What is High Phosphorus Fertilizer | When to Use It?

Gardening enthusiasts, worldwide, are familiar with phosphorus as a necessary component of healthy plant growth and flower production. Whether naturally occurring in the soil or manually added, phosphorus contributes to development and maturity, photosynthesis, and the cache and transfer of energy. 

The phosphorus we use as fertilizer began as rock phosphate. Which is mined from land deposits that were once ancient sea beds. Phosphate molecules in the seawater combined with the fossilized remains of aquatic creatures. Enabling us to use it to nourish our plants, millions of years later.  

For decades, it’s been common practice to use lots of phosphorus to force an abundance of blooms on ornamentals, fruit trees, and vegetables. However, several new studies have put this old practice to the test.

Revealing that while phosphorus is, no doubt, a vital component of plant health, most don’t need excessive amounts in order to thrive and produce. Forcing abnormal growth might actually weaken plants. Not to mention, potentially contaminate the soil and nearby water sources. 

NPK fertilizers contain phosphorus in varying amounts. But, what is a high phosphorus fertilizer? And when is it beneficial to use so much?

What Does Phosphate Do for Plants? 

Phosphorus plays a vital role in the functionality and structure of a plant’s DNA. From this base, cells are formed that depend on this element to create strong cell walls. Grouped cells than from root structures, stems, foliage, buds, and fruit. 

Once plants begin to form, phosphorus then aids in the process of photosynthesis and food production. Using energy from the sun to transform sugars and starches into self-made nutrients. An ability that is then transferred from one generation to the next via seeds, rhizomes, tubers, or bulbs. Provided with sufficient doses, all plants should grow in a robust and vibrant manner.

Increased phosphorus won’t necessarily improve these processes unless plants are grown in deficient soil. In which case, it may appear that your plants have exploded with surprising growth after use.  

Without adequate access to phosphorus, plants will show signs of deficiency in a number of easy-to-recognize ways. These signs, if caught in time, will give you the opportunity to make necessary nutritional adjustments toward health and vitality. 

Identify Phosphorus Deficiency in Plants 

How can you tell if your plants aren’t getting sufficient amounts of phosphorus? One way is to carefully observe the growth of your plants and look for the following symptoms:

  • Smaller than normal plant size
  • Few to no flowers or fruit
  • Thin root systems
  • An abnormal, purple-ish hue to stems and foliage
  • Curling leaves
  • Leaf drop

These symptoms will typically show on older growth first. And could all be indications that more phosphorus is temporarily needed until the soil is fertile enough to support your plants’ nutritional needs.

The most definitive way though is a good soil test. These results will reveal if your growing medium is phosphorus-deficient and how much the soil needs to be added.

Testing Soil for Phosphorus 

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

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

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

  • Bray P1 – will tell you approximately how much soil phosphorus is available in a form that your plants can absorb. 
  • Bray P2 – tells you the total amount of soil phosphorus present. Both in proper form for intake and not. 
  • Mehlich 3 – is commonly used for phosphorus testing. As well as potassium, calcium, magnesium, sodium and the micronutrients.

Impact of Soil pH 

Most plants thrive in a pH range of 5.5 to 8.5. The lower end of the Ph scale indicates acidity and the higher end, alkaline. 

Hydrangeas, azaleas, and many berry varieties prefer the lower end of this range. Many popular ornamentals and shade lovers prefer it closer to the high end, while vegetables 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.

When too much phosphorus is introduced, soil pH levels will often rise. Studies have shown that an increased pH level, in turn, blocks phosphorus availability. Adding well-aged compost to your soil can assist in making phosphorus available to your plants in high pH soil.

Understanding Fertilizer Labels 

With so many options out there, choosing the highest performing fertilizer may seem daunting. But, you can easily match a fertilizer to your specific needs by understanding the labels. 

Most fertilizer manufacturers will state how much nitrogen, phosphorus, and potassium (NPK) their product 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.

