156 PHOSPHATIC FERTILISERS [chap. 



purchased as are contained in the ton of dissolved 

 bones, and the phosphoric acid would be wholly soluble 

 instead of partially, as in the bone manure ; there is a 

 saving of los. 6d. a ton, against which would only 

 have to be offset the greater carriage on the extra 

 8 cwts. the mixture weighs. 



This, of course, is only an example of the general 

 fact that the longer a fertiliser has been known, and the 

 greater the number of people who have had experience 

 of its value and learnt how it can be profitably employed, 

 the better will be its standing in the market, and the 

 higher its price per unit of nitrogen, phosphoric 

 acid, etc. 



It has already been stated that finely-ground rock 

 phosphates are occasionally emplo)ed as fertilisers 

 without any treatment with acid. In America they 

 are often obtainable very cheaply in comparison with 

 superphosphate and arc ground to an extremely fine 

 powder, so that on the basis of equal money value they 

 give more favourable results on many soils than the 

 acid phosphates. In Britain they have proved most 

 effective on wet and peaty soils ; they require a soil 

 containing plenty of organic matter to generate a 

 comparatively strong solution of carbon dioxide in the 

 soil water, in which they must become dissolved. P'or 

 the same reason their action is forwarded by the 

 ploughing in of green crops or by the use of sulphate 

 of ammonia as a source of nitrogen, but on the generality 

 of soils they form but an ineffective source of phosphoric 

 acid. 



It will thus be seen that it is impossible to arrange 

 the phosphatic fertilisers in a scale of value depending 

 upon the relative availability of the phosphoric acid 

 they contain, for this availability is mainly determined 

 by the soil, and will vary for the same manure from 



