Negative ions saturation ratios with

Condensation can take place on ions as well as on aerosol particles. In his classic cloud chamber studies. Wilson (1927) found that a rain of relatively large droplets at low concenuation formed at a. saturation ratio of about 4.2 compared with a dense fog of smaller droplets at saturation ratio.s above 7.9. Wilson hypothesized that ions continuously generated in the air by natural processes served as nuclei at the lower saturation ratio he verified this hypothesis using ions produced by an x-ray source. In later experiments he showed that condensation took place on negative ions at saturation ratios near 4 at about —6°C and on positive ions at a saturation ratio near 6 at a slightly lower temperature. Similar results were obtained by later investigators. 

Discussion. The colloidal clay and humus soil fractions are negatively charged and therefore attract and adsorb positive ions (cations) on to exchange sites. These may be the so-called basic cations defined above, or the acidic cations H+ and Al +. These cations are not soluble in water when in the adsorbed state, but can exchange with H+ which is present in the acidic vicinity of the plant root system. They are now in solution and able to be absorbed into the plant. The extent to which the exchange sites are saturated with cations, together with the ratios of the cations to each other, indicates the nutrient supplying power of the soil. 

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