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an index of exhaustion in other elements of plant food. But manuring 

 with lime alone will be found advantageous, mechanically in improving 

 the texture of heavy clays, and chemically in unlocking stores of plant 

 food at present insoluble and so incapable of being utilised. 

 ^ Dr. Phipson has come to the conclusion that the " degree of exhaus- 

 tion which a cane soil has undergone can, to a great extent, be ascer- 

 tained by comparing the relative amounts of lime and magnesia yielded 

 on analysis." He gives four examples " from the same estate in British 

 Guiana, from various portions of which the sample were taken : 



Cultivated. 



10 to 15 years. Upwards of 60 years. 

 Lime (per cent). ... 0.44-0.64 0.11-0.40 



Magnesia ... 0.32-0.50 0.36-0.51 



It will be noticed how the lime has disappeared (from the same soil) 

 by prolonged cultivation of the cane, whilst the magnesia has remained pret- 

 ty much as it was." Dr. Phipson concludes that : " when the quantity of 

 lime has diminished so much by prolonged culture as to be present to 

 the extent of only 0.1 per cent., and then only one-third that of the mag- 

 nesia present (knowing that in the origin the lime was not only equal to, 

 but higher than the magnesia), we may rest assured that the crops of 

 cane on this soil will fall off year by year, and that the most careful 

 system of manuring will be necessary to place it again in its former lu- 

 crative condition." 



Dr. Griffiths in his " Treatise on Manures" discusses the value of mag- 

 nesia in addition to lime. He says, " The office of magnesia is to assist 

 in the starch forming process and the development of chlorophyll, and 

 its presence is necessary to healthy growth and colour. Magnesia forms 

 a very important constituent in all soils in which the French vine resists 

 the attacks of Phylloxera vastatrix. We find, also, that the American 

 vine flourishes best in those soils containing a high percentage of mag- 

 nesia. The amount of magnesia in the ash of the Styrian vine, according 

 to an old analysis, is 6.55 per cent. Therefore, from soils deficient in 

 this ingredient, it is impossible to obtain full or healthy crops. Pro- 

 bably the ravages of the Phylloxera may be traced to the growth of sickly 

 plants season after season, the unhealthy nature being due to the want 

 of small quantities of such mineral ingredients in the soils cultivated as 

 iron and magnesia. Magnesium sulphate has also proved a beneficial 

 manure for clover and potatoes, as well as for corn crops. It would be 

 well for agriculturists to try the use of magnesium sulphate along with 

 nitrates of soda and phosphates for corn crops. By this means, most 

 likely, many crops may be saved from the attacks of parasitic mildews. 

 The author has found this to be so in the case of wheat grown with small 

 quantities of iron sulphate. 



"It has also come to the author's notice that many of the sugar cane 

 soils in British Guiana and the West Indies suffer more or less exhaus- 

 tion from an insufficient amount of magnesia and lime. Both lime and 

 magnesia play an important part in the growth of the sugar cane. Sul- 

 phate of ammonia has been, and is, largely used in Demerara as a manure 

 for the sugar-cane, but its action is the production of a juice very deficient 



