The Science of Slow Release Fertiliser

Slow release fertilisers are prominent in the agricultural industry for their ability to provide plants with a consistent supply of nutrients over extended periods. Slow and controlled release fertilisers are designed to meet the nutrient uptake dynamics of crops, promoting sustained growth without the surge-and-deplete cycles associated with conventional fertilisers. By mimicking natural soil processes, they offer a smarter, more efficient and sustainable approach to crop nutrition, aligning with modern agricultural practices that focus on nutrient use efficiency (NUE) and environmental stewardship. Most dedicated slow and controlled release fertilisers are developed around nitrogen delivery, given its high solubility and demand, but differ according to manufacturer. 

The Science Explained

Upon application, conventional fertilisers react immediately to the soil’s environmental conditions, with rapid nutrient release determined by exposure to moisture, temperature, sunlight, wind or soil pH. While these fertilisers can have high soluble nutrient concentrations, their unpredictable release patterns commonly lead to high environmental losses, both via atmospheric volatilisation and subsoil leaching, while also exceeding the plants nutrient uptake capacity in their short release threshold. 

The core functionality of slow release fertilisers lies in their ability to control the rate at which nutrients are available to plants. This is typically achieved through one of three methods: physical encapsulation (slow release), chemical modification (controlled release) or biological processes (microbial slow release). Physical encapsulation involves coating the fertiliser granules with materials like polymers, gradually releasing nutrients via soil moisture and temperature, while also mitigating environmental losses from sunlight or wind. 

Controlled release fertilisers often have manufactured chemical modifications to alter their solubility, delaying its availability without the need for a coating. These have pre-determined release patterns ranging from 2 to 12 months, facilitated by soil moisture and temperature.

Biologically assisted release involves the facilitation of soil microbiology, namely bacteria and fungi, and the selection of compatible nutrient types. These methods mimic natural plant behaviours, with a healthy microbiome acting as a symbiotic extension to the plant root system, enhancing nutrient access, storage and uptake. 

These technologies are particularly important in regions with variable rainfall or irrigation practices, as they can buffer nutrient loss in a dry season or irrigated run-off. By ensuring that nutrients are available in the soil longer, or greater utilised, fertilisers requirements and application frequency can be reduced, saving time, fertiliser inputs and labour costs. Furthermore, with a more consistent nutrient delivery, there is less leaching (water soluble losses beyond plant access), volatilisation and denitrification (losses into the atmosphere as gases) thereby enhancing nutrient use efficiency and reducing environmental pollution.

Grow Safe® employs these techniques in products like Potassium Humate Nitrogen (KH-N), which combines a polymer coating and potassium humate to slow nutrient release. This integration not only stabilises nitrogen delivery but also improves handling and application efficiency. All of the Grow Safe® mineral fertilisers are biologically compatible and have the option of the Grow Safe® microbial coating, adding beneficial bacteria and fungi to improve access to a host of trace elements not found in traditional soluble fertilisers. 

Benefits in Agriculture

The adoption of slow release fertilisers offers multiple advantages over traditional fertilisers. Firstly, they enhance nutrient use efficiency, allowing plants to absorb more of the applied nutrients, which minimises waste and decreases the cost per unit of nutrient utilised. This efficiency also translates into reduced leaching, where fewer nutrients are lost to runoff, making these fertilisers an ideal choice for environmentally sensitive areas or where groundwater pollution is a concern.

Moreover, the steady nutrient release helps maintain optimal soil nutrient levels, promoting healthier plant growth and potentially increasing crop yields. This is particularly beneficial in intensive cropping systems where soil fertility must be maintained at high levels to support continuous yields. Additionally, the use of biologically assisted release fertilisers, such as the Grow Safe® mineral fertiliser range, can improve the physical properties of the soil, enhancing water retention and aeration, and promoting beneficial microbial activity, all of which contribute to sustainable soil health.

When selecting fertilisers such as nitrogen, there are a number of factors to consider beyond total analysis and price; nutrient availability, release pattern, handling characteristics and soil health response can drastically impact final fertiliser effectiveness. Grow Safe® often utilise nitrogen sources like Sulphate of Ammonia (SOA) and Mono Ammonium Phosphate (MAP) in their fertiliser blends, that while they have a lower analysis than urea, are highly effective, biologically compatible and not prone to the same environmental losses. When urea is requested for nitrogen-intensive crops, Grow Safe® offers slow release products such as KH-N (Potassium-Humate Nitrogen), ESN (Environmentally Smart Nitrogen) or controlled release MU-38 (Methylene Urea 38). 

Environmental Impact

The environmental impacts of slow release fertilisers extend beyond reducing runoff and leaching. By providing a steady supply of nutrients, these fertilisers help maintain ecological balance in the soil, supporting diverse microbial ecosystems that are essential for soil health and plant growth. This reduced fluctuation in soil nutrient levels can prevent the overgrowth of algal blooms in waterways, a common issue with high nutrient runoff, thereby protecting aquatic life and water quality.

Additionally, the precise nutrient management possible with slow release fertilisers aligns with global efforts to combat climate change. Efficient nutrient management can significantly lower the carbon footprint of farming operations by reducing the need for frequent fertiliser applications and the heavy machinery use that often accompanies it. This method of fertilisation, therefore, plays a crucial role in sustainable agricultural practices that seek to mitigate environmental impact while enhancing food production.

Moreover, Grow Safe®’s inclusion of Minerals & Microbes in their products enhances both Nutrient Use Efficiency (NUE) and Water Use Efficiency (WUE), further solidifying the role of these fertilisers in sustainable agricultural practices. Grow Safe® Minerals and Microbes are available as a standalone granular product, NueGen, for standalone application or blending with slow release fertilisers, or pre-blended such as the Cropping, Pasture or Horticulture product ranges. 


The strategic use of slow release fertilisers is transformative in modern agriculture, offering enhanced crop productivity while significantly reducing environmental risks. As the agricultural sector continues to evolve towards more sustainable practices, the role of technologies such as slow release fertilisers becomes increasingly critical. These products not only support the economic goals of higher yields and lower costs but also advance the environmental objectives of reduced pollution and better resource management. Embracing such innovations is essential for the future of farming, ensuring it remains viable and beneficial for generations to come. To find out more regarding slow release fertilisers or tailored blends, check out the Custom Blending page or Contact Us for more.  

Christmas Closure

Seasons greetings from the team at Grow Safe® – Australian Mineral Fertilisers will be closed from midday the 22nd of December 2023 till the 8th of January 2024. Any enquires or online purchases during this period will be processed in the new year.