118 The Plant World. 



not because the water in the soil is no longer available but be- 

 cause it is no longer supplied with sufficient rapidity. The 

 matter is really one of rates,— a matter with which pure quantity 

 factors have nothing to do. It is possible of course ^and indeed 

 probable, as stated above on pagel I3;that there is really a point 

 at which the capillary pressure in the soil balances the osmotic 

 urge into the plant, and at which water is actually no longer 

 absorbed. Water below such a content would be really ''non- 

 available." As yet, however, the very existence of such a point 

 is unproved and the first experimentation on this phenomenon 

 should be the investigation of its reality. 



It must be remembered too that there is a possible non- 

 availability of soil water with which the plant has nothing to do, 

 but which is caused by a breaking of capillary contact between 

 the soil grains immediately in contact with the plant roots and 

 the main body of the soil. This, if it occurs, is a purely physical 

 matter and is physically determinable. The dragging of the 

 plant into the question is neither necessary nor desirable. It 

 has nothing to do with the case. 



I'he Optimum Water Content. It has long been observed 

 that too much water and too little are alike injurious to plants. 

 The most healthful conditions and the best growth are obtained 

 when the soil possesses an intermediate water content the actual 

 value of which varies for different soils and for diflferent plants. 

 This water content is known as the optimum. In the first paper 

 of this series it was noted that for most plants this optimum 

 corresponds to that water content * at which the physical prop- 

 erties of the soil employed show an inversion and at which the 

 soil is in, or in condition to take on, its most favorable phvsical 

 structure. It may be that this physical factor is alone sufficient 

 to explain the existence of the biologic optimum. Plants may 

 perhaps grow liest at this water content simply because at higher 

 or lower contents the physical soil conditions are less favorable. 

 It is more probable, however, that the phenomenon is more 

 complex and depends upon other factors also, and especially 

 upon the water supply and the amount of air space in the soil. 

 The water supply furnishes a cause for decreasing productivity 

 when the water content is too low. There is simply a lack of 

 water. On the other hand the air supply of the roots may 



♦Called the "critical moisture content''. 



