When surface soils are dry, O? and N? are adsorbed on the external
mineral surfaces. In the process of wetting the soil, water
molecules displace the adsorbed O? and N? molecules to the gas
phase where they can be measured, as was done in this study. These
gases, released from the adsorbed phase, join entrapped air in the
gaseous phase as the primary factor disintegrating aggregates when
soils are wet quickly. Adsorption of N? and O? occurs on surface
soils during hot dry afternoons as the water molecules leave the
surface. During cool nights, relative humidities commonly rise above
50%, allowing more strongly adsorbed H?O molecules to displace
adsorbed O? and N?. Release of this adsorbed N? and O? causes
aggregates wetted by immersion during hot afternoons to be less
stable than aggregates of the same soil wetted in the morning
