I have read that the Earth has undergone five ice ages, the latest one is the Quaternary glaciation. I know that in the ice age there are glacial periods and interglacial periods, and we are currently in an interglacial, I want to make sense what is the effect of those succsessive periods on the life on Earth. Because in my text book I read that the soil has grown and become fertile since 20 thousand years ago, since the interglacial period started. But what made this change in the soil? and why it became suitable for farming? And is it a costant thing that the soil become rich and good in the interglacials of all ice ages?
First a comment on the number of ice ages - one could say 5 to 7 major ice ages, depending upon how one defines an ice age, and several more minor ice-ages, with some debate as to whether some are sufficiently separated in time to be regarded as two ice ages. Most of the northern hemisphere has been emerging from the most recent ice age over the past 10000 to 12,000 years, but by no means at a constant rate, with some short-term reversals. As regards soil formation, it is only the most recent ice age that is of relevance - older palaeosols have been either eroded or lithified.
How long does it take for soil to form? That is primarily a function of climate, and secondarily a function of weathering lithology. That is, many millennia for cold dry climates, or as little as a decade or so for hot wet conditions. Soils take many years to form on limestone, quartzite, granite and some other resistant rocks, but are rapid on recent basic rocks such as volcanic ash. The weathering rate of different minerals varies over many orders of magnitude, and is a combination of chemical and biological processes which is far too complicated to discuss here. I suggest you read a good book on weathering processes.
Regarding soil fertility, this is also rather complex, but obviously continuous cropping will tend to remove organic carbon, organically available phosphate and potassium, not to mention other trace elements, thereby leaving the soil less fertile unless very well managed.
Ice ages are not 'normal' in the geological history, and it will take tens to hundreds of millennia for a normal soil quasi-equilibrium to be reached. This 'equilibrium will also be lost both by human intervention and by accelerated climate change.