Wadleigh and Richards 441 



solvent properties, i.e., "unbound" water. Reitemeier (98) determined 

 the dissolved ions in solutions extracted from six soils at four moisture 

 contents, and found that the concentration of nitrate and chloride in- 

 creased as the moisture content decreased. He explained this effect on 

 the basis of the existence of "unfree" water in the soil or negative ad- 

 sorption of monovalent anions, or a combination of both. This relative 

 concentration of chloride and nitrate ions in the outer layers of thin 

 moisture films may also be involved in the usually observed accumula- 

 tion of nitrogen in plants on dry soils. In general, Reitemeier found 

 the opposite effect for cations and polyvalent anions; that is, as the 

 thickness of the moisture films decreased, there was a relative dimi- 

 nution of the concentration of these ions in the outer layers. It may be 

 coincidental, but plants subjected to low soil moisture supply also tend 

 to have a relatively low content of these ions (52, 55). 



The diminished amplitude of the cationic swarm about an adsorption 

 surface under a thinning moisture film appears to be conducive for 

 potassium ions present to enter the lattice of the clay crystal and become 

 fixed. Volk (i2j) found that alternate wetting and drying of soils 

 treated with soluble potassium salts caused rapid fixation of potassium 

 in a nonreplaceable form, and that very little fixation of this kind took 

 place when the soils were kept continuously moist. This has been veri- 

 fied many times (97, 133, 4, ///). Acid soils fix relatively little potassium 

 when moist, but drying effectively increases potassium fixation. Calcare- 

 ous soils fix potassium when moist and the extent of fixation increases 

 on drying. Martin et al. (83) suggest that the effect of drying on potas- 

 sium fixation is that of increased concentration of ions at the adsorption 

 interface, and that dehydration, per se, is not involved. It is logical to 

 conclude that the relatively low potassium content of plants subjected 

 to low soil moisture supply is related to the increased intensity of potas- 

 sium fixation under such conditions. 



Soil moisture depletion is also conducive to the fixation of phosphorus 

 (88, 119). Trumble (119) concludes that this explains the relatively 

 low phosphorus content of plants under inadequate soil moisture sup- 

 ply. Neller and Comar (88) report that the extent of phosphorus fixa- 

 tion on drying is directly related to the clay content of soils. 



