1883.] SOME OF THE CIIABACTEBISTICS OF SOILS. 105 



S03IE OF THE CHARACTEIIISTICS OF SOILS. 



f|N last month's number of Forestry,' Mr. 1). M'CorquoJale calls 

 ^ in question a recent remark of one of your contributors to the 

 ^ effect that the annual growth of * grass, brackens, and other weeds,' 

 in plantations is liable to further the exhaustion of the soil in such 

 places, and he slyly suggests that if the grasses and weeds bring 

 about exhaustion, the growth of 100 tons of saleable timber to the 

 acre must be a serious matter to the soil. But such sliort and close 

 herbage as * grass, brackens, and other weeds,' if allowed annually to 

 wither and decay where it has grown, adds considerably to the 

 fertility of the soil ; and laud under timber stores up each year in 

 the surface soil the food constituents most required by our croppin*-'- 

 plants. And this we shall endeavour to make plain. Out of the 

 sixty-four yet known elements that together go to form our earth, sea, 

 and air, only some twelve or so are found in the analyses of plants, 

 and of course these must be present in company with other matter 

 in all soils capable of sustaining vegetation. Of some, however, the 

 plant requires but little, while of others it has to absorb a consider- 

 able amount. Those few elements combining one with another, or 

 one with several, and in various proportions, go to form tlie manv 

 different compounds which are required to constitute a soil fit for 

 cultivation : oxygen and hydrogen unite to form water, hydrogen and 

 nitrogen to form ammonia, calcium and potassium each unite with 

 oxygen to form respectively lime and potash ; carbon, nitrogen, and 

 phosphorus unite with water severally to form carbonic acid, nitric 

 acid, and phosphoric acid ; and these three with lime, potasli, and soda 

 to form the carbonates, nitrates, and phosphates of the latter three, 

 and so on. And out of those few the vegetable kingdom elaborates 

 innumerable substances ranging from simple starch to the subtle 

 poisons of the alkaloids. The plant cannot lay hold of and assimilate 

 to its substance any one free and uncombined element, unless perhaps 

 we except the free oxygen of the atmosphere ; when the plant is under 

 certain conditions of growth it cannot avail itself of the free nitrogen 

 of the surrounding air. Nor can it absorb any uncombined carbon — 

 the element that forms the great bulk of the plant. Carbon when by 

 itself is a solid and almost insoluble, but combined with oxygen it 

 forms carbonic oxide, a gas, and plentiful in the atmosphere. Under 

 certain conditions, owing to some mysterious action of sunhght on the 

 green colouring matter of the tender parts of plants, the plant is able to 

 decompose this oxide by which it is surrounded and to turn the carbon 

 to its own uses, and also to avail itself of the free oxygen if required. 



