27 
to draw from and made a better growth than those fertilized with 
soluble phosphates. The most convincing proof of this statement is 
furnished by comparing the results of P 1/2 with those of P t , P x 1/2 , and 
P 2 . In the second millet crop P 1/2 phosphate rock gave the best 
growth, P 1/2 soluble phosphate being comparatively more exhausted. 
By comparing P x 1/2 soluble phosphate with P 2 phosphate rock the 
former is found to have been considerably more effective. Addi- 
tion of nitrogen and potash fertilizer failed to prevent the decrease 
in plant growth due to removal of available phosphates, but upon a 
further addition of phosphoric acid a marked stimulation resulted. 
It has been claimed that lime has a depressing effect upon the 
assimilation of phosphoric acid from phosphate rock. The results 
obtained in sand culture did not support this view. It is possible 
that the apparent depressing effect of phosphate rock in the red-clay 
soil is due primarily to the conditions in the soil unfavorable to the 
assimilation of the insoluble phosphates. This kind of phosphate 
fertilizer is rendered soluble and available as plant food mainly by 
the action of carbon dioxid and organic acids produced by the micro- 
organisms of the soil. Aeration in this soil is very low and carbon 
dioxid is present in almost negligible quantities. Hence if the 
amount of carbon dioxid present is a measure of bacterial activity, 
which is known to vary widely in soils according to physical and 
chemical conditions, then the primary agent upon which phosphate 
rock depends for its availability is lacking. A further factor may be 
found in the fact that the acidity of this type is not due to the presence 
of organic acids, but to conditions the nature of which is yet to be 
determined. 
The table shows the marked increase in assimilation of phosphate 
rock in the green-manured pots as compared with those unmanured, 
indicating that the fermentation of green manure and loosening of the 
soil assisted the millet in assimilating the phosphate rock. The same 
factors influence the availability of bone meal, which is greater in 
humic soils where acid-forming bacteria are present. 
The amount of phosphoric acid present in each pot, according to 
the absolute analysis of the soil, was approximately 18.0 grams, while 
that added as fertilizer ranged from 0.19 gram (P^) to 0.76 gram 
(P 2 ). The analyses of the plants show that the total amount of 
phosphoric acid removed by the two crops of millet and one of buck- 
wheat, in most instances is equal to that added at the rate of F 12 , 
in several instances to that at the rate of P 2 , and in one case to that 
at the rate of Pn/ 2 . The third crop, which was not analyzed, was 
the most stunted in growth of all the series, indicating, as the analyses 
show, that the major part of the available phosphoric acid had been 
removed by preceding crops, leaving only that naturally occurring 
in the soil. It is plainly evident that the millet was unable to 
