Scientific Agriculture. 



564 



[December, 1908 



irrigated tracts in India gave full oppor- 

 tunity for a number of observations to 

 be taken, and on the strength of these, 

 experiment was resumed both in the 

 field and in water culture. 



Conclusion. 



The bearing of the phenomena des- 

 cribed in this article on the question of 

 rotation of crops is obvious. 



The question may, however, be put 

 why cotton, for instance, which grows 

 so feebly near sorghum, growsjat least 

 as well, if not better, after sorghum than 

 after cotton. From experiments now in 

 progress it appears that this is explicable 

 as follows : — 



When cotton is growing near sorghum 

 the roots of the latter exude the toxic 

 substance into the soil in large quanti- 

 ties. This spreads rapidly through the 

 soil into the subsoil especially during the 

 rainy season, and neighbouring cotton 

 plants are not protected by the fact 

 that their tap roots go down far below 

 the zone in which the sorghum roots are 

 situated. When cotton follows sorghum, 

 however, the condition of affairs is 

 different ; the toxic substance remaining, 

 at the time of harvesting, in the roots of 

 the previous sorghum crop is now being 

 given out slowly in the course of the 

 decay of those roots,* and is held and 

 entangled in the organic matter of the 

 roots, largely in the zone of soil in which 

 the roots of sorghum spread. Each crop 

 thus fouls the soil for a crop of the same 

 variety, whose roots will take the same 

 course as the previous crop, more than 

 for a crop whose roots spread in another 

 layer of the soil. 



The precipitation of the toxic substance 

 by most of the mineral manures in 

 common use indicates the manner in 

 which many manures act in increasing 

 crop yields. 



While this note has been going through 

 the press, I have received Bulletin No. 40 

 of the Bureau of Soils (United States of 

 America), by Messrs. Schreiner and Reed 

 on " Some factors influencing soil ferti- 

 lity." In this Bulletin the authors come 

 to the conclusion that " the excreta of 

 the cow pea roots are very slightly toxic 

 to roots of wheat seedlings," and that 

 "the excreta of oats are more toxic to 

 the roots of wheat seedlings, than those 

 of corn or cow peas — a conclusion that 

 is substantiated by the results obtained 

 in crop rotations." 



* That the roots of sorghum and other crops 

 exert an extraordinarily toxic effect when mixed 

 with soil in which plants are then grown has bean 

 proved by the writer in a set of pot experiments. 



The experimental data given in the 

 Bulletin do not justify these conclusions, 

 but only indicate that the excreta from 



cow peas when in the arbitrary concen- 

 tration obtaining in their experiments 

 are very slightly toxic to roots of weed 

 seedlings when these latter are at the 

 stage of growth oj those used in the 

 experiment. 



I find that very young plants are not 

 affected by a toxic solution of given 

 strength so rapidly as older plants, doubt- 

 less because the latter, owing to more 

 rapid transpiratiou, take in the toxic 

 substance in larger quantities. 



Again, the impossibility of the state- 

 ment made with regard to the excreta 

 of oats being more toxic than that of 

 cow peas or corn is self-evident, im- 

 plying as it does that any quantity 

 however great from oats is more toxic 

 than any quantity however small from 

 coav peas or corn. 



The media that were compared con- 

 tained quite arbitrary amounts of 

 excreta from an arbitrary number of 

 plants growing tor an arbitrary period, 

 it being stated that " the agar containing 

 their excretions was obtained in each 

 case by planting a large number of 

 seedlings in a dish of soft agar and 

 allowing them to grow for eight to 

 fifteen days according to the kind of 

 plant employed." 



There are no data in the Bulletin under 

 reference which indicate that the excreta 

 from all the plants tried are not identical 

 in character. — Memoirs of the Department 

 of Agriculture in India, Vol. II., No. 3. 

 April, 1908. 



LINING OF DITCHES AND RESER- 

 VOIRS TO PREVENT SEEPAGE 

 LOSSES. 



In a recent bulletin of the California 

 Station, E. Mead and B. A. Etcheverry 

 call attention anew to the enormous 

 waste of irrigation water due to seepage 

 from ditches and reservoirs. They show 

 from general observation and from a 

 large number of careful measurements 

 that "the water Avhich sinks into the 

 soil from ditches and reservoirs is one 

 of the chief sources of waste in irriga- 

 tion. In gravelly soils, or where ditches 

 cross gypsum strata, the losses some- 

 times amount to more than half the 

 total flow." Measarements made on a 

 large number of ditches in the course 

 of the co-operative irrigation investiga- 

 tions of the Office of Experiment Sta- 

 tion and the California Station "show 



