March 30, 1917] 



SCIENCE 



309 



deficient, but this, it seems to me, gives no 

 justification for the following statement, which 

 I quote from their paper : 



An impartial statistical study of the data from 

 the individual orange groves shows that approxi- 

 mately one half the mottling can ie accounted^ 

 for by the low humus content of the soO. 



3. That all or nearly all citrus soils in 

 southern California are deficient in organic 

 matter has long been known. But to state that 

 half of the mottling " can be accounted for " 

 by deficiencies of the soil in humus when the 

 other half of the mottling is not at all ac- 

 counted for seems to me to be an unusual pro- 

 cedure. 



4. Moreover, the method employed by 

 Briggs, Jensen and McLane for determining 

 humus, upon which much of their discussion 

 depends, has already been pointed out by 

 Gortner^ to be insecure if not entirely inaccu- 

 rate. In the writer's laboratory it has also 

 been found that intensity of color is no crite- 

 rion of the amount of humus. Moreover, no 

 one has yet proved, and there is no justification 

 for believing that the humus portion of the 

 soil organic matter, as determined by any of 

 the arbitrary methods in vogue, is of any 

 greater value to plants or to soils than the rest 

 of the soil organic matter. 



5. That as the paper under discussion points 

 out the total nitrogen content of soils is not 

 related to the amount of mottling should be 

 no cause for surprise since it is the amount of 

 " available " nitrogen as the writer has on 

 many occasions pointed out rather than the 

 amount of total nitrogen that should reason- 

 ably be assiuned to affect plant growth. This 

 is especially true under arid soil conditions, in 

 which, moreover, the term " available " pos- 

 sesses more than the usual significance. 



6. It seems to the writer that we need a 

 theory or theories on some definite and specific 

 cause of " mottle-leaf " in citrus trees and not 

 a description of some general condition like a 

 deficiency of organic matter which can affect 

 soils in many different ways, not always in the 



5 Italics mine. 



e"Soil Science," Vol. 2, No. 5, p. 395, Novem- 

 ber, 1916. 



same direction, and which besides is uni- 

 versally recognized to constitute the most un- 

 desirable feature of arid soils. 



7. As Briggs, Jensen and McLane point out, 

 however, something which affects chlorophyll 

 formation in the leaves of the citrus tree is 

 responsible for the trouble. That factor, in 

 my opinion, is a lack of usable nitrogen, and 

 in view of the peculiar mineral conditions of 

 our soils, it may in many instances also be due 

 to a lack of usable iron. 



8. The writer does not wish to be understood 

 as denying the effectiveness of a lack or of a 

 sufficiency of organic matter in the produc- 

 tion or eradication, respectively, of mottle-leaf 

 in citrus trees. He does desire, however, to 

 deny that there is anything specific about the 

 organic matter factor, since it can affect 

 plants in one of so many different ways; that 

 the portion of the soil organic matter known 

 as humus is any criterion as to the activity 

 and value of the soil organic matter; that the 

 " mottling of orange trees has been definitely 

 correlated with the low humus content of the 

 soil per se; and that soluble organic matter 

 placed in the zone of the feeding roots prom- 

 ises any better for the eradication of " mottle- 

 leaf " than the practise of green manuring 

 which, to put it mildly, has thus far fallen far 

 short of the expectations originally entertained 

 for it. 



9. As I have pointed out in the papers above 

 cited, we shall probably be compelled not only 

 to supply sufficient available nitrogen to eradi- 

 cate the physiological troubles of our citrus 

 and other crops, but we shall have to make it 

 usable by some method of soil protection 

 which will make it possible for roots of plants 

 to make use of the surface soil. The most 

 promising method of soil protection now seems 

 to be complete straw mulching. 



Chas. B. Lipman 

 Soils Research Labohatort, 

 Universitt of California 



lord lister on the value of vivisection 



To THE Editor of Science: The enclosed 

 rough draft of a letter to " Dr. Keen " (as the 

 envelope was endorsed) was found among the 

 late Lord Lister's papers by his nephew and 



