338 



THE AGKICULTUKAL NEWS. 



November 3, 1917. 



the tamarind leaves to the soil did not seem to have 

 had any influence one way or the other on the growth 

 of the plants. It was thus pla^p that the tamarind soil 

 •contained something harmful to the growth of the crop. 



The soil was therefore analysed and was pi-oved to 

 contain an excess of soluble salts known to possess 

 a harmful effect on vegetation when present in such 

 amounts. 



Other observations confirmed this reasoning. 

 A row of bamboos was cut down, and the land brought 

 under cultivation, but wherever the clumps of bamboo 

 had stood the soil was infertile, and the surface in some 

 places was covered with saline deposits. 



A sample of soil was taken from a spot where 

 a tamarind tree had been cut down. The soil was 

 analysed at different depths with the result that sodium 

 sulphate was found in large percentage, but decreasing 

 with the depth. 



As has been noticed above, some trees are com- 

 paratively harmless to vegetation growing beneath 

 them. It seemed probable, therefore, that the roots of 

 some trees might contain more alkalies than others. 

 An analysis accordingly was made of the roots of 

 bamboo, which acts deleteriously on the soil in its 

 neighbourhood, and of Dalhergia sissoo, which was one 

 of the comparatively harmless trees, with the result 

 that although 'sissoo' is more innocuous in effect, its 

 roots contained more alkaline constituents than those 

 of bamboo. Evidently, therefore, the cause of the in- 

 fertility did not lie in the alkalinity of the root 

 constituents of the tree. 



Further experiments were made in the direction 

 of a suggestion that possibly the explanation of the 

 phenomena might be in the movement of the solution 

 •of salts in the soil towards the tree. A growing tree 

 transpires an enormous amount of water, thus exercis- 

 ing a great power of suction, so to speak, on the soil 

 particles, because the roots have to abstract the water, 

 which is transpired through the leaves, from the soil 

 with which they are in contact. The .soil therefore 

 immediately near the roots is rendered drier than the 

 rest of the surrounding earth, and therefore a move- 

 ment of water from the wetter to the drier portions 

 is automatically set up. This movement goes on as 

 long as the tree grows, and along with the water 

 the dissolved salts in the soil also move towards the 

 tree in obedience to the influence of the roots. 



To find out how far the infertility of soils under 

 certain trees was due to the over-accumulation of 

 soluble salts, samples of such soils from various places 



were examined, and the content of soluble salts together 

 with the amount of m' isture in the soils determined. 



From this examination it was clearly shown that 

 infertility was not caused by a deficiency in moisture, 

 for in some cases, owing to the nature of the soil, the 'bad' 

 soil contained more moisture than the 'good' soil. The 

 amount of water retained by a soil, and the rate of 

 movement of that water are chiefly dependent on the 

 nature of the soil. 



The result of the analyses of the soils seems to 

 show that the greater amount of soluble salts is found 

 in the upper layers of soil nearest the trees exercising 

 the deleterious influence, and that there is a distinct; 

 movement of soluble salts in the soil towards such 

 trees. Samples of soil tiken in a straight line from 

 just beneath a tamarind tree to .50 feet away showed 

 a regular decrease in the content of soluble salts as 

 the distance from the tree increased. 



In estimating the causes of stich harmful accumu- 

 lation of soluble salts many points have to be considered. 

 In the first place it mu*t be remembered that only the 

 dissolved salts can practically exert any influence on 

 the roots in the suil, and that therefore the moisture 

 content of the soil is to be taken into account, a-, given 

 the same amount of salts, the less moisture, the more 

 concentration, and vice versa. 



The texture of the soil has also weight in this 

 connexion. Loiimy soils can contain innocuously 

 a larger amount of salts than sandy soils can. And from 

 the point of view of getting rid of soil toxicity, the 

 texture of the soil is of great importance, because an 

 open soil can much more easily be rid of saline accumu- 

 lations by drainage and irrigation. Again, the nature 

 of the accumulated salts is of importance. Some salts 

 are more deleterious than others, and some crops can 

 tolerate and thrive upon amounts of alkaline materials 

 which would mean death and destruction to others. 



As will have been gathered, the problem of the 

 cause of infertile soil by the growth of certain trees 

 is complicated, and by no means fully explained. It 

 would seem, however, that this infertility is caused by 

 the presence of soluble salts in injurious amounts. Ifc 

 would also appear likely that, though this accumulation 

 of soluble salts may be due in some measure to the 

 leaching out of the mineral matter in the decayed 

 leaves and roots of the tree, yet that the transpiration 

 of the tree, causing a flow of moisture and its contained 

 dissolved .salts towards the tree roots, is the most 

 important factor. If this be so, plants having thin leaves 

 like the tamarind, and therefore possessing a high 



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