EXPERIMENTAL WORK WITH RAW ROCK PHOSPHATE. 119 
(1) The conventional laboratory methods so far proposed for de- 
termining the availability of phosphoric acid in various phosphates, 
while of some use, do not necessarily serve as an index to its avail- 
ability under soil conditions. 
(2) Field experiments conducted for only one or two years, where | 
@ithe various fertilizer treatments are not replicated or where no index 
is given to the relative natural fertility of the various plots employed, 
have little or no meaning. 
(3). The application of liberal and even medium quantities of 
raw rock phosphate to most soils produces an increase in the yields 
of many crops the first year. 
(4) The effectiveness of raw rock phosphate depends largely on 
its thorough distribution in the soil, this distribution being brought 
about by liberal applications of very finely divided material and | 
thorough cultivation. 
(5) The presence of decaying organic matter in the soil increases 
the effectiveness of ground raw rock phosphate, owing probably both 
to greater bacterial activity and the higher content of carbon dioxide 
in such soils. 
(6) As a corollary of (4) and (5) the effectiveness of raw rock 
phosphate is usually increased after remaining in the soil for a 
year or more. 
(7) Most crops respond more quickly to applications of acid 
phosphate than to bone, basic slag, or raw rock phosphate. ‘There- 
fore, where the early Simulation and quick maturity of the crop are 
the main considerations, acid phosphate is probably the best form of 
phosphoric acid to apply. 
(8) Field experiments in which raw rock phosphate and acid 
phosphate are compared on the basis of equal applications of the two 
materials or on equal applications of phosphoric acid in the two 
forms result often in favor of acid phosphate (particularly when such 
experiments are conducted for a short period), as in order to get the 
maximum benefit from the natural phosphates they must be applied 
at a rate far exceeding that at which acid phosphate proves effective. 
(9) The question whether increases in yield can ordinarily be.pro- 
duced more economically by applications of the soluble or relatively 
insoluble phosphates must be considered in a measure a separate 
problem for each farmer, since it depends on a number of factors 
of which the most important are the nature of the soil, the crop system 
employed, the price of the various phosphates in each particular 
locality, and the length of the growing season. 
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