Scientific Agriculture. 



130 



[August, 1910. 



symbiosis with higher plants (Pseudo- 

 monas), or exist free in the soil (Azoto- 

 bacter, Clostridium)- 



(3.) Another group of bacteria in the 

 process of breaking down organic matter 

 liberate the nitrogen in the free state, 

 and so reduce the stock of soil nitrogen. 



(4.) Natural drainage waters contain 

 nitrates which have been derived from 

 the soil nitrogen by bacterial oxidation. 



(5.) The rain annually contributes a 

 certain amount of combined nitrogen to 

 the soil. The amount is greater in the 

 proximity of towns ; the average amount 

 at Rothamsted is 3 - 84 lbs. per acre per 

 annum, and other results would show 

 that this is a very representative figure 

 for ordinary country air. 



In practice most of these factors 

 giving rise to gain or loss are at work 

 together ; which of them will predomi- 

 nate will depend upon the style of 

 farming and cultivation the land re- 

 ceives. Some of the Rothamsted plots 

 with their long-recorded history afford 

 an opportunity of estimating the inter- 

 play of the various factors, 



(a.) The simplest case to take is that 

 of land under arable cultivation when 

 nothing is restored to the soil. The un- 

 manured plot on the Broadbalk wheat- 

 field affords a useful example, and we 

 have figures which show the change 

 in its fertility between 1865 and 1893, a 

 period of 28 years. 



Broadbalk, Plot 3. Nitrogen, lb. per acre. 



In soil In soil Loss in Added Keinoved Unaccounted 

 1865. 1893. 28 years, by rain, by crop. for. 



2,722 2,437 285 107 428 +36 



In the first place it will be seen that 

 the nitrogen in the soil declines when 

 the crop is wholly removed and no 

 manure is added, though the decline 

 becomes slow after the first large loss 

 of condition has taken place. It will be 

 remembered that the yield of the un- 

 manured wheat plot at Rothamsted fell 

 off pretty quickly for a few years, but 

 for the last fifty years has remained 

 almost constant at an average of about 

 12-5 bushels per acre. 



If we make out a balance-sheet and 

 set off the nitrogen removed in the crop 

 against that which the soil has lost, as 

 shown by analysis, together with that 

 which has been brought down by the 

 rain, we find that tha soil contains about 

 36 lb. per acre more nitrogen at the end 

 of the period than we should have 

 expected. This quantity is too small to 

 be significant ; it would be more than 

 covered by the experimental errors in 

 the determinations ; we may, therefore, 

 conclude that the nitrogen required by 



the crop has just about been supplied 

 by the soil aud the rain. Thei'e must, 

 however, have been other losses from 

 the soil ; a good many weeds are annu- 

 ally removed, and they contain some 

 nitrogen ; drainage water flows away 

 containing, as we know by trial, some 

 nitrates ; there must also have been 

 some bacterial liberation of nitrogen gas. 

 These losses of nitrogen may not be 

 large in the aggregate, but as there is 

 no such falling off in the stock of nitro- 

 gen in the soil as would balance them, it 

 follows that some recuperative agencies 

 must ha^e been at work in the soil restor- 

 ing the stock of nitrogen. We know that 

 Azotobacter aud similar bacteria are 

 present in this soil ; we know also that 

 there is a certain amount of weed of a 

 leguminous nature growing every year 

 among the wheat ; thes8 are the two 

 sources of combined nitrogen which we 

 may credit with the restoration of the 

 stock of nitrogen in the soil. 



However, their action is not sufficient 

 to maintain the stock unimpaired, even 

 in this impoverished soil, when the crops 

 are wholly taken away. 



(&.) We may now take another case, 

 that of land very rich in organic matter 

 and under arable cultivation, the crop 

 as before beina; wholly removed. An 

 example of this kind is afforded by the 

 farmyard manure plot on Broadbalk, 

 where 14 tons per acre of farmyard 

 manure containing: about 2001b. of nitro- 

 gen are applied every year. 



Broadbalk, Plot 3, Nitrogen, lb. per acre. 



In soil In soil Gain in Added in 



1865. 1893. 28 years. manure. 



4,343 4,976 633 5,600 



Added by rain. Removed in crop. Unaccounted for, 



107 1,361 —3,713 



Under these conditions the losses of 

 nitrogen are enormous ; of the yearly 

 supply of nitrogen not quite a quarter 

 has been recovered in the crop, and less 

 than a quarter remains behind as an 

 enrichment of the soil ; more than half 

 has been permanently lost through the 

 destructive agencies enumerated above 

 (3 and 4). 



The production of this plot of land 

 with its annual application of manure 

 greatly in excess of the requirements of 

 the crop still tends to reach an equili- 

 brium ; after the first few years the crop 

 does not increase any further, nor does 

 the soil become any richer, because the 

 agencies destructive of the combined 

 nitrogen increase at an accelerating pace 

 until they balance the greater intake 

 of nitrogen. Of course, the equilibrium 

 thus attained is at a much higher level 



