BIOLOGY OF SOILS 161 



Liming has a good effect if the soil is markedly deficient 

 in that element ; a very slight acidity arrests the process 

 altogether. A comparatively small proportion of lime 

 less than 1 per cent. is, however, sufficient for the pur- 

 pose, and the addition of more produces no improvement. 

 Fresh lime is soluble in water, and has an alkaline reaction ; 

 it has, therefore, a tendency to arrest nitrification, but if 

 applied in the back end of the year this effect is not noticeable, 

 as it is soon converted into the neutral insoluble carbonate. 



Nitrification takes place very slowly during the cold 

 winter months, but proceeds more rapidly as the soil 

 becomes warmed by the summer heat. It takes place very 

 fast during the month of July, and reaches a maximum in 

 September (p. 104). There is thus a tendency for nitrates 

 to accumulate in the soil towards the autumn. 



The Nitrogen of the Soil. Agricultural crops, except 

 those which belong to the order leguminosae, do not obtain 

 nitrogen directly from the air, but absorb it, like other 

 elements, from compounds pre-existing in the soil. 



Nitrates are the most suitable compounds of nitrogen 

 for the nourishment of plants. It has been shown by 

 experiments that plants can assimilate nitrogen in the form 

 of ammonia and other compounds, but these substances 

 undergo nitrification in the soil very rapidly, and are rarely 

 present in any considerable quantity. Under ordinary cir- 

 cumstances, therefore, the available nitrogen consists 

 almost entirely of nitrates. 



Unlike other elements of plant food, nitrogen is not 

 originally a constituent of the parent substance of the soil. 

 It is all derived, ultimately, from the atmosphere, (1) by 

 the direct absorption of ammonia vapour ; (2) in the form 

 of ammonia, nitrous and nitric acids, and organic com- 

 pounds brought down by rain ; (3) by the action of algae 

 and bacteria directly ; (4) by the action of bacteria in 

 association with leguminous crops. 



S.M. M 



