected to repeated leachings and drying at 110° C, 

 but there were striking differences in the amount of leachings 

 from samples of known productive capacity and others of low 

 production. 



Snyder " found that soil leachings failed to supply a sufficient 

 amount of plant food to produce normal plants of wheat, oats, 

 and barley and that the plants obtain a large portion of their 

 food from forms which are insoluble in water. 



(3) Those soil ingredients that cannot be brought into solution 

 except by alkali carbonate fusion or hydrofluoric acid treatment 

 constitute the unchanged minerals, and under ordinary condi- 

 tions are of no practical value as a source of plant food supply, 

 at least, for many years. 



The water- and acid-soluble ingredients include the supply 

 of plant food which presumably "wdll become available in a 

 period of time in which we are interested" by the reactions 

 which take place in the soil by the application of scientific 

 management and cultural methods. These have been separated 

 and determined. The methods used were substantially those 

 of the Association of Official Agricultural Chemists.'" The 

 results are given on the basis of a sample dried to a constant 

 weight at 105°C. 



All chemical analyses were made on that portion which passed 

 a 1-millimeter screen ("fine earth"). The sand grains larger 

 than this are considered to be composed of quartz more or less 



" The equilibria between the solid, liquid, and gaseous components of 

 soils, although they have been extensively studied, are so complicated that 

 much remains to be discovered. Calcium and magnesium carbonates and 

 phosphates are acted upon by the weak acids in the soil and the roots of 

 plants, and are thereby made available as plant food. Tricalcium phosphate 

 is difficultly soluble in the soil moisture, and is generally considered not 

 easily available. Phosphates of aluminium and of iron are extremely 

 slightly acted upon by soil moisture, and are usually not considered as a 

 source of plant food within a reasonable time. These equilibria are im- 

 portant in the fixation of the water-soluble fertilizers applied upon the 

 land, and prevent waste from leaching. 



'^Loc. cit., Hilgard, Soils, 323. 



''Bull Minn. Agr. Exp. Sta. (1905), 89, 198-200. 



"Bull. U. S. Dept. Agr., Bur. Chem. (1908), 107 (Revised). 



