POROUS CONES FOR AUTO-IRRIGATION 



203 



mercury column used, as a barostat. If a sufficiently 4arge num- 

 ber of cups are used, to give the requisite rate of water movement, 

 the apparatus functions with relatively very great precision, auto- 

 matically adding water to the soil about as rapidly as the plant 

 absorbs it, and the soil moisture content fluctuates over a range 

 of only a few per cent., on the basis of soil weight or soil volume, 

 no matter what may be the fluctuations in the rates of trans- 

 piration, evaporation and root absorption. 



It is clear that this arrangement requires, at all times, an ade- 

 quate capillary continuity between the water of the soil and that 

 of the porous porcelain, which means that the instrument may- 

 fail to operate if the soil retreats away from the external periph- 

 ery of the cup so as to break the necessary capillary connections. 

 In short, the outer surface of the cup must make good contact 

 with the soil about it, or water movement will be hindered. Now, 

 as is well appreciated by all students of soils, a three-phase soil 

 (containing both w-ater and gas, as well as the solid particles) is 

 practically never a static system, even with reference to the solid 

 phase alone. Especially when there is rapid fluctuation in* the 

 soil temperature and in the evaporation and root absorption 

 rates (which is the case under ordinary greenhouse conditions), 

 the solid particles of the soil are being moved about more or less 

 continually by capillary forces and by gravitation. When the 

 soil loses water it usually shrinks, and when the lost water is re- 

 placed the resulting swelling often fails to bring the soil mass 

 back to exactly the form that it had before. It thus comes about 

 that this shifting of the solid particles of the soil may often break 

 the capillary continuity originally existing between the porous 

 cup and the soil in auto-irrigated pots, at least for a portion of the 

 area of contact. Of course such a reduction of the surface of 

 adequate continuity betw^een the water of the cup and that of the 

 soil must reduce the rate of water movement from cup to soil 

 under any given capillary gradient, and if the reduction is suffi- 

 ciently pronounced a pronounced lag in the operation of the auto- 

 irrigator must become evident. This difficulty has been en- 

 countered in practice, where cylindrical porous cups have been 

 used for the auto-irrigation of potted plants, and the practical 



