What is High Potassium Fertilizer | How and When to Use It

Most gardeners are familiar with potassium as a necessary component of healthy plant growth and flower production. Whether naturally occurring in the soil or supplemented, potassium regulates the distribution of nutrients and moisture and contributes to photosynthesis. 

Potassium, in common fertilizers, began as a potash salt. Which is mined from now-dry, ancient seabeds. The potassium in fertilizers cannot be made synthetically, so these natural salts are used. Enabling us to use it to nourish our plants, millions of years later.  

For decades, it’s been common practice, when growing crops for competition, to use lots of phosphorus and potassium to increase size. However, several new studies have revealed that amending soil with organic compost and manure contributes to size and flavor far more than synthetic fertilizers.

Demonstrating that while potassium is, beyond doubt, a vital component of plant health, organic supplements produce more successful crops along with fertile soil. 

But, what is a high potassium fertilizer? And when is it beneficial to reach for fertilizers that contain a higher amount?

What Does Potassium Do for Plants? 

Potassium plays a critical role in the creation, distribution, and metabolism of the food that plants make through photosynthesis, by triggering dozens of “worker” enzymes to focus on each of these processes. 

High potassium fertilizers will next move their attention to supporting phosphorus in bud, flower, and fruit production. Viable seeds are then formed for plant reproduction. This is particularly important in vining crops like tomatoes, squash, and melons. Whereby, access to potassium is critical to the formation of large, juicy fruits.

Root systems also highly benefit from this macronutrient. It not only increases root growth, in order to sustain and support plants as they grow larger but improves drought resistance too by conserving moisture when a temperature increase is detected. 

Identify Potassium Deficiency in Plants 

How can you tell if your plants need more potassium? One way is to carefully observe the growth of your plants and look for the following symptoms:

  • Scorch marks on leaves
  • Curling leaves
  • Yellowing between leaf veins (chlorosis)
  • Purple-ish spots on the underside of foliage
  • Stunted growth

These symptoms will usually show on older growth first. And could all be indications that more potassium is needed.

The most definitive way though is a good soil pH test kit. The results of which will reveal if your growing medium is macronutrient deficient and how much the soil needs to be amended. 

Testing Soil for Potassium 

If you’re applying sufficient potassium and your plants are still not responding, consider these potential factors:

  • Soil pH could be too low
  • Soil is too dense for proper nutrient intake
  • Iron levels in soil may be too high and interfere with nutrient intake

There are a few different tests available that can help narrow down the cause of deficiency:

Digital testing kits – tests for current pH levels and available amounts of nitrogen, phosphorus, and potassium.

Analog testing kits – tests for moisture content and pH levels.

Chemical testing kits – measure nitrogen, phosphorus, potassium, and soil pH. 

Impact of Soil pH 

Most plants thrive in a pH range of 5.5 to 8.5. Hydrangeas, azaleas, and many berry varieties prefer acidity of the lower end range of this scale. 

Many popular ornamentals and shade lovers prefer alkaline soil which is closer to the high end of a pH range.  Vegetables (in the main) do very well somewhere in the middle. 

Above 8.5, nutrients will be blocked entirely. Lower than 5.5 and plants become vulnerable to heavy metals in the soil that inhibit photosynthesis.

Luckily, an abundance of potassium in the soil has little effect on soil pH, if any.  Nonetheless, excessive potassium can still harm plants. 

Understanding Fertilizer Labels 

With so many options out there, determining the best fertilizer may feel unnerving. But, you can easily match one to your specific needs by understanding the labels. 

Most fertilizers state how much nitrogen, phosphorus, and potassium (NPK) it contains on the label. On the back, will be a list of all secondary ingredients and fillers (if any). This will often include trace elements like calcium, magnesium, iron, and other micronutrients that support plant health.

High potassium fertilizer is commonly used to encourage high yields on flowering fruit and vegetable plants. 

With all this talk of an NPK, you may still be wondering what it is. How do you decipher an NPK ratio? What do all those numbers mean for your plants? 

N-P-K Ratio 

Most fertilizers have a three-number formula. This is the N-P-K ratio of that fertilizer, indicating its proportions of nitrogen, phosphorus, and potassium. For example: a 4-4-7 NPK means 4% nitrogen, 4% phosphorus and 7% potassium. 

Every plant requires these three elements in order to thrive. Just in different proportions or NPK ratios. High potassium fertilizer is easy to recognize by the higher 3rd number, for example, 0-0-60.

There are also a few organic materials that naturally add potassium. These include chopped banana peels (.06-.04-11.5 NPK), apple skins (0-3.08-11.74 NPK), and cantaloupe skins (0-9.77-12.21 NPK). 


Nitrogen plays a vital role in encouraging the growth of healthy, vibrant foliage, by fueling the production of chlorophyll. This gives foliage its green color and ensures that photosynthesized energy is available to convert starches and carbohydrates into food. 

This multi-tasking macronutrient also fuels the proteins and enzymes that regulate water and nutrient uptake and disbursement. High nitrogen fertilizer is typically formulated for lawns and other plants where foliage is the primary feature. 


Potassium is primarily responsible for the circulation of water, nutrients, and photosynthesized food throughout your plants. Secondly, it activates those proteins and enzymes that are formed using phosphorus and nitrogen to build a healthy plant structure. 

Proper amounts of potassium contribute to a plant’s resistance to disease and extreme heat and drought by helping them to conserve moisture. And is especially effective in encouraging large, well-formed fruit on vining crops like tomatoes, squash, and melons.


Phosphorus is key in the process of photosynthesis. Once chlorophyll cells are produced, this macronutrient triggers the energy captured by the chlorophyll to convert starches and carbohydrates into food. 

An important (and more visible) function of high phosphorus fertilizer is the creation of abundant flowers and fruit, rather than foliage. 