A high phosphorus fertilizer (or superphosphate), more commonly used for agricultural applications will have a much higher middle number in the NPK ratio,  A standard superphosphate feed contains 20% phosphate. While triple superphosphate contains 48%. 

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 

Every fertilizer label reflects 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 10-30-10 NPK means 10% nitrogen, 30% phosphorus and 10% potassium. Every plant requires these three elements in order to thrive. Just in different proportions or NPK ratios.

There are also a few organic materials that naturally increase available phosphorus. These include a steamed bone meal (3-15-0 NPK), made from mammal bones, and a fishbone meal (3-18-0 NPK).

Bear in mind that no matter how much phosphorus you apply to your soil, it’s the mycorrhizal fungi (beneficial soil bacteria) that convert that phosphorus into a form that your plants can use. Without them, our plants wouldn’t stand a chance. 

Nitrogen

As the first element in an NPK ratio, nitrogen plays a vital role in encouraging the growth of healthy foliage, by fueling the production of chlorophyll. High nitrogen fertilizers and indeed nitrogen in general, give foliage its green color and ensure that energy (acquired through photosynthesis) is available to convert starches and carbohydrates into food. Which, in turn, maximizes flower and fruit yield. 

This multi-tasking macronutrient is also present in root systems, as an integral part of proteins and enzymes that serve to regulate water and nutrient uptake and disbursement. High nitrogen fertilizer is typically formulated for lawns and other plants where foliage is the primary focus.

Phosphorus

Phosphorus is key in the process of converting sunlight into food that is necessary for healthy growth and the formation of flowers, fruit, and seeds. Once the chlorophyll is produced, aided by nitrogen, the baton is then passed to phosphorus. Which then triggers the energy from photosynthesis to make food. Unlike nitrogen, the focus of phosphorus is on flower and fruit production, rather than foliage. 

As the middle number in an NPK ratio, 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.

Potassium

Potassium, the third number in an NPK ratio, is the driving force behind the circulation of water, nutrients, and internally produced food throughout a plant. Once, nitrogen opens the floodgates. This macronutrient also activates those proteins and enzymes that were formed with the help of phosphorus and containing nitrogen. 

Adequate amounts of potassium aid in increasing a plant’s resistance to extreme heat and drought by helping to conserve moisture when temperatures soar. 

This collective effort, by all three macronutrients, allows for the creation of new, healthy cells within your plants that efficiently utilize photosynthesis to feel healthy and vigorous new growth and optimize yield.

What is High Phosphorus Fertilizer Used For?

So, given that adequate amounts of accessible phosphorus are essential for robust growth and a bountiful harvest, why use higher doses? What plants require more than others to achieve optimum growth and yield?

We’ve discussed superphosphate fertilizers that are more suitable for large, agricultural applications rather than home gardens or allotments. So, in this case, a ‘high phosphorus fertilizer’ will be one with an NPK like 15-30-15 for poor soil and larger plots. Or a 2-8-4 for small applications.

Annual flowering plants (with a single-season lifespan) are typically heavier phosphorus feeders simply due to the short time in which they have to bud, flower, and produce seed.

Cold weather crops, like lettuces and root vegetables, depend on phosphorus to be winter-hardy. In summer, vining crops like tomatoes, peppers, squash, and melons need a vegetable garden fertilizer with increased phosphorus in the blooming stage to produce large, well-formed fruit.

Trees and Shrubs

Phosphorus is used by fruiting and flowering trees and shrubs to establish strong root systems and produce abundant blooms. It also supports the transfer of energy and resistance to environmental stresses. Evergreen trees and shrubs accomplish this with a more balanced NPK.

Despite the warning that too much phosphorus leads to dangerously high pH levels, recommended doses of a standard, off-the-shelf, high phosphorus NPK can actually lower soil pH and increase acidity. For this reason, some are tempted to add more to acid-loving shrubs.

This is not recommended as too much can block the absorption of other vital nutrients. Compost, leaf mold, or fertilizers specifically formulated for acid-loving plants will yield far better results.

