58o 



NATURE 



[January 20, 19 16 



whilst the others each had one constituent left out. 

 Thus for wheat he obtained the following results, and, 

 therefore, concluded that on this soil wheat requires 

 a good supply of nitrogen, less phosphorus, and still 



less potassium : dop p r acre. 



Hu>iids. 



Normal manure ... ... ... 43 



Manure without lime 41 



,, ,, potash 31 



,, ,, phosphate ... ... 265 



,, ,, nitrogen 14 



Soil without manure ... ... ... 12 



- The method, of course, is perfectly sound, and it 

 has been very widely adopted. It is, however, frankly 

 empirical, and empirical work is never very inspiring, 

 so that for a long time soil. work came rather to a 

 standstill. 



It has several times happened in the history of agri- 

 cultural chemistry that the new illuminating idea 

 wanted to revivify the subject at a stagnant period 

 has come in from some outside technical problem that 

 had to be solved. So it was here. The growth of the 

 towns and of stricter ideas on public health had 

 brought into prominence the need for better sewage 

 purification, and it was imperative that the problem 

 should be dealt with somehow or other. 



Schloesing and Miintz found that satisfac- 

 tory purification involved the conversion of 

 ammonia into nitrate, and by a brilliant in- 

 vestigation they found that this process was 

 neither chemical nor physical, but biological. 

 Their work was extended to the soil with 

 remarkable results. It was seen that the soil 

 was not a mere inert mass, but that it was 

 teeming with life and pulsating with change. 

 The number of bacteria is enormous, running 

 into millions per gram, and the question is 

 raised : How do these organisms live? Thev 

 must have food, and they must have energy. 

 We are therefore forced to go back to the 

 soil and study it as a medium for the life of 

 a soil population. 



A very cursory examination shows that 

 the soil forms only a thin layer; under- 

 neath it lies the subsoil, which is wholly 

 different in colour, texture, and especially 

 in its behaviour towards the plant 

 (Fig. 2). 



Yet there was not always this differ- 

 ence. When the soil was first laid 

 down it was all like the subsoil, and 

 whenever a new surface becomes exposed, either 

 by landslips, cliff-falls, etc., it is always the subsoil 

 type that appears. The first vegetation has no great 

 supply of plant nutrients, but plants suited to the 

 conditions nevertheless spring up. They take what 

 they can from the crude soil, they take carbon dioxide 

 from the air, they synthesise sugars, starches, cellu- 

 lose, proteins, etc., deriving the necessary energy from 

 sunlight. When the plants die they fall back on the 

 soil and return to it all that they took, and a good 

 deal more of new material besides. That introduces a 

 fundamental change. 



The new material thus added contains stores of 

 energy and food substances suitable for the bacterial 

 population, which forthwith flourishes. Decomposi- 

 tion goes on, nitrates and other substances are pro- 

 duced, and the conditions are made more favourable 

 for the growth of a new race of plants. One of the 

 most obvious changes is the formation of nitrates, 

 but other products are formed as well. It is proving 

 exceedingly difficult to trace out full details, but the 

 following is probably in the main accurate:— 



NO. 2412, VOL. 96] 



— >- Protein 



Carbohydrates 

 Cellulose 



Amino-acids 



NH3 Hydioxy-aciils 



I I 



Nitrites Calcium salts 

 Y I I 



Gaseous ^ ■^ 



N Nitrates CaCOa CO2 



I I 



Other com- 

 pounds 

 " Humus" 



Acids 



I 



Calcium 

 salts 



i ^ 



Oils Waxes 

 / 



decom- 

 posed 



CO2 CaCO;, CO.^ 



Unfortunately, not much is known about the details, 

 but the reaction is extremely important. The initial 

 products are of little value to the crop or the soil. 

 The final products are invaluable for plant nutrition, 

 and some of the intermediate products are very valu- 

 able for the soil. This, therefore, is the reaction on 

 which plant nutrition depends, and it is of the highest 

 importance that it should proceed rapidly and smoothly. 

 Where for any reason it does not, the soil becomes 

 unproductive. 



Fi( 



Plants grown in «oil, sand, end subsoil respectively, all without manure, showing 

 tlie marked differences in behaviour towards the growing plant. 



Scientific crop production depends largely on con- 

 trolling this reaction. Three things are necessary : 

 the conditions — the air supply, water, temperature, etc. 

 ^must be favourable ; the organisms must be of the 

 right kind ; and the supply of raw material — plant 

 residues — must be kept up. 



We shall see later on how the favourable conditions 

 are obtained. Hitherto little has been done to control 

 the organisms beyond improving the conditions, but 

 beginnings have been made in the direction of inocu- 

 lation and partial sterilisation. The supply of rav 

 material is kept up in several ways ; probably th' 

 oldest is to leave the ground alone, so that it covei> 

 itself with wild vegetation, which is then ploughed in. 

 This formed part of the Mosaic law ; it was the regular 

 medieval custom in our own country, and it is prac- 

 tised to this day in Connemara. It is too haphazard 

 for modern use, however, and so nowadays the farmer 

 grows a special crop with the express intention of 

 ploughing in all or part of it, or of feeding it tc 

 animals and ploughing in the excretions. 



The second broad principle of crop production i- 



