EESISTANCE TO ALKALI. 75 



rect idea of the alkali condition of the soil in relation to crop produc- 

 tion, since the most important factor in reference to plant growth is 

 the degree of concentration of the soil moisture. Inasmuch as the 

 water capacity varies greatty in different types of soils it is easily 

 possible that ,two soils having the same percentage of alkali by weight 

 may differ very greatly in their ability to support crop plants sensitive 

 to alkali. Thus in a coarse sandy soil having a low water content the* 

 concentration of the soil moisture may be three or four times as great 

 as in a heavy clay soil having a correspondingly greater water capacity. 



Fortunately it is now possible to determine quickly and accurately 

 the degree of concentration of the soil moisture with tho ingenious 

 instrument devised by Professor Whitney and Mr. Briggs, by meas- 

 uring the electrical conductivity of a column of soil saturated with 

 water. a While this method shows approximately the degree of con- 

 centration of the soil water to which the roots of plants would be 

 exposed, it gives no indication as to the composition of the alkali, 

 which often varies greatly in soils only a few rods apart. Inasmuch 

 as different sorts of alkali vary greatly in their poisonous action on 

 the roots of plants, the needs of the biologist and agriculturist would 

 be served best by the employment of both methods, the electrical giv- 

 ing the concentration of the soil water; the analytical, its chemical 

 composition. At the same time a physical analysis of the soil show- 

 ing the water capacity would be useful in forecasting the danger of an 

 increase in alkali content through the evaporation of saline irrigation 

 water or by a rise of alkali from the subsoil. 



The soils secured from the Sahara, with the exception of the one 

 above mentioned from Biskra, were all similar in nature, being com- 

 posed of sandy loam or fine sand. In consequence the results of the 

 anatyses reported in this bulletin are fairly comparable one with 

 another and are not likely to lead to an overestimate of the alkali- 

 resisting power of the date palm, since the water capacit}^ of these 

 soils is low, and as a result of this the concentration of the soil water 

 is high in proportion to the percentage of alkali present in the soil. 

 The limits of alkali resistance worked out in this bulletin are then 

 directly applicable to the soils best adapted to date culture, viz, sandy 

 loams, and for all other heavier soils are below rather than above the 

 true limit/' 



a This method depends upon the degree of ionic dissociation, rather than the total 

 content of dissolved substance, and gives the best physical measure of the relative 

 concentration and toxicity of solutions of similar composition. 



& Very coarse sand would- have a lower water capacity than the Saharan soils here 

 studied, but alkali leaches out of coarse sand very easily, so that in such soil a dan- 

 gerous accumulation of alkali is not common, though if present the limits here 

 determined for the alkali resistance of the date palm would be too high because of 

 the excessive concentration of the soil water in proportion to the percentage of alkali 

 present. 



