86 



SOIL CONDITIONS AND PLANT GROWTH 



TABLE XXXVII. GAINS IN NITROGEN IN SOILS PERMANENTLY COVERED WITH 

 VEGETATION ROTHAMSTED SOILS LEFT TO RUN WILD FOR 22-24 YEARS. 

 HALL (122). 



LAND LAID DOWN TO GRASS IN 1856 AND MOWN ANNUALLY (DR. GILBERT'S 

 MEADOW, ROTHAMSTED). 



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, 2 in modern rotations the clover or 

 " seeds " mixture is left for two or three years before it is ploughed up, 

 so that the enrichment may become more marked. Mr. Mason at 

 Eynsham Hall 3 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 content 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 : 



1 Estimated. 



2 It was known to the Romans that vetches were a good preparation for wheat (cf, 

 Virgil, Georgics, Book I., lines 73 et seq.), 



s jfourn. Roy. Agric. Soc. t 1904, Ixv., 106-24, 



