116 



CASSELL'S POPULAR GARDENING. 



but from the absence of what has been taken out 

 of it. 



Applying- these facts to the garden, it follows that 

 crops of the same general character, and requiring 

 almost the same manurial constituents for their 

 development, should not succeed each other too 

 rapidly ; a sufficient time being allowed to elapse 

 between the growth of similar crops to enable the 

 soil to recover from its exhaustion, whether it be 

 of minerals, or of nitrogenous organic plant-food. 



Most garden crops have two great advantages 

 over those grown on the farm, from the fact that the 

 period of their growth takes place at a time of year 

 which is not only more favourable for active nitrifi- 

 cation, but also when 

 the soil is less ex- 

 posed to loss of the 

 soluble nitric acid 

 by excess of rain. 

 Garden crops have 

 still one other ad- 

 vantage over those 

 of the farm ; nitric 

 acid contains an im- 

 mense amount of 

 oxygen, and as it is 

 only those particles 

 of carbon which are 

 close to the surface 

 of the soil that can 

 have access to oxj-- 

 gen, the constant 

 stirring which the 

 soil receives during 

 the growth of most 



vegetables is doubtless the means of continually ex- 

 posing fresh surfaces to the action of the atmosphere. 



Assimilation of Tfitrogen by Plants. — 



Having so far considered the amount of plant-food 

 derived from rain, and the minor aqueous deposits, 

 and the amount of nitrogen which various plants 

 are able to take up or assimilate from the atmosphere 

 and soil combined, when grown without any manure 

 at all, or with a supply of the ash constituents of 

 plants only, we may briefly consider the question 

 whether plants assimilate the free nitrogen of the 

 air, which is such an obvious and easy way out 

 of so many difficulties in connection with veget- 

 able economy, that this assumption has from time to 

 time been freely made. As about four-fifths of the 

 atmosphere which surrounds growing plants consists 

 of nitrogen, it is an important question, why should 

 not this be a source to them of the nitrogen they 

 evidently require ? 



It has most abundantly been established, as we 



-Wheat. 



shaU have cause to show, that ammonia in its com- 

 pounds is appropriated by plants as food, and that 

 ammonium salts applied as manure, in actual practice 

 have produced the most striking results. Further, 

 by watering potted plants with very dilute solutions 

 of ammonia, their luxuriance is made to sui'pass in 

 a most surprising manner similar plants placed 

 in precisely the same conditions, save that they are 

 simply supplied with common water. 



Yille has stated that vegetation in conservatories 

 may be remarkably promoted by impregnating the 

 air with gaseous carbonate of ammonia. For this 

 purpose lumps of solid ammonia salt are so disposed 

 in the heating apparatus of the green-house as to 



gradually vaporise; 

 or vessels contain- 

 ing a mixture of 

 two parts quicklime 

 and one part sal- 

 ammoniac ma}'- be 

 employed. Care, 

 however, must be 

 taken that the air 

 does not contain 

 too much ammo- 

 nia vapour, other- 

 wise the foliage of 

 tender plants is in- 

 jured. 



Although such 

 facts indicate very 

 clearly that am- 

 monia is directly 

 absorbed by plants, 

 they most decidedly 

 fail to prove that the soil is not the medium through 

 which the absorption really takes place. 



Again, it is considered an essential part of good 

 tillage to break up and loosen the soil, in order that 

 the air may have access to the decaying vegetable 

 matter, as well as to the living roots which descend 

 to considerable depths beneath the sm-face of the 

 soil. When air is thus admitted to the roots, it is 

 not impossible that some of the nitrogen of the 

 atmosphere, as weU as its oxygen, may be directly 

 absorbed and appropriated by the plant. 



In order to test the fact whether plants do assimi- 

 late the free nitrogen of the air or not, Lawes, 

 Gilbert, and Pugh arranged an elaborate series 

 of experiments at Rothamsted to determine the 

 question. 



The enclosing apparatus consisted of a large 

 glass shade, resting in a groove filled with mercury 

 in an earthenware stand, upon which the pan, with 

 the pot of prepared soil, was placed. The atmo- 

 sphere supplied to the plants was washed free from 



Fig. -2. -Barley. 



