ON MANURES. I II 5 



and porosity of soil are all favourable, that the soil contain, within 

 reach of the roots of the crop, a sufficient supply of all the food- 

 constituents, both of minerals and of nitrogen, which the plant 

 cannot obtain from the atmosphere ; also that these be in an 

 available and assimilable form. In other words, this food must 

 be capable of being dissolved by soil-water during the growth 

 of the plant. 



It is not merely essential that the soil contain so much 

 nitrogen, potash, phosphoric acid, and lime, but rather that it 

 should contain enough plant-food to yield up to the water 

 percolating through the soil during the period when the crop is 

 in active growth, and that the growing plant can take these 

 elements through its rootlets. 



The real point at issue, then, is to learn to what extent the 

 gardener may call in the aid ■ of discoveries of modern chemical 

 science in making more effective the empirical methods of his 

 forefathers, or whether he may substitute for those old systems of 

 feeding plants others yet more effective and certain. 



Constituents and Sources of PIant=food. 



The carbon of all green-leaved plants is absorbed directly, and 

 practically exclusively, from the atmosphere through the medium 

 of the foliage. At least the soil supply of carbon is a matter of 

 minor importance. In fact, Sir John Lawes and Sir Henry 

 Gilbert have found in their invaluable experiments at Rothamsted, 

 Hertfordshire, that an average of about 25001b. of carbon can be 

 annually assimilated by growing plants over an acre of land 

 without an ounce of carbonaceous manure being applied to it. 



The oxygen of green-leaved plants is chiefly absorbed in like 

 manner by the foliage, or is taken up by the roots in com- 

 bination with hydrogen, in the form of water, although a small 

 and comparatively unimportant source of oxygen and of hydro- 

 gen may be found in the breaking up of nitrates and ammonia 

 within the soil. , 



The nitrogen of most garden plants is obtained invariably 

 from the soil, either directly from compounds of nitrogen wich 

 oxygen, or from mineral and organic compounds — such as nitric 

 acid, ammonia, nitrates, and humus. Or it may be obtained 

 indirectly through symbiotic growth of micro-organisms living in 

 the soil, which have the power of assimilating the free and 

 uncombined nitrogen of the atmosphere. This symbiotic growth 

 is apparently almost altogether confined, so far as is yet known, 

 to species of plants belonging to the leguminous family, of 

 which Beans, Peas, Clovers, and Lupins are examples ; and . 

 possibly Orchids may be included in the list. 



