312 MESSRS. J. B. LAWES AND J. H. GILBERT ON THE RESULTS OF 
unmanured plot, is approximately the same as that obtained in wheat or barley 
grown on arable land of somewhat similar character for many years in succession; 
and the leguminous and miscellaneous herbage together contribute about two-thirds 
as much; making, therefore, in all a much greater total annual yield in the mixed 
herbage than in the cereal crop. That it should be greater, at any rate for a 
considerable period of time, would seem only natural to expect, when it is borne 
in mind that the surface-soil of permanent meadow land is much richer in nitrogen, 
due to vegetable debris and previous accumulations, than arable land of otherwise 
corresponding character; and, further, that many of the components of the mixed 
herbage have possession of the soil, and are more or less growing, almost the year 
round. But, under the influence of the mineral manure, the total yield of nitrogen 
per acre per annum averaged one-and-two-thirds as much as without manure, and 
about two-and-a-half times as much as would be yielded in a cereal crop similarly 
manured on arable land ; and, without direct evidence on the point, it would hardly 
be concluded that this largely increased yield could be accounted for in a similar way 
to that of the lesser amount without manure. 
In seeking for an explanation, it had to be borne in mind, that such mineral 
manures did also considerably increase the growth of leguminous crops grown on 
arable land; and, further, that when a leguminous crop is grown on arable land 
it not only removes much more nitrogen from a given area than a gramineous one 
grown under ecpial conditions as to soil and season, but it leaves the upper layers 
of the soil so much richer in nitrogen, and in such condition that the increase may 
be determined by the soda-lime method, and the growth of the succeeding cereal is 
considerably augmented. The question, therefore, suggested itself—how far the 
increased growth of the grasses proper in the mixed herbage was due to an increased 
accumulation of combined nitrogen available to them, in the upper layers of the soil, 
the result of the increased growth of, and accumulation by, the Leguminosm, induced 
by the mineral manure? Whether, in fact, where the plants are thus growing in 
association, there is a parallel action to that which takes place when they are grown 
in alternation ? 
This explanation obviously itself left unsolved the question of the source of the 
nitrogen of the Leguminosce, besides involving other difficulties. Still, it seemed to 
accord with other known facts, and to be at any rate the best that could be offered 
in defect of more satisfactory or countervailing evidence. However, as already 
referred to, after the experiments had been continued for 20 years, samples of the 
soils and subsoils were taken, from each plot, at different depths, and determinations 
of nitrogen have been made in them. The results show a considerably lower per¬ 
centage in the first nine inches of depth of the mineral manured plot 7 than in that 
of the unmanured plot 3 ; and so far as the calculation of such results into quantities 
per acre can be relied upon, the difference is more than sufficient to account for the 
increased yield of nitrogen over the 20 years in both the gramineous and the 
