How to determines the actual availability of nutrients

The amount of a particular nutrient determined analytically does not necessarily represent what is available but is merely taken to be a rough measure of it.  For a plant the quantity of available nutrient present does not matter so much.  But the rate at which the required quantity is supplied to the root system is very important.  The intensity i.e., rate and capacity (amount analysed) determines the actual availability of nutrients to crops and therefore the knowledge of both is necessary.  The availability will further depend on the nature and characteristics of crop grown, growing conditions and soils characteristics. No extractant can take care of all the variables and in that sense the methods are empirical.