PHOSPHORUS AND PHOSPHORIC ACID 93 



combined nitrogen most easily absorbed by the roots of plants, the import- 

 ance of cultivating the soil and admitting air is understood. One of the 

 chief advantages of a long summer's fallow^, in fact, is that nitrification 

 can go on during a long period, and a large amount of nitrate formed for 

 the ensuing crop. Since warmth favours fermentation, these changes take 

 place principally in the summer months, so that crops grov^^ing in the 

 summer and autumn are, other things being equal, more independent of an 

 artificial supply of combined nitrogen than a crop growing in the spring. 

 A fourth condition favourable to nitrification is a sufficiency in the soil of 

 a base able to neutralise the nitric acid produced, for this acid, being a 

 powerful oxidising agent, would otherwise destroy the nitrifying germs and 

 therefore stop further nitrification. The base usually applied for this pur- 

 pose, if there is insufficient in the soil, is lime. 



The nitrates being all soluble in water, their loss by drainage is very great. 

 At Rothamsted it was found that drainage water from uncultivated land 

 contained on the average (19 years) as much as 5 grains per gallon of 

 nitrates calculated as nitrate of sodium, a loss which amounted to 214 lb. 

 per acre per annum. For this reason a long summer fallow should never 

 be carried out on light soils through which water can easily pass, as the 

 nitrate produced would thus be lost. It is moreover desirable, as far as is 

 possible, that crops should always be growing on such land so as to utilise 

 the nitrate as soon as formed. Salts of ammonia are not found to an 

 appreciable extent in drainage waters, owing to their reaction with the 

 double silicates of alumina and lime, magnesia or soda of the soil, the 

 ammonia being fixed and a salt of lime, magnesia or soda, set free 

 (see p. 99). For this reason sulphate of ammonia is sometimes preferred 

 as a nitrogenous manure to nitrate of soda. It must, however, be remem- 

 bered that since the ammonia has first to be nitrified, it is slower in its 

 action on crops, and that owing to the formation of these insoluble com- 

 pounds with soil constituents, it chiefly acts, when applied as a top dressing, 

 on shallow-rooted plants. 



XXIII. PHOSPHORUS AND PHOSPHORIC ACID 



The element phosphorus, like nitrogen, is an essential con- 

 stituent of all animal and vegetable organisms. When organic 

 matter containing much phosphorus decays, the hydride of 



