64 KEPOET OF CHEMICAL LABOEATOKY 



require no special discussion here. A fact Avhich is, however, very generally overlooked in 

 the Sudan, is that the penetrabilitij to roots should extend as far heloio the surface as possible. 

 Gypsum, like other soluble lime compounds, has a marked effect in loosening heavy clayey 

 soil. Being more readily soluble than, e.g., calcium carbonate, it is more quickly removed in 

 the drainage water and the effect is, therefore, less lasting. At the same time it must be 

 remembered that once the soil is made more penetrable and the roots have reached a greater 

 depth, the fact of their presence, and the introduction of more organic matter into the soil, 

 renders the penetrability easier to maintain. 



The effect of clay in lessening the cohesion of heavy soils was clearly shown by 

 the results of trials made recently in this laboratory. Several soils were made into bricks, 

 in the one case with pure water, and in the other with water containing gypsum. The clay 

 in the soil was, of course, partially puddled in the operation. After these bricks had 

 thoroughly dried, their transverse breaking strain was determined. In three out of four 

 cases the breaking strain of the bricks which contained gypsum was very markedly less 

 than that of those made from the untreated soil. It is evident, therefore, that a plant root 

 would usually lind a gypsum-treated soil more readily penetrable than the original soil. 



(d) Gypsum shares the effect, exhibited by certain otlier soluble compounds, of 

 rendering the potash of the soil constituents more available. The gypsum, being 

 comparatively soluble, easily penetrates the soil and acts upon the jaotassium compounds, 

 especially the hydrosilicates (zeolites) formed as the result of rock decomposition under 

 atmospheric influences. The result of this action is a partial substitution of calcium for 

 potassiiim, the latter being set free in soluble form, available as plant food. 



(e) It has already been mentioned that lime (or magnesia), in the form of carbonate, 

 is essential to nitrification, and that these act by neutralising the nitric acid as fast 

 as it is formed, thus preventing its accumulation and the consequent inhibition of the 

 growth of the nitrifying micro-organisms. Calcium sulphate has been found to have a 

 remarkably stimulating effect on the growth and multiplication of these bacteria. In 

 this respect it has been stated by Pichard ' to be more effective than any other 

 substance known. Taking the effect of gypsum as the maximum, he found that, other 

 things being equal, the amounts of nitrates formed were as shown in the table below — 



Gypsum 100 



Sodium sulphate ... ... ... ... ... 47'9 



Potassium sulphate. . ... ... ... ... 35-8 



Calcium carbonate ... ... ... . ... 13-3 



Magnesium carbonate ... ... ... ... 12-5 



Hilgard has noted that the above results are confirmed by his observations on the 

 soils of California. The explanation of the increased nitrification in tlie j'l'esenee of 

 gypsum appears to be due to the fact that the latter acts as a food for the micro- 

 organism. 



The importance of this effect of gypsum in the treatment of many Sudan soils may 

 be very great. There is considerable evidence to show that certain of them are, from 

 one cause or another, markedly deficient in nitrifying power. The application to these 

 of gypsum is well worthy of a trial from this point of view alone. 



(/) The partial exhaustion of soils by the continuous removal of the same variety of 

 crop, without rotation, cannot be explained simply as the result of abstraction of plant 

 food. Several causes are probably at work and there is evidence to show that among 



' Quoted in Soils by Hilgard 



