62 BEPORT OF CHEMICAL LABOEATOUY 



the chemical and [)liysical differences in the soils. The results are nevertheless interestinjj 

 and suggestive. Whether anything like the same benefit will accrue from the application of 

 gypsum to Sudan soils, can only be determined by trials on a practical scale ; but it may be 

 mentioned in passing that a number of pot cultures in the laboratory, with heavy soils, 

 showed marked improvement to result from the addition of moderate amounts of gypsum. 



The following is a brief account of the various effects which have been noted as 

 resulting from the application of gypsnm to soils, and which may account for the increased 

 yields which have been recorded in the table given on 2'«;/e CI. 



(rt) The harmful action of sodium carbonate on soils and on plant life is well known. 

 It appears to be the most injurious mineral ingredient found in otherwise good arable soils. 

 Its action is both direct, on the plant, and indirect, on the soil. On the plant it acts by 

 corroding the root, crown or stem and in some cases the latter is girdled, the bark being 

 completely destroyed. This effect is especially liable to take place when the salt accumu- 

 lates near the surface as the result of evaporation of the soil water. The effect on the soil 

 is no less injurious, a proportion less even than one tenth per cent being sufficient to destroy 

 the tilth of the soil and to render it unproductive. The effect is due to the defloceulation 

 (puddling) of the clay,, with the result that the soil becomes almost or quite impermeable to 

 water. 



In cases like the above, gypsum acts as a veritable specific. The impermeability of the 

 soil renders the simple washing out of the salt impossible ; but the addition of a small 

 amount of gypsum to the soil water suffices to convert the sodium carbonate into compara- 

 tively harmless sodium sulphate, the clay being coagulated and rendered permeable at the 

 same time. 



The fact that too little attention has commonly been paid to the drainage of irrigated 

 lands is a truism. When this lack of drainage results in the stagnation of the soil water — 

 i.e. insufficient aeration — sodium carbonate is almost invariably formed. Treatment with 

 gypsum should be the first step in the correction of such a condition. 



(b) The harmful effects of ordinary salt — in fact of practically all soluble salts, even 

 those employed as fertilisers — when present in any but very small proportions, are only too 

 well known. The case of one of these, sodium carbonate, has just been discussed, and its 

 destruction by the use of gypsum noted. The effect of gypsum in this case is usually 

 explained by a consideration of the chemical change which takes place when these two 

 bodies are brought together. It has, however, been discovered that the inhibition of the 

 injurious action extends to other salts as well, a fact all the more remarkable as no 

 adequate explanation can as yet be given for it. In an investigation' of the effect of alkali 

 soils on vegetation carried out in 1902 by Kearney and Cameron, of the United States Dept. 

 of Agriculture, a very important fact was recorded in connection with calcium sulphate. 

 It was found that while certain very small jsroportions of salts, such as common salt, 

 sodium sulphate, magnesium chloride and sulphate, were distinctly injurious to plant life, 

 in the presence of calcium sulphate these proportions may be greatly, in some cases even 

 enormously, exceeded, without apparent injury. Thus, under the conditions mentioned, the 

 maxima of concentration of the salts endurable by the plant were increased as follows : — 

 Magnesium sulphate ... ... ... 480 times 



chloride 80 ,, 



Sodium sulphate ... ... .. ... GG 



chloride ... ... ... ... 10 ,, 



' "Snnio nintii.ll relations between alkali soils and vegetation." Bulletin No. 71, 1902 