Phosphorus begins its work at the cellular level. Contributing to the construction of those same proteins and enzymes that nitrogen uses to regulate water and nutrients. 

What is High Potassium Fertilizer Used For? 

So, given that most NPK fertilizers have potassium in them, intended for robust growth and vitality, why use higher doses? What plants require more than others to achieve optimum growth and yield?

Annual and perennial, flowering plants need a bit more phosphorus and potassium to produce abundant blooms. Higher amounts are also needed to produce viable seeds, for future planting. As well as rhizome, tuber and bulb replication. 

This holds true for flowering vegetable and fruit plants, too. If necessary amounts of potassium and phosphorus aren’t available for these plants, blossom drop and bottom-rot on crops will follow. 

No matter the plant variety, they all benefit from increased disease resistance, strong stalks for upright growth, and drought tolerance. 

Trees and Shrubs 

Fertilizers for evergreen trees and shrubs use potassium to maintain a healthy and consistent flow of water and nutrients throughout their structures. In which the focus is to generate lush, green foliage. 

This process starts when potassium redirects energy, captured by the foliage through photosynthesis, to the conversion of starches and carbs into glucose. A plant’s primary food source.

That metabolized food circles back and fuels healthy growth which manifests as new, green foliage. 

Potassium, in flowering trees and shrubs like magnolias and azaleas, is also used for nutrient and water circulation. But, instead of supporting foliage, flowers abound.  


Now, to be clear, potassium does not directly influence flower production (a common misconception). That’s the job of phosphorus. What potassium does is create an environment full of hydrating moisture and healthful micronutrients to support the role of phosphorus in flower production.

It also fuels other enzymatic processes that support the efforts of other nutrients. Without potassium, phosphorus wouldn’t be able to generate flowers, or a bountiful harvest, at all. 

In flowering veggie and fruit plants, potassium’s efforts also lead to sweeter, juicer berries, melons, and stone fruits. As well as beautifully formed, nutritious vegetables that may actually display a longer shelf life. 

Vegetable Gardens 

All vegetables require potassium for optimal plant growth, enzyme activation, glucose and calcium metabolism, and of course, photosynthesis.

Some will grow well with a balanced NPK. But, fertilizers for pumpkin, squash, and melons contain higher amounts of potassium and are especially beneficial once buds set. This also supports phosphorus in its efforts to steer energy toward large and well-formed crops. 

More effort is needed in a veggie patch, than in other areas of our gardens, to ward off the effects of extreme temperatures, and drought conditions. Potassium not only effectively circulates water around plants but assists in conserving it when high temperatures are detected. 


Our indoor plants and trees need potassium to form strong roots and healthy foliage. When grown in fertile soil, a balanced NPK (ie. 10-10-10) works well. 

In potassium deficient potting soil, adding a few pieces of chopped banana peel can help balance out any nutritional shortages.

While flowering plants like African violets, begonias, and bromeliads use phosphorus to form lots of lovely flowers, they do well with an equal balance of nitrogen and potassium.

When adding extra potassium to houseplants, please do so with caution. Just as with outdoor plants, too much can cause irreparable damage to your plants. 

Banana Trees

As these heavy feeders progress through the growing season and commence fruit production, they require potassium in abundance to protect them against disease and improve drought tolerance. 

Banana tree fertilizer can help to improve the quality of the fruit harvest. The addition of high amounts of potassium also enables them to regulate their water intake.

Lawn and Grass 

Turfs and wild grasses utilize potassium for effective water and nutrient uptake. This macronutrient also plays a vital role in constructing thick cell walls in each blade of grass. Creating a robust, overall structure that’s resistant to environmental stresses and wear-and-tear. 

Applying just enough potassium for these functions is important. Using an appropriate feed such as a liquid lawn fertilizer will lead to healthy turf, while also preventing phosphorus from contaminating your soil and nearby water sources. 

Excessive amounts of potassium on lawns and different grass types may end up denying them access to much-needed nitrogen, contaminating the soil beneath and ultimately killing it. 

How to Use High Potassium Fertilizer 

This type of fertilizer can be applied as a liquid or by a slow-release granule method. In container gardens, fertilizer spikes can also be very effective. 

When planting new trees in potassium-deficient soil, a handful of granular potash in the hole prior to planting will help roots establish more quickly and encourage new growth. 

In new garden beds, work potash into the top few inches of soil or water with a liquid option, just after planting. 

On vegetables, a higher potassium/phosphorus NPK, just as buds set, will promote an abundant harvest.

On lawns grown on a potassium-poor substrate, scattered potash, followed by a good watering, will provide the roots with the right dose for a robust, green turf. 

Can You Use Too Much? 

Applying too much potassium can have fatal consequences. This is why doing an analog, digital or chemical soil test, prior to fertilizing, is such a wise decision. It’s important that plants are only receiving what they truly need. 

Excessive amounts can:

  • block the absorption of nitrogen and other essential nutrients required for healthy plant growth, causing a yellowing (chlorosis) in the foliage.
  • eliminate beneficial soil bacteria that are essential contributors to soil fertility.
  • be detrimental to the environment by allowing phosphorus to contaminate soil, nearby water sources, and the aquatic life that lives in them. 

Re-check Soil pH After Application 

Research has shown potassium to have little to no effect on soil pH. But, because this nutrient and phosphorus (which does affect pH) work together to increase bloom quantities, it’s important to re-test soil 30 days after application.

If your soil pH is too low, phosphorus will react with iron and aluminum in the soil to block the absorption of other nutrients. Too high and phosphorus, together with calcium, will do the same. 

Plants experience a ‘goldilocks’ period of healthy, robust growth, with abundant flowers and fruit, when potassium, phosphorus, and pH are just right.