How to Use Rock Phosphate Fertilizer In Your Garden

Rock phosphate or soft rock phosphate fertilizer is a fantastic way to add - you guessed it - more phosphorus to your garden soil.

A rock phosphate mine


Rock phosphate is a long-standing organic fertilizer for gardens. It’s known for keeping plants healthy and encouraging new growth. It does this by adding phosphorus, thereby helping them to make other plant nutrients more accessible. But what exactly is rock phosphate, and how should it be used for your plants?

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What is Rock Phosphate?

A rock phosphate mine
Example of the mines where rock phosphate fertilizer is derived from. Source: davidstanleytravel

Also known as phosphorite, rock phosphate is a sedimentary rock that contains high amounts of phosphorus. The rock is mined and also contains clay and limestone. Phosphate rock is, of course, an essential source of phosphorus – the essential nutrient ‘P’ from the three vital nutrients “NPK.” It’s used for making organic phosphate fertilizers for gardens. 

In the olden days, gardeners used rock phosphate as a fertilizer for plant growth. However, its low phosphorus concentration and lack of supply have led most gardeners today to use the processed version. 

Modern fertilizers contain soft rock phosphate, which is often confused with hard phosphate rock. Both sources contain high amounts of phosphorus and calcium. However, these nutrients are more accessible in soft rock phosphate, making application easier. As a slow-releasing fertilizer, it provides your plants with a consistent source of essential macronutrients.

What Is Rock Phosphate Used For?

Rock phosphate fertilizer works with the soil, unlike chemical fertilizers that work against it.

Phosphate rock is extremely rich in essential nutrients to ensure vibrant, healthy plants. These fertilizers are also known as “rock dust” due to the rock minerals in them. Using phosphate rock fertilizers is a common practice for encouraging healthy blooms and vegetables. 

If you have flowering plants that are perennials in your garden, they’ll love an application of phosphate rock at the beginning of the season in early spring. The high mineral content will result in huge, colorful blooms.

Flowers like roses love phosphate rock fertilizers as it helps them develop more buds and a stronger root system. The phosphates present in the formula are also used for a healthy lawn and tree root system development. It serves as an all-purpose fertilizer for different kinds of perennials and trees in the garden. 

If you’re growing a vegetable garden, you will have to take stronger measures to ward off pests and diseases. Thankfully, phosphates help in reducing pests and enhance the flavor and yield of your vegetable crops. Powdered rock phosphate is excellent for crops like coffee, tea, apples, rubber, and citrus. Vegetable garden soil conditions can tolerate the direct application of the fertilizer. 

Rock Phosphate’s Effect on Plants

Rock dust or rock phosphate usually has a slow-releasing effect on plants. It is best applied in early spring, a little before the flowering season. You should ideally aim for 10 pounds of rock phosphate fertilizer for every 100 square feet.

Rock phosphate requires specific soil conditions under which it performs the best. It is typically more accessible in acidic soils with a pH of less than 5.5. This way, the phosphorus is soluble and can be absorbed by plant roots.

Is it Good for Plants?

Definitely. As a natural source of phosphorus, rock phosphate is essential for the maturation and healthy growth of plants. Phosphorus also plays a crucial role in photosynthesis. This nutrient is present from the seedling growth to the maturation of grain. Every plant, therefore, requires phosphorus in the early years. The nutrient supports its vigor and overall health. 

Rock phosphate also provides phosphorus from the energy storage and transfer, cell enlargement, and respiration in plants. Without the application of rock phosphate, plants can not grow fully nor regulate their metabolic pathways. 

Using Soft Rock Phosphate Fertilizer Properly

Soft rock phosphate is insoluble if the pH of your soil is above 5.5. It needs acidic conditions to be fully absorbed by the plant roots. To use soft rock phosphate properly, always incorporate it into the soil before planting. You can also add it to a planting hole when repotting or planting trees. Adding it to the soil surface may not be as effective.

If you have a well-established lawn and trees, scatter the rock phosphate over the landscape. Gently rake it into the soil to incorporate it better into the root zone. For transplanting and growing trees, use 2-10 pounds for every planting hole. If your soil is particularly deficient in phosphorus, you can increase the uptake to 50 pounds for every 100 square feet. Depending on how healthy your plants are, a single application of soft rock phosphate can last for 5 years. 

