Page and Bod man 141 



Penetrability by roots 



The impenetrability of unfractured, dense rock to plant roots needs 

 no comment. In shallow residual soils plant growth may be seriously 

 restricted by the presence of continuous rock masses. Dense layers of 

 soil also may be expected to resist root penetration provided that the 

 density is sufficiently high or the pores sufficiently small. For example, 

 restricted root growth may be observed over the surface of puddled 

 cultivation-soles in citrus orchards and within crevices over the sur- 

 faces of dense structural columns in the subsoils of some alfalfa fields. 

 Uptake of water and other nutrients will thus be restricted. In Cali- 

 fornia there is now being followed a practice of noncultivation of 

 orchards whereby danger of puddling is diminished and previously 

 puddled soils are allowed, by natural processes of wetting and drying 

 in the undisturbed state, to recover their former more open structure. 

 Root development is thus enhanced. 



Soils of low total porosity and relatively high microporosity may be 

 almost impermeable to water. The B horizon of the San Joaquin series 

 (California) has been shown (//) in some profiles to have a total 

 porosity (0.26 to 0.34) inadequate to accommodate its moisture equiva- 

 lent water, the magnitude of the latter being determined in the stand- 

 ard way on disturbed and crushed soil lumps. This horizon, assuming 

 its penetrability to both roots and water, would be deficient in the total 

 water available for plant growth if for no other reason than that the 

 upper limit of growth water would be significantly less than in the 

 case of the same material in the less compact state. Field observation 

 reveals very few roots in the B horizon of these soils. 



An apparent anomaly was observed by Veihmeyer and Hendrickson 

 (yo) in the seasonal change in moisture content of the subsoil of the 

 Bale gravelly loam (California) under grapevines, and in that of a 

 certain primary soil supporting a chaparral plant association. Despite 

 water additions and plant transpiration these subsoils showed but slight 

 changes with time in the content of total water, which suggested lack 

 of absorption by roots. A density of 1.83 g. per cc. was observed in the 

 Bale subsoil. Later work (7/) was conducted in which the penetrability 

 to sunflower roots was examined for several different manually com- 



