CONSTITUTION OF SOIL. 14t 



into carbonate, it has the advantage of being in a much finer state 

 than when ground and first applied. It promotes, during the decay of 

 organic matter, the formation of nitric acid, which is very influential 

 in vegetation, forming with it nitrate of lime. This salt is very solu- 

 ble in water and therefore enters readily into the roots of plants, pro- 

 ducing the same effects upon them as the nitrate of soda. 



Irrigation of the soil is another mode of improving or manuring it. 

 This is extensively practised in South America and the East, though, 

 for no other purpose generally than to moisten a dry soil. But in 

 more northern climates, it washes out the acid and other noxious mat- 

 ters generated in the soil. Its favorable effects are seen in peat soils, 

 which contain much matter unfavorable to vegetation, and in subse- 

 quent drainings. The deposits also of productive organic and inor- 

 ganic matter in the form of mud, in addition to the soluble matter, 

 proves generally of great advantage to plants. They absorb from the 

 water the salts which, as we have noticed, are so valuable in their 

 growth. Draining and burning of pure soils is also much practised. 

 When the vegetable matter of the sod is burned the ash of the plants 

 is left to blend with the calcined earth. The ash supplies the soil, 

 like dressings, with inorganic matter, and the carbonaceous matter at 

 the same time contributes to correct and improve the soil. The 

 advantages of burned clay are not that it contains any organic matter 

 useful as the food of plants, but that it will crumble into a friable 

 powder ; and instead of a paste-like substance, it becomes comminuted 

 and equally diffused, so as to give a due consistency to the soil. Thus 

 it will render even clay soils more open and modify the texture of most 

 others, not chemically but mechanically. Burned bricks render soils 

 more porous, and are supposed to imbibe and condense air, which 

 facilitates the decay of vegetable matter and aids the early growth of 

 plants. They are thus believed to imbibe ammonia and nitric acid 

 from the air. Charcoal has a similar effect, though the entire opera- 

 tion of this as well as of burned clay, is to be further and better un- 

 derstood by repeated experiments. 



The productive powers of soils are mostly modified by the plants 

 grown on them. Lands unfit for arable culture, may be made to pro- 

 duce profitable arable crops by the continued growth of wood. But 

 there is a difference in the kind of trees thus calculated to improve 

 the soil. The pines may not improve it at all, and the beech and 

 sycamore may render it even less valuable than at first. Oaks render 

 it better only after 15 or 18 years. The larch renders it still better, 

 though in a still longer time. This improvement is the result of addi- 

 tions to the soil by the leaves of the trees. But the green foliage 

 delays the decomposition of the fallen leaves by shading them from 

 the favorable influence of the sun. Some leaves decay more readily 

 however than others the beech and oak, for example, sooner than 



