Dear Nguyen, I do agree with what Barbara has suggested, however this in only possible where the material is available and from my experience these rock materials are not available every where. The most traditional, cheap and readily available source of K in rural set-up is "Kitchen Ash". This is available I think in every household. This source however can have riots levels of K depending on the source of fuelwood. Studies show that ashes from hardwood have high K contents. Hardwood ashes can be applied directly as fertiliser or can be added into a compost pile to improve K levels. This is very useful especially for home/Kitchen gardens. Ashes have also effect of raising soil pH and can also regulate chemical and biological processes in the soil biosphere

If you mean organic sources, I would recommend the following: wood ash, compost made with fruits/vegetable wastes especially banana peels, manure from cattle or fowl, and fresh green manuring.

For all practical purposes, K in animal manure is fully plant-available. Generally, my experience is that when manure is applied at rates adequate to meet the N requirements of crops, P and K are more than adequately supplied as well.  

The previous answers cover the topic quite well. Most 'organic' farming groups ban the use of natural potash (potassium chloride) because its high chloride content is believed by some to be harmful to soil fauna and micro-organisms. There is little evidence for this belief. They usually allow the use of potassium sulphate instead, even though most of the potassium sulphate sold these days is produced by reacting potash with sulphuric acid. Most other products are low in K content, and transport costs and/or availability are usually major limitations. Some countries (eg Egypt) have deposits of 'high' potassium feldspars. These are effective as very slow-release sources of K if very finely ground, but are only about 5% K.

Dear Dr. Hue,

You are right, compost or vermicompost content, on average, 1 % or less of K2O. As you mentioned, seedweeds should be available in Hawai. You can dry and transformed them into a powder to avoid their possible regrowth as invasive weeds. Also, you can place them into water to create slurry (after decomposition). All the K would be into the liquid solution. You can even compost them.

A traditional source of K is fish wastes.

Some used alfalfa pellets (3-0.5-3) or you could try a green manure.

Off course the mineral forms suggested by Barbara Sawicka such as organic potassium sulphate, powdered kainite powdered langbeinite, K Max Lite (up to 15.5% K), greensand (0-0-3), some marine sediments (up to 7% K2O), Great Salt Lake minerals, etc.

In all cases, you should check if they are authorised by your organism of certification. Good luck,

Dear, in general, application of composted animal manure has sufficient quantities of potassium too. In exceptional cases where K demand of the plant is too high, crop residues rich in K may be used for recycling. An organic Banana farmer utilizes all the banana wastes other than the fruits for recycling in his farm in india.

We are trying to fertilize are squash plots with the recommended amount from the soil test (200LBS/AC of K2O). Our composted chicken litter is 4-3-4 and we will apply it according to the Nitrogen requirements of 80 lbs/ac. That leaves us with 120 lbs/ac of needed K. So we are looking for a source of organic K any suggestions?

Organic banana peels, which are kitchen and processing plants' wastes, as well as vermicompost can certainly help.

To add to the list, seaweed-based composts are worthy too. Seaweed is a rich source of bioavailable potassium.

Thank You

Compost made primarily from food byproducts is an excellent source of potassium. In particular, banana peels are very high in potassium. Wood ash can also be used, but make sure that you apply wood ash only lightly, as too much can burn your plants.Potassium sources such as wood ash, greensand, and seaweed can also supply K but require special management because of their low nutrient content, their effect on soil pH, low solubility, or bulky nature. Typical sources include mined rock powders and wood ash. Of course, manures, compost and other organic materials are potassium sources, too, because even though the concentration of potassium in them is pretty low, typically a lot of material is applied to a field.

Potassium requirements of most crops are rather high, comparable to nitrogen . Providing this amount of potassium through compost is rather difficult. As suggested by Rk Naresh , some high potash bearing sources may be tried. An added advantage of application of composts is that it helps to retain more amount of potassium in available ( water soluble as well as exchangeable) forms in soil. This helps to reduce actual requirement of potassium application by preventing the large scale loss of this nutrient.

In my humble opinion, One option is soils with a heavy texture without conducting the improvement operations. In fact, It can be suitable pi-loads (case study) for conducting such an experiments due to the presence of preservative colloids and an appropriate granulation/granular (intake and uptake of the K element from the base source). Of course, It is better to the whole variables ought to considered alone or together and the comparison analysis was done for receiving the best results. This works to some extent in natural ecosystem and as above said it needs to more studies in the crop lands and agricultural ecosystems.

Regards