What Is Rock Phosphate and How to Use It in Your Garden?


Rock phosphate has long been one of the most widely used soluble organic fertilizers known for promoting healthy growth and development of plants.

Ever since it was first discovered in central Florida, in the 1880s, it has been widely used in the United States – the country is the world’s leading producer and consumer of phosphate rock. 

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However, it is also commercially mined, on a large scale, in France, Belgium, Spain, Morocco, Tunisia, and Algeria.

In some parts of the world, phosphate rock is also used to make calcium phosphate nutritional supplements for animals.

What is Rock Phosphate, Exactly?

Rock phosphate also referred to as phosphate rock or phosphorite, is a type of non-detrital sedimentary rock packed with phosphate mineralization, including apatite. 

In addition to being an essential organic source of phosphorus, phosphate rock also contains limestone, clay, and some other essential trace minerals.

Traditionally, gardeners used the rock, on its own, to add phosphorus to the soil. 

However, most of them now use the processed rock phosphate fertilizers available on the market due to the lack of supply of rock phosphate and its low concentration of phosphorus.

Then modern phosphate fertilizers are made of soft rock phosphate. 

While the nutrient composition of both soft and hard rock phosphate is similar, they are more accessible for plants in soft rock phosphate. 

Furthermore, since it works as a slow-release fertilizer, it ensures a consistent supply of the nutrients.

The commercial rock phosphate fertilizers are commonly known as rock dust.

There are also many phosphorus-only fertilizers available on the market. 

They are called superphosphates and are derived from acid-treated minerals.

How Rock Phosphate Helps Plants?

Phosphorus, the second element in NPK fertilizers, is one of the basic elements of plant life. 

In addition to playing a number of key roles in metabolism, phosphorus is also an essential nutrient required for photosynthesis, nitrogen fixation, and carbon metabolism. 

It’s also a primary component of plants’ nucleic acids (RNA and DNA) and supports energy transfer.

Phosphorous also plays a key role in seed formation as well as promotes healthy root growth and flowering.

Rock phosphate, being one of the best natural sources of phosphorus, offers all these benefits to plants.

What Happens When Plants Do Not Get the Required Amount of Phosphorus?

Phosphorus deficiency in plants leads to:

  • Slow or stunted plant growth
  • Delayed maturity
  • Slowing down the process of carbohydrate utilization, which leads to carbohydrate accumulation.
  • Dark-green leaf color. 
  • This is the result of a higher concentration of carbohydrates in plants. 
  • Older leaves in phosphorus-deficient plants may also turn purple.
  • Increased susceptibility of plants to disease.

Why Do Plants Need Phosphorus Fertilizer?

Although phosphorus is naturally present in the soil, it tends to get depleted with time, like most other nutrients. 

It either gets washed away with heavy rains, lost through de-liming, or gets attached to the eroding sediments. 

Hence, plants need to be regularly supplemented with soil amendments for this essential nutrient or they would suffer from phosphorus deficiency.

What Makes Rock Phosphate the Best Phosphorus Fertilizer?

Although phosphorus is present in every balanced all-purpose fertilizer, the reason why many gardeners prefer to use a rock phosphate fertilizer is because it doesn’t work against the soil, like most chemical fertilizers. 




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Instead, it works with the soil and offers a steady and even supply of the nutrient to plants for a long time.

Under normal circumstances, a single application of soft rock phosphate can last for up to five years. 

However, the application rate may vary depending on soil conditions.

In addition to rock phosphate, many gardeners also swear by the benefits of Azomite and bone meal for replenishing soil phosphorus.

Which Plants Benefit from Rock Phosphate Application?

All plants benefit from an application of rock phosphate, or any phosphorus fertilizer, as the nutrient is essential for a variety of plant functions. 

However, perennial flowering plants benefit the most from the use of phosphate rock as it encourages more and healthier blooms. 

Roses, in particular, love rock dust application. 

When supplemented with rock phosphate, rose plants not only develop strong roots systems and exhibit healthier overall growth but also produce more flowers.

Rock phosphate is also highly beneficial for vegetable gardens. 

It ensures healthy crop production, greater yields, more flavorful produces, and fewer pests.

Powdered rock phosphate is the best fertilizer for apple, coffee, rubber, tea, and citrus trees.

How to Apply a Rock Phosphate Fertilizer?

Phosphorite works best in early spring. 

While the exact amount may vary according to the soil type and their nutrient composition, 10 pounds of rock dust per 100 square feet is used, on average. 

For soils highly deficient in phosphoric acid, the amount is increased to a maximum of 50 pounds of fertilizer per 100 square feet.

Add the fertilizer to compost and then use it in your vegetable and flower garden to make up for the lost nutrients.

However, the direct application of powdered rock phosphate is performed for coffee, tea, citrus, and apple trees.

If using soft rock phosphate, always add it into the potting soil or planting hole before planting. 

Depending on your needs, use 2 to 10 pounds of the organic fertilizer for a planting hole.

To replenish soil nutrients and to augment the growth and health of plants in your already well-established garden, simply scatter the rock phosphate on the ground surface. 

Now rake it gently into the soil so it gets incorporated into the soil and becomes available to roots.

In general, rock phosphate works the best in acidic soils. 

Since phosphorus is largely soluble in acidic soils, it becomes more accessible for plants and hence, easily and more readily absorbed by roots. 

However, for non-acidic soils, mix it in compost first. 

Since the organic matter is acidic in nature, it will make up for the soil pH difference.

Use Phosphorus Fertilizer with Caution

Just like with other nutrients, you need to be careful with adding phosphorus to the soil. 

Always perform a soil test to determine how much phosphorus needs to be added. 

One of the most common signs of phosphorus over-fertilization is the yellowing of leaves between the veins. 

This happens because when present in too much quantity, phosphorus makes zinc, iron, and manganese unavailable to plants.

Do not feed the plant with a phosphorus fertilizer during the rainy season or when heavy rain is forecasted. 

This is because phosphorus gets dissolved in water, leading to the loss of plant nutrition as well as pollution of waterways.

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