Now that you understand how mobile nutrients are in the soil, it'll make more sense when I write about how those nutrients get into roots.  Root hairs (tiny roots) are constantly growing, searching for new areas full of soil nutrients so they can trade for cations and suck up anions.  The roots use calcium as a way of determining whether they're growing in a good direction --- if calcium stops coming in (for example, if the root hits a rock), the root hair stops growing.

At the same time they're growing, the root hairs are exuding substances that help ease their way between soil particles and that also help in other ways.  The root exudates ooze between soil particles and help dissolve phosphorus, aid in the uptake of metals, and feed microorganisms that make other nutrients more available.  As I mentioned in yesterday's post, the roots are also pumping out hydrogen ions to trade for cations in the soil; and (less importantly), roots pump out hydroxyl ions to trade for negatively-charged nutrients.

Mycorrhizal fungi act like extension of the root hairs during this nutrient-uptake process.  The tiny fungi are thin enough and long enough to vastly extend the reach of the plant roots, and they're also good at bringing in immobile nutrients like phosphorus, copper, and nickel that would be tough for plants to suck up on their own.  Mychorrhizal fungi also help out by digesting organic matter to free up nutrients, making yet more nutrients available for plants.

As if all of the above methods of getting nutrients aren't complicated enough, plants also have to deal with weeding out nonessential nutrients (like heavy metals) that would compete with essential nutrients in the plant.  Some of the exudates released by roots are used to bind these nonessential nutrients into the soil so they become unavailable, and mychorrhizae also help out with this binding.

Why should the gardener care about all this microscopic action?  First, it's handy to understand how important microorganisms are in teaming with your plants so you'll spend energy keeping the microbes happy (with lack of tilling, the addition of organic matter, etc.)  But you should also be aware that the work of soil microorganisms and mychorrhizal fungi slows down drastically during cool weather, so your plants will have more trouble finding nutrients in early spring.  That might be a good time to pay more attention to ensuring easily-soluble nutrients are available right up against seedlings' roots so they can suck up nitrogen without bothering their sleeping fungal neighbors.