18 



The carbon is obtained from the air by the process of assimilation. 

 From water-culture experiments (that is, by growing young plants 

 with their roots in jars containing water with various known salts 

 in known quantities), it has been found that nitrogen in compounds, 

 phosphorus, sulphur, magnesium, calcium, and iron all necessary 

 constituents of a plant must be taken in through the roots only. 

 Though always present in the ash, sodium, silicon, and chlorine are 

 not necessary. The experiments also show that no particles of food 

 in a solid state can enter the plant's roots, but must first be dissolved 

 either before application or in the soil water. Hence, the importance 

 of having plenty of humus in the soil to retain the water. The salts 

 in solution enter through the root-hairs, passing through the cell-wall 

 into the cell-sap. If, however, the salt is in a stronger solution than 

 the cell-sap, the reverse occurs, and the plant loses water and flags. 

 Thus great care must be taken not to apply too strong a solution of 

 any liquid manure to the active roots of plants. A similar result 

 is seen in the scorching or burning of leaves, with which some too 

 strong liquid manure has been in contact. 



Nitrogenous Manures. These are generally considered the most 

 important, seeming to. have a more distinct influence on an ordinary 

 soil than any other fertilizer. Nearly all plants have to obtain their 

 supply of nitrates from the soil, although there is such a large pro- 

 portion of nitrogen in the air. Moreover, water-culture experiments 

 show that exactly the same amount of nitrogen is removed by the 

 plant when fully grown as was supplied to the water. Leguminous 

 plants, however, such as beans, clover, and furze, are exceptions, and 

 have the power of fixing the free nitrogen of the air. This they are 

 enabled to perform by means of the nodules on their roots, the nodules 

 being colonies of a certain bacterium, which provides the plant with 

 nitrogenous matter from the air, and obtains from the plant in return 

 the carbohydrates necessary for the fixation of nitrogen. The only 

 limit to the quantity of nitrogen that can thus be supplied to the 

 soil is the difficulty of cropping the ground with leguminous crops 

 in frequent succession. 



As regards the effect on a plant, nitrogenous manures serve to 

 promote vegetative growth ; in fact, the growth of a plant is almost 

 proportionate to the amount supplied. Plants provided with an 

 excess of nitrogen, however, mature late and show but little tendency 

 to produce flowers or seed, and are moreover more susceptible of 

 disease than normally manured ones. 



Nitrate of soda and sulphate of ammonia are the two principal 

 nitrogenous manures. The main sources of the former are the large 

 deposits in Chile, and of the latter coal. As nitrate of soda is soluble 

 in water, and easily washed down into the subsoil, it should be applied 

 only as a top dressing during active growth, care being taken, of 

 course, not to let it fall on the leaves, as it forms a strong solution 

 with water and is liable to scorch them. This remark also applies 

 to sulphate of ammonia. Nitrate of soda can be safely mixed with 



