1 84 



SOIL CONDITIONS AND PLANT GROWTH 



Advantage is taken of this recuperative effect in all 

 rotations by alternating the periods of arable cultivation with 

 periods of "rest" in grass and leguminous crops. In the old 

 Norfolk rotations one year in four was given up to clover,^ 

 in modern rotations the clover or " seeds " mixture is some- 

 times left for two or three years before it is ploughed up, so 

 that the enrichment may become more marked. Mr. Mason 

 at Eynsham Hall ^ considerably enriched in nitrogen some 

 poor Oxford clay by the growth of lucerne. But the gain in 

 nitrogen does not go on indefinitely ; in course of time a point 

 of equilibrium is reached, higher or lower according to the soil 

 conditions, where further gains are balanced by losses, so that 

 the nitrogen content remains constant. 



Thus there is an upper as well as a lower limit to the 

 nitrogen content of the soil, the actual values depending on 

 the soil conditions. Between these limits the nitrogen con- 

 tent may be maintained at any desired level, high when the 

 ground is left in grass and leguminous crops, low when the 

 ground is continuously cultivated. Unfortunately, on our 

 present knowledge it is impossible to maintain a high content 

 of nitrogen on cultivated land except at a wasteful expenditure 

 of nitrogenous manure. 



Tentative determinations of some of these limits are ; — 



The reactions involved in all these changes are obviously 

 complex, but they have been partially disentangled, and we 

 can now pass on to a more detailed consideration of the 

 separate changes. 



1 It was known to the Romans that vetches were a good preparation for 

 wheat (cf. Virgil, Georgics, Book I., lines 73 et seq.). 

 ^yourn. Roy. Agric. Soc, 1904, Ixv., 106-124. 

 ^ Containing less than 10 per cent, of organic matter 



