COLLOIDAL SILICA AND THE EFFICIENCY OF 
PHOSPHATES 1 
By P. L. Gile, Chief, Division of Soil Chemistry, and J. G. Smith, Assistant 
Chemist, Bureau of Soils, United States Department of Agriculture 
INTRODUCTION 
General fertilizer practice recognizes in a vague way that there are 
reactions between soils and phosphatic fertilizers which affect the 
efficiency of the fertilizers. Bone meal, for instance, is recommended 
for use on open soils which are not too dry (39, p. 169), 2 and for light 
soils not too deficient in phosphoric acid (82, p . 320). Floats are 
recommended by Wheeler (39, p. 173) for acid soils high in organic 
matter, and by Schneidewind (82, p. 321) for acid high-moor soils 
but not for mineral soils. Acid phosphate, according to Schneidewind 
(82, p. 297), is adapted to all the “better” and heavier soils. How¬ 
ever, the extent to which a phosphate is dependent on the character 
of a soil is probably not sufficiently appreciated. 
EFFECTS OF SOILS ON THE EFFICIENCY OF PHOSPHATES 
A complicated series of reactions follows the application of a 
phosphate to a soil; the nature and extent of the reactions must 
vary in different soils; and it is to be presumed that these reactions 
will affect the availability or efficiency of the fertilizer. Attempts 
have been made, by extracting the soil with various solvents, to 
determine what happens to a phosphate after it is incorporated in 
the soil (18, 40). But it has not been possible to follow the reactions 
in the laboratory far enough or precisely enough to determine just 
how the fertilizing efficiency of a phosphate will be affected. Evidence 
that the soil influences the efficiency of phosphates is therefore 
derived chiefly from experiments with plants. 
That soils affect the efficiency of the various phosphates differently 
is indicated by many experiments in which the fertilizing values of 
the soluble and insoluble phosphates are compared. It has been 
shown frequently that rock phosphate or bone meal may be almost 
as effective as acid phosphate on one type of soil but quite ineffective 
as compared with acid phosphate for the same crop on another type 
of soil. Variations in the crop increase produced by a given quantity 
of rock phosphate on different soils are so great that it seems quite 
improbable that variations in the relative efficiency of the two classes 
of phosphates are due in all cases to differences in their so-called 
“secondary” effects on the soil; that is, in decreasing acidity, supply¬ 
ing lime, and the like. The low efficiency of rock phosphate in 
quartz sand and its high efficiency in some soils indicate that in certain 
soils reactions take place which enhance the “availability” of the 
phosphate—increase the supply of phosphorus acting as a plant 
nutrient. 
1 Received for publication September 29,1924; issued September, 1925. 
2 Reference is made by number (italic) to “Literature cited,” p. 259. 
Journal of Agricultural Research, 
Washington, D. C. 
( 247 ) 
Vol. XXXI, No. 3 
Aug. 1,1925 
Key No H-ll 
