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.
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.
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.
‘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.
Newswise — March 17, 2022 – 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. The March 15th Soils Matter blog reviews what makes these products enhanced fertilizers and how can they be so long-lasting.
According to blogger Kirsten Verburg, “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.”
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.
These types of fertilizers need water to start releasing nutrients. 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.
To read the full blog, visit https://soilsmatter.wordpress.com/2022/03/15/what-are-controlled-release-fertilizers/ This blog is based on research by Dr. Verburg and her colleagues published in the Soil Science Society of America Journal.
Follow SSSA on Facebook at https://www.facebook.com/SSSA.soils, Twitter at SSSA_Soils. SSSA has soils information on www.soils.org/discover-soils, for teachers at www.soils4teachers.org, and for students through 12th grade, www.soils4kids.org.
The Soil Science Society of America (SSSA) is a progressive international scientific society that fosters the transfer of knowledge and practices to sustain global soils. Based in Madison, WI, and founded in 1936, SSSA is the professional home for 6,000+ members and 1,000+ certified professionals dedicated to advancing the field of soil science. The Society provides information about soils in relation to crop production, environmental quality, ecosystem sustainability, bioremediation, waste management, recycling, and wise land use.
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Credit: Kirsten Verburg
Caption: 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: J.M. Poole
Caption: 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: L.P. Di Bella, with permission
Caption: 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: Kirsten Verburg
Caption: ‘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.
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