Scientific Agriculture. 



550 



[Juke 1908. 



6. There is one other point, however, 

 which must be borne in mind, and which 

 brings ine to the second consideration in 

 my subject. These lands which have 

 been water-logged for an indefinite 

 period may also have suffered from the 

 evils of " alkali." 



7. The term is rather an ambiguous 

 one, but its application in this connec- 

 tion is generally well understood. The 

 salts known as " alkali" are almost 

 all salts of soda — chloride, carbonate, 

 and sulphate. Some salts of potassium 

 also appear, but they need not be 

 considered as a rule. The sodium car- 

 bonate is the " black alkali," and the 

 most harmful of all. I quote below 

 remarks by Mr. Herbert M. Wil-ou, 

 formerly Geographer to the U. S. Geolo- 

 gical Survey. <fec: — 



" Where the natural drainage of the 

 country is defctive, and the strata under- 

 lying the surface are impervious or the 

 soil not deep, irrigation or rainfall causes 

 the sub-suriace water-plane to rise to 

 such a height that finally the soil be- 

 comes saturated. Evaporation then 

 takes place from the surface, and as this 

 process continues there are left on the 

 soil the salts contained in the water. 

 Thus, the more water that evaporates 

 from the surface, the more alkali will 

 be deposited, and increased rainfall or 

 irrigation will increase the amount of 

 alkali. It is thus seen that the direct 

 cause of the production of alkali is the 

 rise of the sub-surface water-plane, due 

 to defective drainage, and the evapor- 

 ation of water from the surface. * * * 

 Professor E. W. Hilgaru's experiments 

 show that the main mass of alkaline 

 salts exists in the soil within a short dis- 

 tance of the surface, and that the amount 

 of these salts is limited. * * * 



Several preventives for the rise of 

 alkali or the excessive soaking of the soil 

 have been recommended, and some have 

 been employed with success. Since 

 evaporation is the cause of rise of 

 alkali, the chief preventive is by reduc- 

 ing this to the lowest point. This may 

 be done by mulching the soil. It is also 

 possible to cultivate deep rooting plants 

 or such as shade the soil and reduce the 

 amount of evaporation or such as are 

 least harmfully affected by alkali, thus 

 mitigating the evil and permitting some 

 use to be made of the land. Irrigating 

 only such lands as have good natural 

 drainage, and exercising care not to in- 

 terfere with this* is one of the best and 

 surest preventives of the production of 

 alkali and water-logging. The introduc- 

 tion of artificial drainage produces the 

 Bame effect, while in a lesser degree the 

 same result may be obtained by the use 



of deep ditches or furrows which them- 

 selves act as drainage channels. 



When the quantity of alkali is small, 

 the evil effects resulting from its pre- 

 sence may be mitigated by the applica- 

 tion of chemical antidotes, and lastly, 

 relief may be obtained in some cases by 

 watering the surface and draining off 

 the water without allowing it to soak 

 into the ground. This system of surface 

 washing and draining off the salt-im- 

 pregnated water is known as 'leaching.' 



* # * 



A cheap antidote for many alkaline 

 salts is common lime, while neutral cal- 

 careous marl will answer in some cases. 



* # * 



Notable experiments have been made 

 by Professor E. W. Hilgard, which 

 prove the value of gypsum in neutraliz- 

 ing ' black alkali ' or carbonate of soda. 



* * * 



An excellent preventive against eva- 

 poiation from the soil surface and the 

 consequent production of alkali is ' mulch- 

 ing.' The best mulch is a well and 

 deep-tilled surface soil, which is kept so 

 constantly stirred that a crust is never 

 allowed to form. Ploughing in large 

 quantities of straw produces also an 

 effective mulch. Tne depth or thickness 

 of this protective tilled layer is of the 



utmost importance After a proper 



tilling to a depth of, say, 10 to 12 inches, 

 it requires a long time for the salts to 

 come to the surface again in sufficient 

 amount to injure the crop.* * * 



Leaching is not infrequently employed, 

 more especially in Europe. This is prac- 

 tised by building temporary embank- 

 ments round the land and then flooding 

 it, after which the salt-impregnated 



waters are rapidly drawn or pumped off. 



* * * 



One of the most effective plants which 

 can be grown on slightly alkaline soil is 

 ' alfalfa,' which, when once established, 

 brings to bear the action of deep roots 

 and dense shade." 



8. It may be generally taken for 

 granted that water supplied to land in 

 the form of irrigation is only the mechan- 

 ical agent in the production of alkali, 

 and those districts which are wind-swept, 

 such as Jaffna, Mannar, Puttalam, and 

 Hambantota, and where evaporation is 

 great, are most likely to suffer from 

 alkali. The soil, however, may modify 

 this to a certain extent, as evaporation 

 varies considerably in extent with the 

 class of soil. In some experiments made 

 in 1873 it was found that where the mean 

 evaporation from water was 20'4 inches, 



