Understanding Soil Nutrients – Potassium

This is the third entry for the Understanding Soil Nutrients Series. Today we will focus our attention on a very important soil nutrient – potassium. If you missed the first and second entries to this series, here they are:

Understanding Soil Nutrients – Nitrogen

Understanding Soil Nutrients – Phosphorus

Why Potassium is Important To Your Vegetable Garden

Potassium (K) is generally called potash when used in forms for a soil fertilizer. The name comes from the collection of wood ash in metal pots when the soil benefits of this material were first recognized centuries ago. Hence, the name “potash” came to be.

This nutrient is essential for vigorous growth, disease resistance, fruit and vegetable flavor and development, and general plant function. Potassium breaks down slowly so you won’t have to add it very often. Deficiency symptoms include yellow areas along the leaf veins and leaf edges, crinkled and rolled-up leaves, and dead twigs. Fruit trees may develop fruit with poor flavor or stunted fruits. Certain animal manures and mineral fertilizers, such as greensand, add potassium to the soil.

There are several different forms of potassium available for use in your vegetable garden, here are a few:

    • Muriate of Potash – Muriate of potash is potassium chloride containing between 50 to 60% potash. Its chlorine content passes off rapidly when applied to soil, and is harmful to certain beneficial bacteria. Some authorities think sulfate of potash makes a better potash fertilizer.
    • Sulfate of Potash – Sulfate of Potash contains 48% potash. It is more expensive than muriate of potash but is considered less harmful to bacteria and plant roots. It also does not contain the chlorine that is in muriate of potash.
    • Wood Ashes – Organic gardeners rapidly accept wood ashes, since it is truly organic in nature. Wood ashes contain about 6% potash, plus good amounts of lime. At one time used corn cobs were put in piles and burn. The ash remnants of the burnt cobs contained up to 35% potash. Almost any ash resulting from burning organic materials that contain some fiber should be a decent source of potash fertilizer. Wood ashes are particularly good to use for adding potash to compost.

When using a fertilizer, check the N-P-K rating on the back of the container. These numbers represent the percentage of each macro-nutrient in the fertilizer. Potassium (or K) is usually the last number in the N-P-K rating. In the picture below, this fertilizer has 2% nitrogen, 1% phosphorus, and 1% potassium.

The Last Number Represents The Percentage of Potassium

Make sure to complete a thorough soil test on your vegetable garden soil before adding potash. If a potash deficiency is detected, potash can be worked into the garden soil usually once a year, when the soil is being readied for planting.

5 Comments on Understanding Soil Nutrients – Potassium

  1. According to our soil test, our potassium level is excessive at 248 pounds per acre. Do I need to correct this, and if so, how? We add goat manure mixed with alfalfa hay scraps to our soil throughout the year. Do we need to do less of that or continue?

    • Hi J.R.!

      That is a high amount of potassium, but I don’t think it will cause any huge problems. The best way to correct it would be to lessen the amount of manure and alfalfa hay and just add more plain compost.

      To be honest what you are adding is very good for your soil, but too much can cause an imbalance.

  2. Hubby just built a 4x8x8 raised vegetable garden. Wanted it to be organic. Used peat moss, garden soil, nitrogen and phosporus (Hi-Yield brand, 3 or 4 lb. bag of each). Also added an entire bag of muriate of potash, enough for 800 SF. Our garden is 32 SF. We have already planted early spring plants, i.e., lettuces, broccoli, onions, asparagus. If they even survive, will they be safe to eat? Can you reach a toxic amount of muriate of potash for consumption? What would you recommend we do?

    • Hi Julie – I don’t think you can reach a toxic level of potash (or muriate of potash). The plants will use what potassium they need and will not intake ny more. The onions will enjoy the potash as it helps develop strong root systems and onions are basically roots. The other plants you listed need more nitrogen as they are considered foliage vegetables and nitrogen promotes greener, stronger foliage.

      I think you will be ok just don’t add any more potash this season. You can help balance it out some by adding compost as a side dressing around the plants once they are past the seedling stage.

  3. my garden soil tests very high . . . P – 173 K – 1100

    What can I do to lower these high levels? What vegetable’ plant do well with such high levels?

    Thanks in Advance,


1 Trackbacks & Pingbacks

  1. Vegetable gardening in arizona | Organic Gardening Magazine

Leave a comment

Your email address will not be published.