106 SOIL CONDITIONS AND PLANT GROWTH 



TABLE XLIII. PORE SPACE, WATER CONTENT, AND AIR CONTENT OF CERTAIN 



SOILS. RUSSELL. 



The actual quantity of water present in the soil is constantly chang- 

 ing from the saturation quantity, which completely fills the pore spaces, 

 to the minimum amounts recorded in Table XLIII., but the extreme 

 values only persist for a very short time in normal soils, the usual 

 fluctuations being between much narrower limits. Thus, a considerable 

 bulk of the soil is really water spread as films over the particles or held 

 up in the pore spaces, but unfortunately the actual thickness of the 

 films cannot be calculated from the available data. 



The whole of this water is not generally available for any one plant. 

 Water must be supplied to the plant at least as quickly as it is lost by 

 transpiration, otherwise wilting sets in. Now the rate of supply of soil 

 water is simply the speed at which water can move in the soil, and this, 

 as we have seen, depends on the amounts of clay and colloidal matter 

 present ; it may easily fall below what is wanted for maintaining equi- 

 librium in the plants growing on soils rich in clay or organic matter. 

 Another factor also comes into play. The soil water is not pure, but 

 contains dissolved substances which do not necessarily enter the plant 

 simultaneously with it. As removal of the water goes on the solution 

 becomes more concentrated, and may finally reach a point where it is 

 too concentrated to enter the plant. Thus, wilting will often set in on 

 clay or humus soils containing several per cents, of water. 



Determinations have been made by Heinrich (127), Briggs (56) and 

 Crump (73) of the water still remaining in the soil when wilting begins, 

 and it is customary to speak of this as " unavailable " water. The ex- 



1 Driest periods of 1909 and 1910. During the abnormal drought of 1911 the numbers 

 fell to 6 and 8 for the first two soils. 



