Cation exchange in soil.
<P>
   Clay  soils  are usually alkaline and bind positively charged minerals
   (cations such as Ca^2+).
<P>
   Hydrogen  ions  (H^+)  help make nutrients available by displacing the
   cations.
<P>
   Plants  secreting H^+ by cellular respiration: CO[2] reacts with H[2]O
   to  form carbonic acid (H[2]CO3) in the soil, which dissociates to add
   H^+ to the soil.
<P>
   Video:  <A ClAsS=ACT Href="http://media.pearsoncmg.com/bc/bc_campbell_biology_7/media/activities/load.html?37&amp;A" oNClick="popact('http://media.pearsoncmg.com/bc/bc_campbell_biology_7/media/activities/load.html?37&amp;A');txtcolor('soil_availability1','gray');return false;" Id=soil_availability1><iMg sRc="/images/tutorial.gif" aLt='' boRDer=0></A>