Q.What is leaching?

Because most soil minerals are charged ions that cannot cross the cell membrane freely, root uptake occurs by two routes. Passive absorption moves ions into root cells by simple diffusion, from higher to lower concentration, without energy expenditure; a special case is Donnan equilibrium, where certain anions become fixed inside the cell membrane and additional mobile cations accumulate to electrically neutralise them, even against their own concentration gradient. Active absorption moves ions against their concentration gradient and requires ATP energy from respiration -- oxygen-deprived roots show a sharp drop in active mineral uptake. Hoagland and Davis (1923) proposed the carrier hypothesis to explain active transport: specific membrane proteins ('carriers') actively pump cations and anions from the soil into root epidermal cells, with Bennet and Clarke (1956) suggesting the carrier is conjugated with lecithin. Mineral ion absorption is independent of water absorption, but once inside the root, mine …