Flowers

As mentioned, annuals (especially those grown in the ground) like a bit more phosphorus in their diet, in order to produce a lot of vivid, summer color. It also helps accelerate plant maturity in annuals. Which results in buds and flowers appearing sooner than on plants grown in phosphorus-deficient soil. Unless, they’re already growing in rich, fertile soil. In this case, a well-balanced feed will support an equally balanced show of healthy stems, leaves, and blooms. This would also apply to those grown in pots or hanging baskets. 

Flowering perennials will typically put on a great show with equal amounts of nitrogen, phosphorus, and potassium. Too much phosphorus may force extra blooms but at the expense of the plant as a whole.

Vegetable Gardens

All vegetables require phosphorus for root development, flowering, and fruiting. Some will do just fine with a balanced NPK. But, climbers like tomatoes, squash, and melons require higher amounts once buds are set. The extra phosphorus will bolster the plant’s energy to turn those buds into fruit. Encouraging large and well-formed crops. 

More effort is needed in a veggie patch, than in other areas of our gardens, to ward off pests and disease. Phosphorus helps out in this area, as well. Kitchen scraps like coffee, tea, and citrus work great for adding a touch of extra phosphorus where needed.

Houseplants

Just like outdoor varieties, our indoor plants and trees need phosphorus to form strong roots and healthy foliage. When grown in fertile soil, a balanced NPK (ie. 10-10-10 fertilizer) works well. In phosphorus-deficient potting soil, a 10-15-10 NPK can help to balance out any nutritional shortages.

Flowering plants like African violets, begonias, and bromeliads use phosphorus to form lots of lovely flowers. These do well with a 7-9-5 NPK.

When adding extra phosphorus to houseplants, please do so with caution. Just as with outdoor plants, too much can block the absorption of other vital nutrients and kill your plant.

Lawn and Grass

Lawns and grasses utilize phosphorus to produce a thick, lush blanket of color and a robust root system. Yet, relying on higher levels of nitrogen to maintain a vibrant structure that is also resistant to wear-and-tear, excessive heat, and drought.

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

The best way to know how much phosphorus to use on your lawn is to do a soil test prior to starting any kind of fertilization schedule.

How to Use High Phosphorus Fertilizer

Depending on the kind you choose, this type of fertilizer can be applied as a liquid when watering or by working slow-release granules into the soil. In some cases, fertilizer spikes can also be very effective. 

When planting new trees in phosphorus-deficient soil, a handful of granular phosphate can be added to the hole prior to planting. This will help roots get established more quickly and encourage new growth. 

In new garden beds, working granules into the top few inches of soil or watering with a water-soluble option, just after planting, will provide plants with a great start. 

With vegetables, applying a higher phosphorus NPK just when buds are setting will promote an abundant harvest.

On lawns grown on a phosphorus-poor substrate, scattering phosphate granules across them followed by a good watering will provide the roots with the right dose for a robust, green turf.

Can You Use Too Much?

As we’ve seen, applying too much phosphorus can have fatal consequences. This is why doing a P1, P2, or Mehlich 3 soil test, prior to fertilizing, is such a wise decision. 

Excessive amounts can:

  • block the absorption of other essential nutrients required for healthy plant growth.
  • eliminate beneficial soil bacteria that are essential contributors to soil fertility.
  • be detrimental to the environment by contaminating soil, nearby water sources and the aquatic life that live in them. 

All plants need phosphorus to grow healthy. It’s important to make sure that they’re receiving only what they truly need. 

Re-check Soil pH After Application

Because phosphorus affects soil pH, it’s important to the health of our gardens to re-test it roughly 30 days after a high phosphorus application.

If the pH drops too low, phosphorus will react with iron and aluminum naturally present in the soil and block the absorption of other nutrients. If the pH becomes too high, the phosphorus will combine with calcium deposits in the soil and do the same. 

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

High Phosphorus Fertilizer FAQ’s