THE SOIL ATMOSPHERE 433 



297. Organic matter. Organic matter being more 

 porous than any size or arrangement of mineral particles, 

 the effect of that constituent is always to increase the 

 volume of air. While this is generally beneficial in a 

 humid region, it is often very injurious in an arid one. 

 Unless sufficient water falls upon the soil to wash the 

 soil particles around the organic matter and to maintain 

 a supply sufficient to promote decomposition, the pres- 

 ence of vegetable matter leaves the soil so open that the 



Fig. 115. Blade cultivator, with hammock seat. For surface work. 



capillary rise of moisture is interfered with, and the large 

 movement of air keeps the soil dry, with the result that 

 the portion of the soil layer mixed with and lying above 

 the organic matter, is too dry to germinate seeds or 

 support plant growth. 



298. Moisture content. It is quite evident that the 

 larger the proportion of the interstitial space filled with 

 water the smaller will be the quantity of air contained. 

 This does not necessarily mean that the higher the per- 



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