I believe that the soil scrkpr referred to in the #1 post was some kind of commercial potting soil. These soils are often loaded with quite a lot of fertilizer and, also, they are usually sterilized. If they were not sterilized, bacteria would oxidize all the ammonium to nitrate. Probably when they are left open to the air, they get recolonized by bacteria that then oxidize all the ammonium. When they dry up the bacteria of course are inactive and can't do anything.
I have found that sometimes dried up soil causes a lot of bacterial growth when it is put under water, whereas moist soil never does. The only explanation I can think of is that drying may have killed a lot of the soil organisms and then they decay rapidly when the soil is covered with water.
I think the beneficial activities of the soil organisms can best be encouraged by putting the soil in a covered container like a sweater box or larger covered plastic container. The cover should be on loosely enough to allow some air exchange, but not so loose that the soil dries up. One or two weeks should be plenty of time for the decay and oxidation of all easily decomposable organic matter.
If there is very much nitrate in the soil there will be bubbling when the soil is covered with gravel and then water in the tank. The bubbles are nitrogen gas produced by denitrifying bacteria that use the nitrate the way we use oxygen in respiration. When the oxygen is used up many bacteria can turn to nitrate as a substitute. They give off N2 in the process. |