MODERN HIGH FARMING. 91 



cent, guaranteed strength is equal to 26.16 per cent, of tribasic 

 phosphate of lime, but not of biphosphate, or monophosphate, as 

 might be easily supposed, and of which no more than about 17 per 

 cent, would be present. 



When buying phosphatic manures, it is always well to have a 

 distinct understanding as to the word soluble, as many makers pro- 

 duce an article which, though only partially soluble in water, 

 readily dissolves in citrate of ammonia. It frequently happens that 

 a superphosphate, even when made with all care and skill, suffers 

 from the subsequent reaction of the iron and alumina, what is called 

 a retrogradation or going back to insolubility, and when this is so, 

 becomes only partially soluble in water, while remaining entirely 

 soluble in the other liquid named. 



INSOLUBLE PHOSPHATES only occur in cases where imperfect 

 decomposition and want of skill have characterized the manipulation ; 

 and as no immediate benefit can be derived from applying them to 

 the soil in this form, which cannot be as readily attained by using 

 raw phosphates, the value should be brought down to the actual 

 market level of the latter. Presuming, for instance, the soluble 

 phosphates to be worth one dollar per unit, these insoluble matters 

 must not be charged at a higher rate than twenty to twenty-five cents. 



SULPHATE OF LIME results from the process which goes on in 

 the mixers already described. It invariably exists in large propor- 

 tions in all superphosphates, and its chemically great affinity for 

 water facilitates the drying of compounds which, in many cases, 

 would otherwise retain the form of paste. Its efficacy is well recog- 

 nized, and, as plaster of Paris, or baked gypsum, it is frequently 

 purchased and applied with good results to grass lands. 



This gypsum is not taken into consideration, in fixing the value 

 of a manufactured manure, and the farmer who has been in the 

 habit of buying plaster of Paris, may cease this outlay from the day he 

 commences to use superphosphates, it being no longer necessary. 



ALKALINE SALTS generally consist of sulphate of soda and potash, 

 in ordinary mineral superphosphates, and in their case are too in- 

 significant to require any special attention. 



