EXPEEIMENTAL 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 

 the 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 

 phosphat«3 than to bone, basic slag, or raw rock phosphate. There- 

 fore, where the early stimulation 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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