A new question dropped into the Ask Gardenerd inbox this week:
“Lovin’ [your] podcast since 2009! My soil test recommends 15-0-15 for potash deficiency. Organic sources would be….what? Wood ash, greensand, kelp meal? Do you have preference? I’m told wood ash (easiest and free) leaches out faster and has other elements too, ones I don’t need to supplement like calcium and magnesium. Just wondering what you think…”
First of all – thanks for listening to the podcast! So glad you like it. If you have a second to post your positive review on iTunes, it would help us a lot.
Okay – on to your question:
Potash, or potassium, is responsible for fruit and flower development in plants. It is an insoluble element, usually found in rock materials like crushed granite, that is invaluable in the garden. Some sources of potash are better than others, some more available to plants than others.
While bloodmeal, feathermeal or bat guano will provide the nitrogen solution (the first number listed above) for those recommendations, potash is a little more tricky.
Wood Ashes – As you mentioned, wood ashes are usually free, but they must be used with caution if your soil pH is over 6.5. Wood ashes tend to make the soil more alkaline, so if your soil is already neutral or alkaline, avoid using wood ashes.
Greensand, a slow-release fertilizer of marine origin, will offer about 3% to 6% of your 15% needed. It’s close, but not quite what we’re looking for.
Animal Manures – You can use dried animal manures (at about 1.5%) to deliver potassium to your garden, but the usual choice for the kind of supplementation you need is rock potash.
Rock Potash – Peaceful Valley Farms offers organic rock potash in 50 lb. bags, but on Amazon you can find rock potash in smaller quantities. One thing to remember is that excessive use of potash can prevent plants from taking up magnesium and other essential minerals, so use as directed.
As with many products that we use in our gardens, rock potash is a mined product. Potassium is the 7th most abundant element on earth, but that doesn’t mean it’s easy to get to. The good news is that these sources of potash remain in the soil for a long time, so you probably won’t have to add potassium too often.
Thanks for writing in, and happy gardening!