84 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



the test is conducted in such manner as to keep the moisture uniform 

 throughout the soil, and hence uniform for the deepest and shallow- 

 est-rooted seedlings. 



It is also self-evident that specific differences may be brought out 

 by one set of conditions, which would not be apparent in another 

 set, particularly conditions which make the requirement for. mois- 

 ture great or small. If transpiration is very rapid, seedlings of a 

 shallow-rooted species may be unable to meet this demand, while 

 deeper-rooted seedlings in the same pan may pull through, because 

 their supply at this stage is somewhat more readily obtained. For 

 an actual test of drought resistance, therefore, it is fundamentally 

 necessary that the transpiration and soil-drying process should be 

 slow enough to permit equalization of the opportunities before the 

 critical test comes. Hence the standard conditions of exposure 

 which have already been suggested. 



The method of recording the death of each seedling in a lot of 

 100, together with the pan weight and calculated moisture accom- 

 panying such death, has a distinct advantage over the method which 

 permits only one determination of the moisture content when all 

 of the seedlings, or a majority of them, have succumbed. It gives 

 an indication of the possible variation between individuals of the 

 same species, and a measure of the probable experimental error 

 due both to this variation and to uneven distribution of moisture 

 in the pan, which is not wholly avoidable. What it really amounts 

 to is practically 100 separate tests on 100 sections of soil. If, on the 

 one hand, the first losses occur in sections of the soil ""which have 

 unavoidably become drier than the average, on the other hand, the 

 last survivors are undoubtedly in areas which are at the opposite ex- 

 treme. These variations should be largely compensated by taking 

 the algebraic mean of all the moisture determinations, a figure in 

 which a great deal of confidence can be placed. 



INDIRECT METHODS FOR WILTING COEFFICIENTS. 



Inasmuch as the direct determination of the wilting coefficient is 

 a process which is likely to require several weeks, at the best is liable 

 to rather large experimental errors, and is also, without question, in- 

 fluenced by the kind of plant used, various methods have been devel- 

 oped by which the affinity of the soil for water may be determined; 

 and the amount of water held by it under certain empiric conditions 

 of the test may be related to the amount which would be held against 

 the pull of plants. 



In addition to furnishing a ready, if only approximate, index to 

 the soil conditions which may be encountered in the field, and espe- 

 cially an index to the danger of early drought, it seems that the use' 

 of indirect methods, employing definite physical forces for the crea- 



