140 Mineral Nutrition of Plants 



Rose (19) that, at a given moisture content and between certain mois- 

 ture potential limits, dispersion by puddling diminishes the moisture 

 potential, i.e., moisture retention forces are increased. Although the 

 diminution appears significant, the magnitude of the effect in some 

 cases may be slight (12). McGeorge and Breazeale (5/) expressed the 

 opinion that puddling increased the moisture content at which plants 

 first displayed symptoms of moisture unavailability. 



Total porosity diminution by puddling proceeds at the expense of 

 the coarser pores, which are destroyed relatively easily. It is largely in 

 these coarser pores that air and water movement occur and that roots 

 find their most favorable environment. The benefit derived from 

 aggregation is largely associated with the increase in the important 

 larger pore spaces in the soil. Sands, of course, may have sufficiently 

 large pores for normal air and water movement but the pore spaces in 

 most silts and clays, if these soils are not aggregated, are usually so 

 small that capillary forces cause them to remain full of water to the 

 exclusion of air. The persistence of smaller pores, despite puddling 

 treatment, can be explained in three ways, according to Rubin (59) : 

 (a) destroyed small pores of a given radius are replaced by others re- 

 sulting from decrease in radius of larger pores; (b) smallest pores are 

 filled with water which resists removal and prevents pore collapse; and 

 (c) small pores owe their existence, in large part, to a more stable 

 particle arrangement in their immediate vicinity, i.e., they may (and 

 probably do) exist in greater abundance within the real or potential 

 aggregates which are present in the soil mass. 



Increase in relative abundance of very fine pores conceivably may 

 affect water availability to plants, directly, by increasing the energy of 

 water retention and, indirectly: (a) by offering greater resistance to 

 root penetration, (b) by decreasing the supply of necessary oxygen 

 to the roots with the result that roots no longer function normally in 

 absorbing water, and (c) by increasing, over short distances, the re- 

 sistance to water movement toward the plant roots. The mechanisms 

 involved should be given more attention than they have heretofore re- 

 ceived, but the net effect to the plant would be qualitatively similar, 

 namely, the availability of the existing water supply would be dimin- 

 ished. 



