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High Phosphorus Fertiliser and When to Use it

Phosphorus, represented by the 'P' in the NPK ratio on fertilisers, is a vital component for plant growth. It plays a significant role in various plant functions such as energy transfer, photosynthesis, and nutrient transportation. In this guide, we will take a closer look at phosphorus and its importance in fertilisers.

Phosphorus is a macronutrient that plants primarily absorb from the soil. It aids in the development of new tissue and division of cells. Being a part of plant DNA, it's crucial in the growth process, specifically during the early stages. A phosphorus deficiency can result in stunted growth and fewer blooms for flowering plants.

Moreover, phosphorus also contributes to the overall health and strength of a plant. It helps with root development, which enables plants to absorb water and other essential nutrients from the soil. Stronger roots lead to better anchorage, making plants more resistant to wind and other environmental factors.

Phosphorus and Plant Growth

Phosphorus is vital for the growth and development of plants, as it plays a critical role in photosynthesis. Photosynthesis is the process in which plants convert sunlight into energy to support their growth. During this process, plants absorb sunlight, water, and carbon dioxide to produce glucose, which serves as the main source of energy for plant growth. Phosphorus facilitates the transfer of this energy, making it an indispensable element for plant development.

In addition to its role in photosynthesis, phosphorus also significantly contributes to root development. Roots are responsible for absorbing water and nutrients from the soil, and phosphorus enhances this process by promoting root growth and improving their nutrient absorption capacity. Furthermore, phosphorus is instrumental in seed development, making it a vital factor in plant reproduction.

The Importance of Phosphorus in Fertilisers

Phosphorus is an essential nutrient for plants, and its availability in the soil directly affects plant growth and yield. However, phosphorus is not readily available in most soils, making it necessary to add it through fertilisers. Fertilisers that contain phosphorus provide plants with the necessary nutrients for optimal growth and development.

Phosphorus is particularly essential in modern agriculture practices where intensive farming leads to soil depletion. As crops absorb more nutrients from the soil, including phosphorus, it becomes necessary to replenish the levels through fertilisers or through more wholistic and less intensive farming practices.

Understanding Phosphorus Levels in Soil

Phosphorus is naturally present in most soils, but its availability for plants can vary depending on the soil pH levels. In acidic soils, phosphorus tends to bind with other elements and become less accessible to plants. On the other hand, in alkaline soils, it may react with other minerals and form insoluble compounds that cannot be taken up by plant roots.

Australian soils are inherently different from many other soils around the world, known for being old, highly weathered, and low in nutrients, including phosphorus. Unlike other soils, the majority of Australian soils are acidic, which as mentioned earlier, can cause phosphorus to bind with other elements, making it less accessible to plants.

Remarkably, Australian native plants have evolved and adapted to thrive in these low phosphorus conditions. They have developed specialised root systems and symbiotic relationships with certain types of fungi to increase their ability to access and absorb the limited phosphorus available in the soil. Consequently, many Australian native plants can be sensitive to high levels of phosphorus. Gardeners growing these plants need to ensure they use low-phosphorus fertilisers to prevent damage to their Australian natives. When fertilising gardens with Australian natives, look for a fertiliser with a phosphorus percentage of less than 3%.

High Phosphorus Fertiliser

A high-phosphorus fertiliser contains a larger proportion of phosphorus compared to other nutrients. The NPK ratio, which stands for Nitrogen, Phosphorus, and Potassium, is typically displayed on fertiliser packages. A high phosphorus fertiliser would have a higher middle number. For example, a product with an NPK ratio of 10-15-5 would be considered high phosphorus as it contains 15% phosphorus.

High-phosphorus fertilisers are commonly used for flowering plants, vegetables, and other crops that require more energy during their reproductive stage. It can also benefit newly planted or transplanted seedlings by promoting root growth

When and Where to Use High Phosphorus Fertiliser

High-phosphorus fertilisers are best used for plants during their budding or blooming stage. They are also beneficial when growing root crops, as phosphorus encourages healthy root development.

In terms of location, phosphorus fertilisers are commonly used in areas with phosphorus-deficient soil. However, it's crucial to conduct a soil test before application to prevent excessive phosphorus levels, which can be harmful to the environment.

Phosphorus and Indoor Plants

Even indoor plants can benefit from high-phosphorus fertilisers. Indoor plants often lack sufficient sunlight and may struggle with nutrient uptake, which phosphorus can assist with. However, caution must be taken not to over-fertilise, as it could lead to strength imbalance among other nutrients, negatively affecting the plant's health.

While high-phosphorus fertilisers can be beneficial for indoor plants, it's also essential to strike a balance in nutrient supply to avoid possible nutrient burn or toxicity.

However, the right phosphorus percentage can vary depending on the specific needs of each plant species. For example, flowering indoor plants or those in their blooming stage may benefit from a slightly higher phosphorus percentage to enhance bud formation. Conversely, plants primarily grown for their foliage might prefer a fertiliser with lower phosphorus and higher nitrogen levels to promote vigorous leaf growth. Our Indoor Plant Food was designed with these foliage plants in mind.

In all cases, it's essential to apply fertilisers in moderation and regularly monitor plants for any signs of nutrient deficiency or excess. Overly high phosphorus levels can hinder a plant's ability to absorb other nutrients and may cause toxicity symptoms. As always, when in doubt, it's best to seek advice from a local garden centre or a qualified horticulturist to ascertain the ideal fertiliser composition for your specific indoor plants.


In conclusion, phosphorus is an essential nutrient for plant growth and development. While high-phosphorus fertilisers can be beneficial in certain situations, it's essential to use them sparingly and with caution. Understanding the role and importance of phosphorus in soil will help gardeners make informed decisions on when and where to use high-phosphorus fertilisers for optimal plant health



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