Chapter VI. 



MOISTURE OF THE SOIL. 



IN GENERAL. 



The soil receives its water supply either by natural rainfall or by 

 irrigation. The plant in successive generations of cultivation adaj^ts 

 itself to the ordinary supply of water, but in order to perpetuate its 

 kind it must have sufficient during the growing season to serve it as 

 a medium for extracting from the soil and air the nutritious sub- 

 stances needed by it for its own development. The water really 

 available to the plant is principally that which is left in the soil close 

 to the roots after the surface drainage has carried off a large per cent 

 of the original rainfall and after the evaporation by the dry wind 

 has taken 20 per cent of the remainder from the surface soil and after 

 a further large per cent of the remainder has by percolation or 

 seepage slowly settled down beyond the reach of the roots of the 

 plant. Thus it happens that the roots rarely have left for their use 

 20 per cent of the original rainfall, and this is the so-called " useful 

 remainder." Generally this remainder is best expressed as a per- 

 centage of what the soil would hold were it completely saturated. 

 Therefore its absolute quantity will vary with the character of dif- 

 ferent soils 



EVAPORATION FROM THE SURFACE OF FRESH WATER. 



MONTSOURIS DATA FROM DESCROIX. 



An approximate idea of the relation between the velocity of the 

 wind, its temperature, and its dryness, on the one hand, and its power 

 to evaporate water on the other, may be obtained by collating the 

 data given by Descroix in his article on " The climatology' of Paris," 

 in the Montsouris Annuaire, 1890, page 121. From the mass of data 

 given by him I select the averages taken according to the direction 

 of the wind, or wind roses, for the three summer months June, July, 

 and August, 1889, as these are the months during which crops are 

 liable to suffer the most severely from droughts and dry winds. I 

 give them in the following table : 



(104) 



