ON THE GROWTH OF LEGUMINOUS CROPS. 55 



Obviously, however, it is more important to consider, what is the 

 probable average loss of nitrogen over a given area by the removal of 

 crops generally, and not by that of grass alone. Moreover, in making 

 such an estimate, it is not the total nitrogen of the crops that has to be 

 reckoned ; but, taking into account the return by manure, only the 

 amount eventually lost to the soil. With the great variation according 

 to circumstances, it is of course very difficult to estimate this at all 

 accurately ; but it may be stated, that two independent modes of 

 estimate lead to the conclusion that, for Great Britain for example, the 

 average annual loss of nitrogen is more probably under than over 20 

 Ibs. per acre ( = 22*4 kilog. per hectare). In fact, the loss by cropping 

 under the usual conditions of more or less full periodical return by 

 manure, is by no means so great as is generally assumed in discussions 

 of this subject. 



The loss of nitrates by drainage may, however, in some cases be 

 considerable/ There may also, under some circumstances, be loss by 

 the evolution of free nitrogen. Such loss may take place in the 

 manure heap ; or in soils very heavily manured, as in market gardening, 

 for example. But in ordinary agriculture such excessive manuring 

 seldom. takes place; and the soil is generally much poorer in nitrogen 

 than in the cases of the experiments which have been adduced as 

 showing great loss from rich soils. Loss may also take place when the 

 soil is deficiently aerated ; but, here again, the conditions of the 

 experiments cited, in which considerable loss by evolution of free 

 nitrogen was observed, are not the usual conditions of soils in actual 

 practice. Indeed, the balance of evidence is against the supposition 

 that there is a constant and considerable loss by the evolution of free 

 nitrogen from arable soils which are only moderately rich in organic 

 nitrogen, and which are fairly drained, either naturally or artificially. 



On this point it may be mentioned that, in those of the field 

 experiments at Rothamsted in which the unusual practice of applying 

 farm-yard manure every year is adopted, it is found that there is 

 considerable loss of nitrogen from the soil, beyond that known to 

 be removed in the crops, and estimated to be lost in the drainage. 



On the other hand, where no nitrogen has been applied for many 

 years, and the amount of nitrogen in the surface soil is only about, or 

 little more than O'l per cent., the loss of nitrogen by the soil over a 

 long series of years corresponded approximately with the amounts 

 removed in the crops, together with those estimated to be lost in the 

 drainage. Again, when ammonium-salts are applied, even so late in the 

 season as October or November, and drainage takes place soon after- 

 wards, the drainage-waters will contain amounts of nitrogen showing a 

 very direct relation to the different amounts of ammonia applied in the 

 manure ; but scarcely any of it as ammonia, nearly the whole existing 

 as nitric acid ; and this is the case although the drainage passes 

 through twenty inches or more of raw clay sub-soil. Lastly, direct 

 experiments have shown that there is a diminution in the amount of 

 nitric acid" in the soil down to a certain depth, varying according to the 



