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 ; 

 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. 



