What are controlled-release fertilizers?

Many of us who enjoy gardening or grow plants in pots on the balcony use fertilizers that are characterized as “enhanced.” They may have descriptions like “improved nutrient uptake,” “6 months feeding” and “feed and forget.” We use the fertilizer to provide extra nutrients to the plants we grow. 

But what makes these products enhanced fertilizers and how can they be so long-lasting?

five small vials on table filled with various types of controlled release fertilizer pellets
Various types of controlled-release fertilizers. The type of coating on the pellets can influence the ease with which water is absorbed. The nutrients in controlled-release fertilizers are released slowly through the coating, such that the nutrients are released just when the plant needs them. Credit: L.P. Di Bella, with permission.

Some may contain a mix of nutrients in just the right proportions to what a particular plant may prefer or include critical trace elements and minerals. They also provide slow or “controlled” release where nutrients are released gradually to the plant, not all at once. This is similar to a timed-release pain reliever or other medication – slower release over more time.

Using a controlled-release fertilizer means that we do not have to top up with fertilizer quite as often and can use that time for other jobs in the garden or to enjoy a cup of coffee while watching the plants grow.

How does the slow-release work? 

Sometimes the fertilizers contain organic sources that are slowly recycled by soil microbes and other life in the soil, releasing nutrients in the process. This too is a slow-release fertilizer.

However, where the fertilizers are sold and referred to as “controlled-release,” the fertilizer granules containing the nutrients are usually covered with a coating. The nutrients are released slowly through the coating. The idea is to make the coating such that it releases the nutrients just when the plant needs it. Or in other words, to ensure that release and plant uptake are “synchronized.”

small tomatoes growing on tomato plant
‘Enhanced’ fertilizers with controlled release can provide extra nutrients for plants we grow in the garden or in pots on the balcony. When the plants are watered, the fertilizer inside coated pellets slowly dissolves and is available to the plant, like these tomatoes. Credit: K. Verburg

From gardens to farms

Many farmers use fertilizers and some of them are also interested in the use of controlled-release fertilizers. Just like in our garden, their use may reduce labor (and fuel) costs associated with fertilizer application.

However, more often it is to avoid losses of nutrients that can do harm to the environment – nitrogen in particular. With large or prolonged rainfall, the nitrogen in the soil can get washed out of the soil and end up in a nearby creek or in the groundwater below.

Alternatively, the applied nitrogen can be lost in gaseous form, with some of it (nitrous oxide) contributing to “greenhouse gas emissions.” Examples of industries where controlled release fertilizers have been tested include corn in the Midwest. Researchers have studied both gaseous losses of nitrous oxide and losses via “tile drains” into waterways flowing into the Mississippi River.

Other research has taken place on sandy soils in Florida on vegetable and sugarcane farms – to improve crop performance and protect the groundwater. In Australia, where I work, the sugarcane industry is interested in avoiding nitrogen losses to waterways that flow into the Great Barrier Reef. 

three side by side photos showing microscopic view of how fertilizer pellets empty fertilizer over time
Figure 1: Dry pellets of controlled-release fertilizer absorb water during rainfall or watering. The coating allows water to diffuse through, wetting the fertilizer. This dissolved fertilizer then moves through the coats into the soil (via diffusion). These photos were taken with a microscope to illustrate how pellets empty of fertilizer over time. Credit: J.M. Poole

Controlled-release fertilizers and the need for water

The instruction on the bag or tub of fertilizer that we use for the plants in our gardens may say “water once or twice to start the feeding process” or “water well after application.” Why do the fertilizers need water to start releasing?

Initially the fertilizer inside the coated pellet is dry. The coating allows water to move slowly inside the pellets, which then dissolves the fertilizer. The dissolved fertilizer then moves through the coating and becomes available to the crop. As time goes on, the concentration of fertilizer inside the granules reduces, which will cause a gradual slowing down of the rate of release.

The result of the three release steps is a release pattern that might resemble that shown in Figure 1. Ideally the fertilizer is released just before the crop needs it. That would reduce the risk of nitrogen loss to the environment because the amount of nitrogen in the soil stays low.

Controlled-release fertilizer products differ in the composition and thickness of the coating, which results in different rates and patterns of release. That is handy, because crops can also differ when they require the nutrients the most. It is also well known that release is temperature dependent, releasing faster when the soil is warmer to match faster growth of plants.

field of sugarcane crops in Australia with hills and sky with clouds in background
Controlled-release fertilizers are also considered in various agricultural industries, including sugarcane in Australia as shown. Fertilizer pellets are coated, and as the coating absorbs rain or irrigation water, the fertilizer inside slowly dissolves, then moves outside the pellet via diffusion., fertilizer is slowly released into the soil and is accessible to crop roots. Credit: Kirsten Verburg

Related research at CSIRO Agriculture and Food, Australia

While we know water is needed to start the release, it is less well understood how wet the soil needs to be and whether that soil wetness may affect release rates later. It is important because for crops that rely on rainfall there is no guarantee that the soil will be wet, unlike in our garden or on an irrigated farm.

As often happens in research, there were some results from different studies that appeared to contradict each other. So, in a recent study we looked at controlled-release fertilizer granules through a microscope. The granules had been in both wetter or drier soils, and we could see when the coated granules had absorbed water and started to release the nitrogen. This helped explain the release data we obtained for fertilizer granules that we had incorporated for different lengths of time in soils of varying wetness.

For one fertilizer product the release was considerably delayed when the soil was relatively dry. Using the microscope photos, we could show that this related to slow absorption of water. As other products had not shown that behavior in earlier studies, we suspect that the characteristics of the coating can influence the ease with which water is absorbed. For example, some coatings may be designed to repel water – to be “hydrophobic.” These may show longer delays until the start of release in dry soil. Whether this is a concern in practice depends on the soil water content of the soil at the time and depth of application. Rain around sowing or subsurface application may ensure sufficient soil water is present. It is important that we test these effects for new products, including novel biodegradable ones. That way we can correctly predict the timing of release under different conditions to optimize plant uptake while minimizing impacts to the environment.

Answered by Kirsten Verburg, CSIRO Agriculture and Food, Australia

This blog is based on research by Dr. Verburg and her colleagues published in the Soil Science Society of America Journal.

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