Growmore

Looks like lots of you have been seduced by the organic lobby. Plants need NPK to grow and they don't care where they get it from, the source makes no difference to yield or quality of growth as long as the quantity is the same. Some organic fertilisers do have to breakdown to release their nutrients so spread their effect a little but bear in mind things grow when they are young not when they are old. The practical way to garden to get good results is feed the soil with as much organic matter as you can get hold off but largely ignore any nutritional benefits that my accrue as a bonus. Feed the plants according to their NPK requirements, if you can get hold of "Know and Grow Vegetables" from the library it has some good tables of rates of use for fertilisers of different NPK contents. Buy your NPK according to price, Growmore is often the cheapest but prices vary widely probably from 60p to £3 a kilo. At the correct rate it will feed a plant for it's growing season, I don't even give over Winter Brassicas anything extra in the Spring. By the way, it isn't made from oil like the organic lobby like to tell you.

Please look at http://www.pda.org.uk/leaflets/8/no8-page1.htm
If you do not want to read it all just read the following quotation

Potash leaching
Potassium does not leach anything like as readily from soils as does nitrate and sulphate. Potash additions not taken up by the crop will be held in the soil by the clay minerals or organic matter as described above. For the great majority of soils, which have a clay content of 5% or more, where normal rates of potash are applied, potassium not used by the crop will remain in the cultivated layer of soil and will not move further down the profile. However, significant losses of potassium can occur when any source of potash (fertiliser, slurry or manure) is applied under adverse conditions i.e. when soil is water-logged, frozen or very dry and deeply cracked. Most of this loss is by surface run-off and can be avoided by following codes of good agricultural practice. Soils with less than 5% clay (the sands and loamy sands) have a much lower retentive capacity for potassium. Such soils, especially if shallow and subjected to rainfall producing large amounts of through-drainage, have a greater risk of potassium loss. On these soils, potash should be applied 'little and often' and applications timed to suit crop uptake and amounts carefully matched to crop offtake.

Yes it can be leached but only under extreme circumstances.