THE ACTION OF ONE CROP ON ANOTHER. 
377 
themselves, but also to that produced by the plants in the trays, which 
must generally amount to two or three times that produced by the 
plants in the pots. 
Ample evidence could be adduced to show that the fertility of soil 
is eventually increased by growing crops in it, provided that the crops 
are not allowed to exhaust the soil : it follows, therefore, that any 
toxic effect produced by the growth of a plant must be of a temporary 
character only, and that the toxin must become changed, probably 
by oxidation, into plant-food. Many general instances might be 
quoted in support of this fact, but this is hardly necessary ; we may, 
however, quote one particular experiment carried out in pots which 
illustrates this point very forcibly. Trees grown in soil which 
had not been grassed were compared with others grown in soil from 
an adjoining plot of ground which had been under grass for many years, 
and the vigour of growth of the latter was found to be over twice as great 
as that of the former ; but when they were grown in this same fertile 
soil with the turf replaced on the surface, their vigour was reduced 
to half of what it was in the less fertile soil v/ithout grass. The soil 
from the grassed land was beneficial, and it was only while the grass 
was actually growing in it that it was toxic. 
The disappearance of the toxic property is apparently rapid: in 
experiments with trees in pots, when the washings from the grass in 
the trays, instead of being allowed to run immediately down to the 
roots of the trees, are collected in a separate pan, and not supplied 
to the trees till some hours afterwards, they are found to have lost 
their toxic properties, and, possibly, to have acquired some slightly 
beneficial property ; even the interposition of a layer of two inches of 
pumice between the soil in the pots and the trays carrying the surface 
crop, when these trays are still in situ, allows sufficient oxidation of 
the leachings to produce an appreciable reduction in the toxic effect. 
A similar result was obtained in some experiments in the field, when 
trees were grown in tilled ground, and in ground which was grassed 
under different conditions. The effect of the grass, when it entirely 
covered the roots of the trees, was to reduce the vigour of the latter 
from ioo to 5 in the course of four years, but when the grass was 
not allowed to come within three feet of the stems it produced an 
actually beneficial effect at first, the toxin formed by the grass having 
time to become oxidised before reaching the tree roots, and it was only 
after two years, when the roots began to extend into the grassed area, 
that the toxic effect began to be felt. 
Relative Vigour of Trees. 
1910. 
1911. 
1912. 
1913- 
Ground tilled ..... 
100 
100 
100 
100 
Ground turfed ..... 
70 
30 
6 
5 
Turfed to three feet from stems 
132 
M5 
78 
61 
Allowed to grass over 
101 
38 
12 
10 
Turfed and fed ..... 
69 
30 
9 
28] 
