■September 16, 1910] 



SCIENCE 



367 



position, that the fertility of soil is often 

 determined by excretions from the plants 

 themselves, which thereby poison the land 

 for a renewed growth of the same crop, 

 though the toxin may be harmless to a dif- 

 ferent plant which follows it in the rota- 

 tion. This theory had also been examined 

 by Daubeny, and the arguments he ad- 

 vanced against it in 18-15 are valid to this 

 day. Schreiner has indeed isolated a num- 

 ber of organic substances from soils — di- 

 hydroxystearic acid and picoline-carboxy- 

 lic acid were the first examples — which he 

 ■claims to be the products of plant growth 

 and toxic to the further growth of the same 

 plants. The evidence of toxicity as deter- 

 mined by water-cultures requires, how- 

 ever, the greatest care in interpretation, 

 and it is very doubtful how far it can be 

 applied to soils with their great power of 

 precipitating or otherwise putting out of 

 action soluble substances with which they 

 may be supplied. ]\Ioreover, there are as 

 yet no data to show whether these so-called 

 toxic substances are not normal products 

 of bacterial action upon organic residues 

 in the soil, and as such just as abundant 

 in fertile soils rich in organic matter as 

 in the supposed sterile soils from which 

 they were extracted. 



As then. we have failed to base a theory 

 of fertility on the plant food that we can 

 trace in the soil by analysis let us come 

 back to Mayow and Digby and consider 

 again the niter in the soil, how it is formed 

 and how renewed. Their views of the 

 value of nitrates to the plant were justified 

 when the systematic study of plant-nutri- . 

 tion began, and demonstrated that plants 

 can only obtain their supply of the indis- 

 pensable element nitrogen when it is pre- 

 sented in the form of a nitrate, but it was 

 not until within the last thirty years that 

 we obtained an idea as to how the niter 

 •came to be found. The oxidation of am- 



monia and other organic compounds of 

 nitrogen to the state of nitrate was one of 

 the first actions in the soil which was 

 proved to be brought about by bacteria, 

 and by the work of Schloesing and Miintz, 

 Warington and Winogradsky we learned 

 that in all cultivated soils two groups of 

 bacteria exist which successively oxidize 

 ammonia to nitrites and nitrates, in which 

 latter state the nitrogen is available for 

 the plant. These same investigators 

 showed that the rate at which nitrification 

 takes place is largely dependent upon 

 operations under the control of the farmer : 

 the more thorough the cultivation, the bet- 

 ter the drainage and aeration, and the 

 higher the temperature of the soil the 

 more rapidly will the nitrates be produced. 

 As it was then considered that the plant 

 could only assimilate nitrogen in the form 

 of nitrates, and as nitrogen is the prime 

 element necessary to nutrition, it was then 

 an easy step to regard the fertility of the 

 soil as determined by the rate at which it 

 would give rise to nitrates. Thus the bac- 

 teria of nitrification became regarded as a 

 factor, and a very large factor, in fertility. 

 This new view of the importance of the liv- 

 ing organisms contained in the soil further 

 explained the value of the surface soil, 

 and demolished the fallacy which leads 

 people instinctively to regard the good soil 

 as Ijdng deep and requiring to be brought 

 to the surface by the labor of the cultiva- 

 tor. This confusion between mining and 

 agriculture probably originated in the 

 quasi-moral idea that the more work you 

 do the better the result will be; but its 

 application to practise with the aid of a 

 steam plough in the days before bacteria 

 were thought of ruined many of the clay 

 soils of the Midlands for the next half 

 century. Not only is the subsoil deficient 

 in humus, which is the accumulated debris 

 of previous applications of manure and 



