Muddstopper Mg is a secondary or Marco element of fertilizer and a SALT. Like all salts it is very Hygroscopic or attracts water. The Mg is call Adsorbent and the the hydra film is that water that coats the Mg molecule which is then called Adsorbate. To release the water or Hydra film the process is call desorption. This chemically differs from water up take by Plants which is absorption. IMHO Mg doesn't have enough Hydraulic conductivity to be of value to increase field capacity. But what do I know? BTW Mg is used as a drying agent in some cases.