332 



NATURE 



[June 15, 1916 



usual. Thus it appears that ampionia is the precursor 

 of nitrates, and is itself preceded by the usual amino- 

 acids. The distinguishing feature of the soil decom- 

 position is simply that it is carried several stages 

 further. 



This decomposition is absolutely indispensable to the 

 plant ; the initial products — the proteins — are useless 

 for plant nutrition ; the intermediate products are not 

 much good ; the ammonia is considerably better, while 

 the final stage — the nitrate — is the best of all. 



During this decomposition also, the energy stored 

 up by the plant during its lifetime is run down, so that 

 there is a transformation both of material and energy. 

 Neither the energy nor the material is wasted ; they 

 go to support a vast population of the most varied 

 kind, ranging from microscopic bacteria to earth- 

 worms. All these depend on the plant residues for 

 their food and their energy. But theirs is no case of 

 taking all and giving nothing in return. 'Their work 

 is nothing less than the production of food for the 

 plant : preparing new plant food out of old plant 

 residues. 



Thus we have a great cycle going on in the soil; 

 dead plant residues mingle with it, and give life to 

 countless micro-organisms, which in turn manufacture 



. Complex pUnt substaT\ccs. 



PI ^ 



^c5 





:3 



P- 3 



&asfous 

 N 



Nitrates 



COa 



iCOi Ccnpounds 

 of p. K. etc. 



out of these residues food for a new generation of 

 plants. 



It is necessary to set some limits to the inquiry, and 

 so we restrict ourselves to the production of nitrates. 

 This process is the work of a great number of organ- 

 isms, some of which carry out the first stages, and 

 others the later stages. It resembles the process of 

 making munitions in that the first stages can be 

 brought about by a large variety of workers, while 

 later stages are much more specialised, and can be 

 effected only by one or two special workers. Indeed, 

 in the wars of the eighteenth century the process was 

 actually under the Ministry of Munitions of the time, 

 and both in Sweden and in Germany elaborate in- 

 structions were drawn up for the working of nitrate 

 beds. 



The process of nitrate formation is not free from 

 waste; starting with loo parts of nitrogen as protein, 

 one never recovers loo parts of nitrogen as nitrate; 

 there is always a loss. But the fault does not appear 

 to be with the special organisms carrying out the last 

 stages of the process, for at least 96 per cent, of the 

 ammoniacal nitrogen reappears as nitrate. It is not 

 clear that it lies with the organisms ^ producing 

 ammonia; at any rate, they can work without loss. 



NO. 2433, VOL. 97] 



The probability is that the loss arises from some of 

 the nitrate that has been actually formed. 



However it arises, this loss, as well as the leaching 

 out of nitrate by rain, would in natural conditions 

 bring the stock of soil nitrogen to a very low level if 

 there were no counterbalancing processes, and for the 

 last fifty years chemists and bacteriologists have been 

 searching the soil very thoroughly to find out how 

 these gains are brought about. Two sources are 

 known : the organisms associated with clover and 

 other Leguminosae, and free-living nitrogen-fixing 

 organisms. These differ very much in appearance and 

 mode of life, but they both require energy for the 

 nitrogen fixation, and this they obtain from the com- 

 bustion of carbohydrate materials. 



It must not be supposed, however, that the organ- 

 isms bringing about these changes are the only ones 

 in the soil, or that they lead their lives quite inde- 

 pendently of the rest of the soil population. Indeed, 

 they could scarcely do so in any case, for there is only 

 a limited store of food and energy, and whatever is 

 not helping is hindering them. Numerous experiments 

 show that there is some factor — neither food, air, 

 water, nor temperature — which is operating to keep 

 down their numbers. As it is put out of action by 

 heating to 55° C, or by traces of volatile antiseptics, 

 and can be reintroduced by adding a little untreated 

 soil, it is presumably biological, and the evidence shows 

 that it consists in part at least of certain amoebae; 

 it is quite possible that other forms are involved as 

 well. But whatever the detrimental organisms may 

 be they impede the work of the organisms producing 

 plant food in the soil. Fortunately they are put out 

 of action more easily, so that we get the apparent 

 paradox that any process fatal to life (but not too 

 fatal) proves ultimately beneficial to fertility, while 

 any process beneficial to life proves ultimately harm- 

 ful. Long frost, drought, heat, therefore benefit the 

 useful makers of plant food, while prolonged warmth, 

 moisture, and treatment with organic manures lead to 

 deterioration or to " sickness," as the practical man 

 puts it. 



Having thus set out the general nature of the cycle, 

 we next proceed to see how and to what extent it can 

 be controlled. 



Control may take place in two directions : the 

 amount of organic matter, i.e. raw material out of 

 which plant food is made, may be increased, or the 

 pace of the manufacturing process may be forced. 



The necessity for increasing the organic matter in 

 the soil was realised very early. Arable farmers soon 

 found that land cannot be cropped indefinitely; sooner 

 or later it becomes "exhausted"; it recovers, how- 

 ever, if it is left to itself for a time, so that natural 

 vegetation can spring up and die again. The Mosaic 

 law commanded the Jews to leave their land for one 

 year in seven and not to reap " that which groweth 

 of its own accord." The system survived in our own 

 land through Saxon and medieval times ; land v/as 

 uncropped one year in three, two corn . crops were 

 taken, then grass was allowed to grow up on the 

 stubble to be ploughed in. The principle still under- 

 lies our modern rotations; crops are grown, then the 

 land is left covered with vegetation, but the process 

 is regulated by sowing a definite mixture of grass or 

 clover chosen to make vigorous growth. 



Another method for increasing the amount of organic 

 matter in the soil consists in growing a crop exclu- 

 sively for the purpose of ploughing it in. This also 

 goes back to ancient times : Theophrastus, 300 years 

 before Christ, tells us that beans were grown in 

 Macedonia and Thessaly expressly to be ploughed in at 

 flowering time, and Varro, about 50 B.C., states that 

 lupins were grown for the same purpose. This method 



