CHAPTER XXI 



DRYLAND FARMING ' 



The term " dryland farming " refers really to water con- 

 servation in farm practice, particularly in arid and semi-arid 

 regions, where irrigation is impossible or not practicable. The 

 term is a misnomer and unfortunate, but, because of its 

 general acceptance, is still adhered to. 



The following is a rough classification of land according 

 to the average annual rainfall : — 



Less than 10 inches — arid. 



10 — 20 ,, — semi-arid. 

 20—30 ,, —sub-humid. 

 More than 30 ,, — humid. 



This is a very general classification, because factors such 

 as run-off, evaporation and percolation, are not taken into 

 consideration. For example, 15 inches of rainfall will pro- 

 vide moisture for much better crops in a part having an 

 annual evaporation of 20 inches, than in a part having an 

 evaporation of 70 inches. 



For a proper appreciation of the subject a brief descrip- 

 tion of the forms of water found in soils is necessary. 



Hygroscopic Water. — This form of water is held in the 

 soil by the forces of adsorption. It is not capable of moving 

 from particle to particle, and can be completely expelled only 

 by heating the soil to the temperature of boiling water. Soils 

 previously dried, so as to deprive them of all their moisture, 

 and then exposed to moist air, absorb water vapour from the 

 atmosphere with great energy at first ; both the rapidity of 

 absorption and the amount absorbed, when full time is given, 



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