THE SOIL AND THE PLANT 137 



of water found in certain soils periodically examined by the 

 writer : 



Lowest amount of Water found in certain Soils after Periods 

 of Drought, per cent, by Weight 



Clay soil, Rothamsted. 



No organic manures 

 supplied. 



ii-g 



Annual dressings of 

 farmyard manure. 



i8'o 



Sandy soil, Millbrook. 



7-6 



Heavy clay soil, Woburn. 



20'0 



The air relationships are bound up with the water content 

 of the soil. Only a limited amount of space is available in 

 the soil for air and water. In some of the above cases the 

 figures are : 



Volume available for Air and Water in 100 Volumes of Soil in — 



Rothamsted. 

 No organic manure. 



Total space unoccupied by soil particles . . 34 



Minimum of water always present ... 23 

 Difference, being maximum volume of air that 



can be present 11 



Rothamsted. 

 Farmyard manure. 



38 

 30 



8 



The volume of air present in the soil fluctuates from the 

 maximum figures given above to the very small volume found 

 during and shortly after heavy rain. Oxygen is continuously 

 being absorbed and carbon dioxide evolved but usually atmo- 

 spheric oxygen diffuses sufficiently rapidly into the soil to 

 maintain there a partial pressure approximately equal to that 

 outside. Thus the difference between the soil air and atmo- 

 spheric air is that the former contains a few more volumes of 

 carbon dioxide in 10,000 parts than the latter. 



Injurious and inhibiting factors are of various kinds and 

 form a highly vague group but some of them must be put out 

 of action by calcium carbonate, because of the striking effect 

 it has on soil fertility. Indeed from the vegetation stand- 

 point, soils can be divided into two main groups — those that 

 contain calcium carbonate and those that do not. So great 

 is the distinction that the practical man has long since made 

 use of a separate name for the latter soils and calls them " sour," 

 a word which many writers naturally but not altogether 

 correctly interpret as "acid." 



These then are the main relationships which agricultural 

 chemists are called upon to investigate. Of the various hypo- 

 theses as to the constitution of the soil that have been framed 



