38 
excess of the aluminum, while in the two latter the opposite relation 
exists. If it is permissible to assume that the phosphate is largely in 
combination with iron in soils Nos. 5 and 6, while that in Nos. 7 and 
8 is largely in combination with aluminum, then the former is more 
soluble and, as results have shown, slightly more available. On the 
other hand, Hawaiian clay soils, in which the aluminum content is 
higher than the iron, possess more marked colloidal properties, and 
hence, due to their physical influences, should contain phosphoric 
acid in a less soluble and less available form. 
It appears that the solubility of the phosphates already present in 
the soil is dependent partly upon conditions of equilibrium which 
influence the concentration of the soil solution. One effect of an 
added fertilizer is to disturb this state of equilibrium. For this reason 
the solubility or availability of an added phosphate will depend upon 
its action following its addition to the soil, and hence is influenced by 
several factors. 
Reference to Tables IX, X, and XI will clearly show that the solu- 
bility of the phosphates before adding to the soil can not be used as a 
criterion of their solubility after addition. According to the results 
tabulated in Table IX, fixation is apparently influenced by the valency 
of the salt. Trivalent potassium phosphate is fixed most strongly, 
divalent sodium phosphate next strongest, while monovalent sodium 
phosphate is most soluble. Superphosphate is much more soluble 
than the sodium and potassium phosphates, as is also acid phos- 
phate, with the exception of monosodium phosphate. These facts 
indicate that the fixing or reversion of the calcium phosphate is much 
less rapid than that of the sodium and potassium salts, or possibly 
that the calcium salt is not so strongly fixed. But this does not cor- 
relate with the availability as measured by the plant growth in this 
soil, where sodium phosphate was the most effective phosphate 
fertilizer. The sodium and potassium salts were least soluble in the 
first portion of water passing through the soil, and in this they dif- 
fered radically from the calcium salts. This indicates the possible 
influence of physical factors upon their solubility. Phosphate rock 
and slag proved to be the least soluble in water. 
The solubility, as measured by citric acid (Table X), indicates that 
citric acid only magnifies the action of water. All forms of calcium 
phosphate were among the most soluble, except Thomas slag (tetra- 
calcium phosphate). The solubility in citric acid is also influenced 
by the valency of the sodium and potassium phosphate, namely, 
the monobasic phosphate is most soluble while the tribasic is least 
soluble. 
Somewhat more complete data are given in Table XI. The solu- 
bility in water is slightly different from that given in the previous 