Getting the Most Out of Your Rock Phosphate 

The phosphorus in rock phosphate requires acidic soil to be accessible. Alkaline soil can make the fertilizer almost useless. To speed up the process, you can add the fertilizer to compost. The acidic nature of compost can allow the chemical bonds to loosen, thereby making phosphorus more accessible to the roots. 

You can add other beneficial microbes like mycorrhizae for the fast release of phosphorus. Always buy high-quality rock phosphate from a reputable company to ensure your plants get the best treatment. 

How Is Phosphate Rock Made?

Phosphate rock is a naturally-occurring fertilizer. In most cases, it reacts with sulfuric acid to make phosphoric acid (P2O5) before it can be used. The acid is added to pulverized phosphate rock. This acid is then processed further to create good fertilizers for gardens, trees, flowers, and vegetables. 

When sulphuric acid is used, the resulting chemical, which is called ‘Normal or single phosphate (SSP),’ is made, which contains 16-21% of phosphorus content. The production of SSP starts by mixing the phosphate rock and sulphuric acid in a reactor. After that, the mixture is transferred to a slow-moving conveyor. It is then cured for at least 4-6 weeks before it is packaged and shipped as a fertilizer. All phosphate fertilizer plants have phosphoric and sulfuric acid production facilities. 

What’s the Chemical Formula for Phosphate Rock?

The chemical formula for phosphate rock is Ca5FO12P3. This mineral formula is also called calcium carbonate-fluorapatite. There are four kinds of phosphate rocks:

  • Hard
  • Soft
  • River pebble phosphate
  • Land pebble phosphate

The chemical formula for phosphorus acid (also known as phosphorite) is P2O5, where the content of phosphates can vary from 2% to 21%.  

Is It an Organic Fertilizer?

Soft rock phosphate fertilizer
Granular form of soft rock phosphate fertilizer.

Phosphate rock can be considered an organic fertilizer, depending on how you define organic. True “organic” fertilizers like compost are derived from organic matter like animals, manure, and plants. Phosphate rock, on the other hand, is a rock mineral, which is processed to be used as a fertilizer. Since it’s naturally occurring, it helps gardeners avoid the use of synthetic chemical fertilizers. So, by that logic, it is “organic.”

As a natural and potent source of phosphorus and calcium, it is widely used in organic agriculture. It contains 30% of phosphates and 48% calcium. It is environmentally friendly as it also reduces greenhouse gases, and helps in better yielding crops and flowers. 

When is Rock Phosphate Unsuitable as Fertilizer?

Rock phosphate might not always be a suitable fertilizer. Since most of it remains insoluble, it can be hard for the roots to absorb the maximum amount of phosphorus. Apart from its bulkiness, it’s relatively slower than fast-acting synthetic chemical fertilizers.

Over-fertilization with rock phosphate is also a common problem that can lead to leaf chlorosis and destroy beneficial microorganisms in the soil. 

Even when it’s relatively eco-friendly, phosphate fertilizers run the risk of water pollution. Phosphates can tightly bind to the soil particles and rarely move out of the soil. If too much fertilizer is used, there are chances of excess phosphorus seeping into the water systems. This can lead to contamination issues as well as bacteria and algae outbreaks. 

Our Top Recommendations for Rock Phosphate

Frequently Asked Questions

Q: What is rock phosphate used for?

A: It is used to add phosphorus to soil.

Q: What is a phosphate rock?

A: Rock phosphate is a sedimentary rock that contains high amounts of phosphorus. The rock is mined and also contains clay and limestone.

Q: Is rock phosphate the same as bone meal?

A: They are two different fertilizers that promote the same nutrient profile in soil. Rock phosphate is a mined mineral, and bone meal is powdered bone.

Q: Is rock phosphate harmful?

A: Only when it gets into mucous membranes, eyes, and onto skin. Use gloves and eye protection when applying.

Q: Does rock phosphate change soil pH?

A: It tends to lower soil pH and make soil more acidic as it breaks down.

Q: How often do you apply rock phosphate?

A: There’s no hard and fast rule as different plants and soils have different nutrient needs. However, apply 50 pounds for every 100 square feet in areas where phosphorus is needed.

Q: Is phosphate rock rare?

A: In terms of the availability of minerals in the earth’s layers, rock phosphate is fairly rare.

Q: Why is phosphate banned?

A: It’s banned in the production of cleaning products as it can lead to eutrophication in waterways.

Last update on 2024-07-15 / Affiliate links / Images from Amazon Product Advertising API

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