70 
JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 
organic matter in the different soils, according to the locality 
from which they have been obtained. Thus an arable loamy 
soil that is under constant cultivation of an ordinary rotation 
of crops contains 34,500 lbs. of organic matter per acre with 
3,360 lbs. of nitrogen. As all arable land was once pasture or 
forest, the loss of organic matter and of nitrogen that has 
occurred during its cultivation becomes obvious when we com¬ 
pare these figures with those that follow in Table III. A 
pasture soil with its undisturbed accumulation of root-fibres 
contains 76,050 lbs. of organic matter per acre, a quantity more 
than double that of the arable soil; and 5,558 lbs. of nitrogen, 
or nearly one ton per acre of nitrogenous plant-food in excess of 
the arable soil. The prairie soil, which has doubtless been 
gathering and storing up organic matter for many centuries, is 
seen to contain 117,225 lbs. per acre of this substance, with a 
correspondingly large quantity of nitrogen, amounting to 
9,630 lbs. or to more than four tons per acre. The forest mould 
contains even more organic matter than the prairie soil, but, 
owing probably to the woody nature of the refuse, the decaying 
material is much less rich in nitrogen than the more fibrous-rooted 
soil of the prairie. The leaf-mould and the peat-mould contain, 
as would be expected, an enormous amount of organic matter, 
averaging, when cut to 9 inches deep, more than seventy tons 
per acre. The proportion of nitrogen, which is lower than in 
some of the other soils, depends in such moulds upon the degree 
of oxidation or decomposition to which they have been submitted. 
The oxidation of the organic matter in a peat-bog may be greatly 
checked by a high-water level, which excludes air from the soil I 
hence an unlimited accumulation of organic matter may take 
place if plants capable of growing under these circumstances are 
present. 
A dark-coloured soil becomes hotter in the sun’s rays than a 
light-coloured one, hence the oxidation and nitrification of the 
organic matter is more active in these richer soils, provided the 
requisite mineral ingredients are not deficient. 
The last item in Table III. relates to a vegetable mould 
existing in Belgium and known in the horticultural trade as 
“ Ghent heath mould ” ; it is the result of the decomposition of 
various species of Erica mixed with sand, and is most extensively 
used in the cultivation of azaleas for market. 
