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

Most gardeners are familiar with potassium as one of the three primary macronutrients that are responsible for plant health and the production of blooms. Regardless of whether it occurs naturally in the soil or is added as a supplement, potassium regulates the distribution of nutrients and moisture and contributes to photosynthesis. 

Potassium, in common fertilizers, began as a potash salt and cannot be made synthetically. Instead, it is mined from now-dry, ancient seabeds. Once obtained, these natural salts are utilized to provide nutrients to plants.

It has been a long-held gardening tradition, when growing crops for competition, to use a plentiful supply both 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 crucial part of the success of healthy plants, organic supplements produce more successful crops along with fertile soil. 

Read on to find out about the benefits of using a high potassium fertilizer and exactly when and how you should use it for the best possible results.

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. 

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Identifying Potassium Plant Deficiencies

The best way to work out if your plants need more potassium is to take notice of their growth and watch out for a number of possible indicators:

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

These indicators tend to appear on older foliage at first and are typical symptoms of a lack of potassium.

To be absolutely certain, I recommend conducting a soil pH test. The results of which will reveal if the soil has a macronutrient deficiency and to what extent the soil needs adjusting. 

Testing Soil for Potassium 

When it comes to soil testing, there are a number of different kits on the market that can assist in narrowing down the root cause of the deficiency issue and helping to establish if the problem is related to a lack of potassium:

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. 

If you have conducted a soil test, amended potassium levels, and your plants are still not recovering, the following may be worth considering:

  • Soil levels may be too acidic (low soil pH)
  • Soil needs aerating to increase the availability of nutrient uptake
  • Iron levels need to be reduced to enable better absorption of nutrients

Impact of Soil pH 

The majority of plants perform best in soil with a pH range of between 5.5 and 8.5. There are exceptions however, blueberries, azaleas, conifers, and rhododendrons all have a preference for acidic soil in the region of 5 to 5.5. 

Most leafy green edibles and many ornamentals including lavender prefer alkaline soil with a pH range of 6 and above. 

A word of caution here though, soil pH over 8.5 will result in nutrients being completely prevented from being absorbed. And where soil pH levels are less than 5.5, plants are susceptible to the effects of heavy metals in the soil that restrict the process of 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 such a vast choice of options on the market, determining the best fertilizer may feel unnerving. But, rest assured, selecting one that meets your fertilizing needs is easy once you unravel the labeling details.

All fertilizers are required to provide details of both the macro and micronutrients that are contained within the product. The primary macronutrients – which are needed for all plants to grow healthily – are nitrogen, phosphorus, and potassium (NPK).

Additionally, the label will state details of the secondary ingredients plus any fillers that have been added. This includes trace elements such as copper, calcium, magnesium, boron, and iron, all of which also support plant health.

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

But let’s delve a little deeper into what exactly NPK is, and look at how to interpret the NPK ratio in any given product.

N-P-K Ratio 

An NPK ratio is easily identifiable on fertilizer labels as three numbers, usually separated by a dash. This series of numbers correlate with the proportion of nitrogen, phosphorus, and potassium contained within. For example, a 4-4-7 NPK means 4% nitrogen, 4% phosphorus, and 7% potassium. 

Plants need nitrogen, phosphorus, and potassium, and without them, are unlikely to remain healthy for too long. The amount each plant needs varies from one species to the next and so different fertilizer products contain varying proportions or NPK ratios. High potassium fertilizer is easy to recognize by the higher 3rd number, for example, 0-0-60.

Some food waste contains naturally occurring amounts of potassium. For example, the NPK ratio of chopped banana peels is .06-.04-11.5, for apple skins, it’s 0-3.08-11.74, and for cantaloupe skins, it’s 0-9.77-12.21 NPK. 


When it comes to growing vibrant, green, healthy leaves and stems, Nitrogen plays a vital role. It is also responsible for fueling chlorophyll production.

It is chlorophyll that provides plants with that unmistakable green foliage. In addition, chlorophyll makes sure that the energy from photosynthesis is available so that starches and carbohydrates can be converted into food. 

The diverse and hardworking nature of nitrogen also ensures that the proteins and enzymes that are required to regulate water and nutrient in plants are fuelled in order to carry out this task effectively.

Gardeners typically use High nitrogen fertilizer for lawn care and for non-flowering plants where foliage growth is most important. 


Phosphorus plays a fundamental role in the photosynthesis process. Once chlorophyll cells are produced, this macronutrient triggers the energy captured by the chlorophyll that then enables starches and carbohydrates to be converted into food. 

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

Phosphorus contributes to the development of the proteins and enzymes that are then fuelled by nitrogen and used to regulate water and nutrients. 


Potassium’s main function is to enable water, nutrients, and photosynthesized food to be successfully distributed throughout a plant. It also activates the proteins and enzymes that phosphorus forms and nitrogen uses to enable healthy and thriving plant structures. 

When applied in the correct quantities, potassium helps plants to conserve moisture which in turn supports the plant’s resilience against diseases, extreme heat, and drought conditions.

In addition, potassium is what ensures large, well-formed fruit on vining crops like tomatoes, squash, and melons.

What is High Potassium Fertilizer Used For? 

Without potassium, plants may suffer from stunted growth and a reduction in crop yield, but only some plants need large quantities of it. Let’s turn our attention now to the types of plants that require higher doses of potassium to achieve optimum growth and yield, and why exactly, that is.

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 producing larger crops that are perfectly formed. 

Vegetable patches can be particularly susceptible to severe frost, extremely high temperatures, and drought conditions. This is where potassium is needed to not only effectively circulate water around plants but also to assist in conserving it when high temperatures are detected. 


Plants and trees that are grown indoors need potassium for establishing healthy roots and an abundance of thriving foliage. If you know that your plant’s soil is fertile soil, then it’s ok to apply a balanced NPK such as a triple 10 fertilizer. 

For potting soil that is lacking in potassium, amendments will need to be made to help balance out any nutritional shortages.

Always do so with caution, and make sure you conduct a soil test once you have amended your soil to avoid overzealous applications which can potentially lead to 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. 

Adding a fertilizer containing potassium is recommended during the summer when temperatures are up and there is potential for drought. Avoid use in the spring, at a time when foliage growth and robust root growth are more important, as this may prevent your lawn from absorbing nitrogen and phosphorous.

How to Use 

High potassium fertilizers are available to buy as fast-acting liquids or slow-release granules. In container gardens, Fertilizer spikes are often preferred for container gardening. 

When planting new trees in potassium-deficient soil,

I recommend adding a handful of potassium granules to a planting hole if you are aware of a potassium deficiency when planting trees. This will encourage faster root establishment and more foliage growth. 

On vegetables, a higher potassium/phosphorus NPK, just as buds set, can help to encourage a better harvest.

If you know that the soil beneath your lawn is lacking in potassium, use a fertilizer containing potassium at the start of the growing season to encourage root growth and protect against drought and extreme temperatures later in the season.

Can You Use Too Much? 

It is possible to apply too much potassium and the consequences can be fatal.

Applying too much potassium can prevent nitrogen absorption which can lead to a yellowing (chlorosis) in the foliage. It can also destroy soil bacteria that would otherwise contribute to soil fertility.

I recommend conducting an analog, digital, or chemical soil test before you begin fertilizing, to avoid potential problems. It’s important that plants are only receiving what they really need. 

    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.

    Soil with a low pH range means that phosphorus along with iron and aluminum may react, causing nutrient absorption to be blocked. High levels of soil pH mean that phosphorus, together with calcium, will do the same.