CARBON AND NITROGEN CYCLES IN THE SOIL 83 



manures to a series of pots, or plots, and then measure the effect on 

 the growth of the crop, and compare it with the effect of nitrate of 

 soda, which is taken as 100. The idea is that the nitrogenous com- 

 pound changes in the soil to nitrate, and this is taken up by the plant, 

 which thereby becomes the agent for measuring the amount of nitrate 

 formed. The method is simple, and gives the kind of information the 

 practical man wants, but, unfortunately, it rarely gives the same 

 results twice. We can now see that it is incapable of accuracy : in the 

 first place the decomposition of the nitrogeneous compound does not 

 merely give rise to nitrates but to gaseous nitrogen also, and the relative 

 amounts of these two products vary with the conditions obtaining in 

 the soil ; in the second place the efficiency of the plant as a nitrate 

 absorber is not constant, but depends on all the various factors in- 

 fluencing plant growth ; finally, only in the rare cases where the ex- 

 periment is conducted in a lysimeter is any allowance made for 

 the unabsorbed nitrates. In spite of these drawbacks, however, avail- 

 ability measurements are of some practical value in classifying roughly 

 the various manures and systems of cropping. 



It is evident that there must be some recuperative agency, or the 

 stock of soil nitrogen, which is never very great, would long ago have 

 disappeared in old countries. Experiment has shown that soil gains 

 nitrogen when it is allowed to remain undisturbed and covered with 

 unharvested vegetation as in natural conditions. On the Broadbalk 

 field a third plot adjacent to the two already mentioned was in 1882 

 allowed to go out of cultivation and has not been touched since ; it 

 soon covered itself with vegetation, the leaves and stems of which go 

 to enrich the soil in organic matter. The gain in nitrogen is very 

 marked, as shown in Table XXXVII. The gain is much influenced 

 by the amount of calcium carbonate in the soil, and is considerably 

 less on another plot in Geescroft field where only little calcium car- 

 bonate is present ; whether this is due to any specific action, or to the 

 changed physical conditions brought out by decalcifying a soil, is not 

 clear. Gains of nitrogen also take place on land covered with perennial 

 grasses and clovers even when the crop is mown or grazed. On wet 

 clay pastures dressings of basic slag have been found to increase the 

 nitrogen content of the soil, whilst potassium salts, such as kainit, have 

 had the same effect on sandy soil. 



In all these cases leguminous plants are present in greatest extent 

 where the gains in nitrogen are greatest, but they are not necessarily 

 the only nitrogen fixers. 



Advantage is taken of this recuperative effect in all rotations by 



