322 University of California Publications in Agricultural Sciences [Vol. 3 



a proper moisture supply in the soil at the blooming and setting period 

 is reflected in the practice of many growers who irrigate their orchards 

 heavily at such times as well as during the periods of hot, dry north 

 winds. 



In this connection it should be noted that Fowler and Lipman 51 

 have recently shown that under conditions of a soil moisture supply 

 somewhat below the optimum the visible effects upon the citrus tree 

 are a great deal less than under conditions of the same percentage 

 above the optimum moisture content. In other words, these authors 

 have shown that the citrus tree does not exhibit the effects of a deficient 

 soil moisture supply to the same extent that it does an excess of 

 moisture in the soil. It may well be, therefore, that many of our 

 citrus orchards are underirrigated and the irregular water relations 

 above discussed accentuated by reason of this fact. The authors feel 

 that many of the orchards studied in this investigation would probably 

 do better with heavier irrigation. Manifestly it would be useless to 

 attempt methods of modifying the climatic complex with the end in 

 view of cutting down daily water deficits, if the soil moisture supply 

 is deficient. Therefore, the grower should first make certain that 

 sufficient soil moisture is available. 



It has long been known that the presence of sufficient moisture in 

 the soil is not conclusive evidence that the plant is enjoying optimum 

 moisture conditions. Plants inhabiting salt marsh regions possess their 

 xerophytic adaptations by reason of the fact that although growing 

 with their roots in water or mud they are unable to obtain water in 

 any large amounts and are forced to economy in the use of it. This 

 inability to absorb water has been traced to the ratio between the 

 osmotic concentrations of the soil solution and the cell sap of the roots, 

 and such a condition is called ' ' physiological drought. ' ' Physiological 

 drought may be induced by the inhibition of absorption through the 

 action of factors other than the osmotic concentration of the solutions 

 involved. 



Among the most important factors conditioning absorption is that 

 of aeration. It has long been known that when grown in water cul- 

 tures many plants make very unsatisfactory growth. Hall, Brenchley, 

 and Underwood 52 have recently shown that this unsatisfactory growth 

 is due to lack of aeration and can be remedied by passing a stream of 



5i Optimum Moisture Conditions for Young Lemon Trees on a Loam Soil, 

 Univ. Calif. Publ. Agr. Sci., vol. 3 (1917), pp. 25-36. 



52 The Soil Solution and the Mineral Constituents of the Soil, Jour. Agr. Sci., 

 vol. 6 (1914), pp. 296-301. 



