Maech 25, 1921] 



SCIENCE 



275 



as this basis of measurement may be, it cer- 

 tainly has some correlation with productivity. 

 Indeed it may be said that chemical methods 

 of soil examination resulted in a great neglect 

 of the study of the more obvious character- 

 istics. Unquestionably the best index of the 

 quality of a soil is its productivity in crops. 

 Analyses of soil from good spots and from 

 poor spots in the same field have sometimes 

 revealed no differences. This fact and others 

 led to the concept that productivity might be 

 lessened not only by the absence of a nec- 

 essary element but the presence of a deleter- 

 ious agent, and that fertilizers were in some 

 cases at least substances that inhabited the 

 injurious factor. It is a long story to con- 

 sider this subject, but viewed purely as a 

 theory it can explain some things not clarified 

 by the plant food theory. It is well to re- 

 member that many investigators who con- 

 sidered the effects of nitrogen, phosphorus and 

 potash as due solely to additional plant food, 

 nevertheless regarded the effects of lime as 

 partly at least due to overcoming an un- 

 favorable factor, and the action of still other 

 materials as stimulants, without clearly de- 

 fining what they meant by stimulants. The 

 available facts were simply the addition of 

 the substance and the end reaction of the 

 plant. The different roles postulated are in 

 the main hypotheses, and the existing body of 

 facts certainly is insufficient to prove any one 

 of the simple theories. A broader view now 

 coming to be widely held is that the soil is a 

 complex of very numerous factors, good, bad, 

 or indifferent so far as a particular plant is 

 concerned, and the end result measured in 

 yield is the balance of the conflicting factors. 

 To state it in another way productivity is 

 probably quite as much influenced by qualiti- 

 tative soil differences as by quantitative diver- 

 sities — ^but the theory that has generally been 

 accepted is purely quantitative. Such a view 

 of " soil fertility " which it must be admitted 

 can now be measured only in terms of yield, 

 means that it is comparable in scope to 

 " weather " as applied to the seasonal complex 

 of climatic factors. The actual knowledge that 

 we have of soil productivity and of fertilizers 



is therefore still almost wholly empirical. 

 The extension and clarification of this knowl- 

 edge is, it seems, most likely to be obtained 

 by a much more intimate knowledge of the 

 plant reactions to each of the soil factors that 

 can be controlled and the different combina- 

 tions of these factors. An exceedingly inter- 

 esting recent contribution is that of Bottom- 

 ley, who presents strong experimental evidence 

 to show that highly organized green plants 

 must have dead organic matter as part of 

 their food. 



The effects of one crop upon another often 

 very marked, sometimes beneficial, more often 

 injurious. The nature of these effects is very 

 obscure, but it now seems clear that it can not 

 be wholly related to the quantitative supply of 

 plant food. These phenomena have been used 

 to lend support to the theory that yields are 

 often greatly reduced by the presence of 

 deleterious substances, in this case supposed 

 to be excreted by the preceding crop. The 

 theory is attractive in its simplicity and there 

 is some evidence in its favor, but there is 

 no clear proof that plants do excrete repeUant 

 substances. The curious way in which certain 

 wild plants occupy areas to the complete or 

 nearly complete exclusion of other species 

 might well be due to such a factor. The 

 effect of one plant upon another is an old 

 observation in plant culture, and appears in 

 botanical literature as early as Von Mohl. It 

 is only in recent years that the actuality of 

 the fact is established beyond doubt. An 

 understanding of its basic causes is manifestly 

 a matter of great importance. At the Rhode 

 Island Experiment Stations, onions varied in 

 yield from 13 bushels to 412 bushels per acre 

 in a long series of plots, the differences being 

 due solely to the effects of the preceding 

 crops. 



The subject of tillage is likewise one much 

 involved. The simplest plant culture requires 

 some disturbance of the soil, even if only to 

 remove stones or roots. But different meth- 

 ods or different degrees of stirring the soil, 

 show marked effects on subsequent yields. So 

 great are these differences that the famous 

 Jethro Tull proclaimed the slogan " Tillage is 



