Dowding says that it is untrue that all soil nutrients are water soluble and can be leached by the rain. I think that he is a little confused here.
Only nutrients in solution have been observed entering plant roots. If they aren't soluble then they aren't nutrients.
None of the nutrients exist in the soil as elements but are always bonded with other elements in molecules. When the soluble molecules dissolve they dissociate into positively and negatively charged particles called "ions".
These positively and negatively charged particles can stay in solution or associate with the opposite charge on the surface of organic matter or some clays. This is called nutrient adsorption and forms what can be called an active nutrient pool. Nutrients can be adsorbed or dissolved depending the concentration of nutrient ions in solution and the soil solution pH.
So the plant can access these nutrients by altering the pH of the soil solution. Plants do this by secreting organic acids from their roots. When acids dissolve in water they also dissociate into positive and negative ions and one of them is always a proton - a hydrogen ion. They can also pump protons out of the root using energy. Protons are little jobs that can get between the nutrient ions and organic matter popping off the nutrients and returning them to the soil solution by replacing them.
Getting nutrients into the root is not an straightforward process. Some nutrients enter down a concentration gradient, where there is a higher concentration in the soil solution than in the root cells. They have to enter root cells through special pores in cell membranes. Other nutrients need to be pumped into the root cells using energy.
If you alter the concentration of nutrients around the root by adding man made fertilisers there may be a net flow of water out of the root due to osmosis. This will damage the root and may be the origin of the "root burning" misconception. However, even when I used National Growmore, I never saw "root burning". Adding fertilisers, whether organic or man made, is an expensive business so I never put very much on.
I presume it would be more likely with the delicate roots of seedlings but with the composts I use - home made or bought - it is never a problem.
I do add lots of manure and compost.
There may be a problem with fresh manure because of the concentration of urine and the production of ammonia, but this is not a problem if you allow urine and ammonia to be leached out, leaving the manure to rot down for a while. This is why it is useful to have the manure storage at a high point on the allotment garden. . A slow leaching will allow the nutrients to become diluted and avoid concentrations around the roots. The urine and ammonia will flow down the slope and be made available to vegetables while it is moving through the soil in the soil water.
Cations are positively charged and anions are negatively charged ions. The cation exchange capacity of the soil can be seen as a measure of the soil's ability to store and pop off plant nutrients. In other words its fertility.
Any nutrients dissolved in the soil solution can be lost due to mass flow of water through the soil pores - what we call leaching.
It is true that some nutrients, like phosphorous, have very low solubility and that is probably why the region around the root is usually deficient in this nutrient. Phosphorus from the active pool is slow to dissolve and this leads to depletion.
Phosphorus can also be combined with elements like calcium and aluminium. Calcium phosphate (bones and a small percentage of egg shell) is not very soluble and aluminium phosphate is not soluble at all. Although calcium phosphate is slow to dissolve, it is soluble and does contribute to the nutrient pool.
So to reiterate; all nutrients are soluble and all nutrients can be leached albeit some slower than others.
If nutrients are adsorbed onto the surface of clays or organic matter then they are a little more resistant to leaching.
Manures and composts contain nutrients which are locked up in large organic molecules. Organic chemicals in Chemistry just mean those with carbon in them. While they are locked up in the organic molecules they are inaccessible to plants. However, these large organic molecules can be used by bacteria and fungi to produce their mass and energy. While this is happening nutrients are released into the soil solution or adsorbed onto charged surfaces. This is called mineralisation.
Adding man made fertilisers bypasses this process.