COMMERCIAL MANURES. 209 



the needs of the plants after being in the soil for a month or two, 

 and fully as fast as they require it ; for, although not soluble in 

 pure water, it is gradually soluble in rain water, which alwaj'^s con- 

 tains some carbonic acid. 



Suppose now another superphosphate is submitted to his analysis 

 which is made from a mineral or fossil phosphate, and which by his 

 process shows the same constituents in the same quantities and 

 proportions as the first. Most people would say at once, it must 

 have the same value ; and so it would so far as the soluble phos- 

 phoric acid is concerned, for this would act with the same prompt- 

 ness and efficiency whatever its source might be, but it would be 

 very different with that portion which he calls insoluble. This is 

 dissolved with ease in the powerful acids which the chemist 

 employs, and so he knows it is there contained, and he sets down 

 its quantity with precision and accuracy, but it is no more soluble 

 in rain water, nor of any more use to the plants than is the potash 

 which is contained in many rocks. 



We may pulverize feldspar, or granite, of which there are mil- 

 lions of tons all about us. Both contain potash in considerable 

 quantity ; but the ground rocks would not help plants grow as 

 ashes would, and why ? Because in one case it exists in a really 

 insoluble condition and not available to the plants, while from 

 wood ashes the plants can get it as fast as they need it. The differ- 

 ence in the effect of two such superphosphates as I have described, 

 provided they contained not less than three or four per cent, of 

 soluble phosphoric acid, might not be noticeable upon the crops 

 during the first year, or at least not until near harvest time, but 

 either at the harvest, or in the second and subsequent years, it 

 would be very marked. 



Take another illustration. Suppose a sample of pure, finely 

 ground raw bone be sent to the chemist for analysis. He will 

 report about 23 per cent, insoluble phosphoric acid in its earthy 

 portion, and in its animal matter, nitrogen equal to about 5 per 

 cent, of ammonia, and he says truly. Now let him analyse a 

 mixture of ground apatite (mineral phosphate) and leather chips. 

 He finds the same constituents as in the bone and in as large 

 amounts ; yet the latter will have about the same effect on crops 

 as so much gravel, and would be about as worthless, while pure 

 fine bone dust everywhere readily brings a price nearly equal to 

 that of good superphosphate. The reason is simply this, that the 

 bone, if fine enough, gives to the plants what they want, nearly as 

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