COMPOUND AND MISCELLANEOUS MANUEES 291 



Gypsum. This substance may be used to supply 

 calcium salts to soils that are deficient in that constituent, 

 if any such exist, and it is supposed to be good for legu- 

 minous crops. In no sense is it a substitute for lime, 

 and the author has never seen any beneficial effect 

 produced by it. 



Copperas. It is very difficult to believe that copperas, 

 the common sulphate of iron, has any direct manurial 

 value. Nevertheless, some remarkable effects have been 

 produced, apparently, by the application of this substance 

 to potatoes. It has also been recommended for cabbages, 

 cereals and grass. As a reducing agent it might be ex- 

 pected to prove harmful, and only small quantities 

 about -J cwt. per acre must be used. On open soils 

 very little danger is to be apprehended from this cause 

 as it is rapidly oxidised. The fertilising properties 

 claimed for ferrous sulphate may, perhaps, be due to 

 catalytic action of the ferric oxide, so formed, upon the 

 humus of the soil. 



Magnesium Salts. Magnesium sulphate has been 

 occasionally used, but the cases in which this element is 

 deficient in the soil are so rare that the subject need not 

 be further discussed. Kainite and most of the potash 

 salts contain quantities of magnesium compounds. 



u2 



