The Rothamsted Agricultural Experiments. 175 
tinuously under wheat for sixty-one years, parallel strips having been 
treated with different manures. 
Even the unmanured plot still continues to produce a crop 
every year, and its average crop for the last half century (thirteen 
bushels to the acre) is about the same as the world’s average, 
though the English farmer expects a much heavier crop than this. 
The plot receiving a mixed mineral manure without nitrogen, 
produces very little more than the unmanured plot; with nitrogen 
the crop increases greatly, and a progressive increase of nitrogen 
leads to a corresponding increase in the crop. Thus we see, as we saw 
in the case of hay, that combined nitrogen is the great requirement of 
the grass-plant. One of the great objects of changing the crop on 
a piece of land is to get rid of the weeds especially associated with 
a given crop, which would otherwise increase to a harmful extent. 
This is seen on Broadbalk field, where the wheat had become so 
“ foul ” with Alopecurus ngrestis, the Black Bent Grass, that the 
experiment is being tried of fallowing half of each plot in alternate 
years, in order to get rid of the pest. 
One end of Broadbalk field was left entirely to itself many 
years ago. In comparatively few years the wheat entirely dis¬ 
appeared, its place being taken by a miscellaneous mixture of weeds, 
among which were a number of seedling bushes and trees. In 
another few years the bushes formed an impenetrable thicket, 
through which the young trees, largely oaks, were beginning to push 
their way. In another thirty years there would have been a young 
oak wood with rich undergrowth, but the bushes were in the way 
and have been recently stubbed up. The succession of events 
illustrates the constant tendency of good land to go back to the 
condition of woodland (no doubt its primitive state throughout the 
temperate regions) a tendency only arrested by the constant cutting 
of crops or by grazing. 
Clover-Sickness. 
A very interesting experiment with leguminous plants has been 
made in Hoos Field. 
Various leguminous crops have been grown continuously since 
1848 with various combinations of mineral manures, but after the 
first few years they became a complete failure, owing to an unknown 
cause which nearly always prevents the continuous growing of 
leguminous crops and is called by farmers “clover sickness.” The 
bulk of this area was ploughed up in 1898 and sowed with wheat. 
Five crops of this were taken without manuring in succeeding years 
to test the amount of nitrogen accumulate d by the leguminous 
