THE NUTRITION OF THE ROOT 6i 



s^ 9. How can the soil best meet the requirements of the 

 roots for air ? 



It is not only because the soil contains an extraordinarily 

 large quantity of the nutritive solutions in its pores that we 

 have compared the ideal soil with a sponge, but also on account 

 of its permeability for air. The roots must breathe, and there- 

 fore the soil on which our crops grow, must be of such a nature 

 that down to a certain depth there must be a sufficient inter- 

 change between the air contained in the soil and the outer 

 atmosphere. This explains the advantages of the addition of 

 organic humus-forming substances to all soils, whether they be 

 light or heavy. The nature of the air contained in the soil is 

 not, however, immaterial. If the processes of decomposition 

 in the soil take a wrong course, gases detrimental to the root 

 may be formed ; and as we are able, whenever we work or 

 manure the soil, to change its structure and its porosity, we 

 have every reason to pay attention to the interchange of 

 gases. 



The air contained in the soil has not the same composition 

 as the atmospheric air ; for within the soil oxygen is continu- 

 ally being used up by the processes of decomposition of organic 

 substance, and the roots themselves constantly absorb some 

 and give off other gases. Besides these two causes, various 

 gases are physically fixed (absorbed) by the small particles 

 of the soil. Of these gases, we are at present only interested 

 in two — oxygen and carbonic acid. Nitrogen, and its com- 

 bination with hydrogen (ammonia), we have previously dealt 

 with. 



We may assume that, generally speaking, the air of the soil 

 is richer in carbonic acid and poorer in oxygen than the 

 atmosphere. The rapidity of exchange depends on the atmos- 

 pheric pressure, and upon the moisture of the soil. The 

 greater the water capacity of the soil, the greater are the 

 variations in its permeability. If much water is present, the 

 permeability for air is greatly reduced. A loosening of the 

 soil will increase the permeability to a greater extent, the 

 finer the particles of the soil are. 



