What are those green things in my houseplant’s soil?
The answer: fertilizer prills!
There’s a lot that goes into keeping a plant alive and healthy. Sure, they make their own food using pretty much just sunlight, air, and water. But there are plenty of interactions a plant has with the substrate that it roots into. Below the surface of the soil, plant roots can create microscopic relationships with bacteria and fungi, take up nutrients and minerals from the soil, and even participate in making more soil for future plants to come. What about a plant growing in a container? A container plant is isolated from these natural processes. Supplementing them with nutrients by applying fertilizer can help a plant thrive when it’s separated from these natural processes.
Nitrogen, potassium, and phosphorus are the three key nutrients that make up the main components of every fertilizer. These three macronutrients can have different ratios to better support the needs of specific types of plants. For example, a plant that has lots of foliage may grow healthier when given a fertilizer with a high ratio of nitrogen. A high-quality fertilizer will also supply micronutrients such as magnesium and boron. It is important to not apply too much fertilizer — this can lead to plant burning and other damage. Having the responsibility to properly feed and fertilize a plant can seem daunting, but it can be as simple as following the instructions on the fertilizer packaging.
There are plenty of different formulations of fertilizer available, but they can come in a variety of forms as well. Water soluble fertilizer is most used in nursery production, and it allows fertilizer to be delivered to the plant on its normal watering schedule — which is referred to as fertigation. Many growers prefer this because of the control they have over how much fertilizer is applied and when. It is a good choice for fast growing plants that live in containers, because it makes the nutrients available immediately. However, nutrients that are not used will be washed away with a frequent watering schedule. Prills are a type of controlled release fertilizer application. They look like small beads that are either mixed into the potting substrate or applied on the surface. Prills can be confused with granular fertilizer, which is also shaped into small beads but do not have a slow-release coating.
This coating can be made of polymers or oil-based resin, and it allows for the gradual dissolution of the fertilizer salts inside the prill. Water enters through this semi-permeable coating, dissolves a small portion of the fertilizer inside, then exits the coating due to a buildup of osmotic pressure. How fast the fertilizer can release depends mostly on the thickness of the coating and the external temperature. The coating can increase its porosity as temperature rises, which correlates to the outdoor growing season. As the weather warms and plant growth takes off, the prills can release more fertilizer.
To make a prill, a solution of fertilizer is highly concentrated. Then, the liquid is pumped up to the top of a tower, where it is sprayed down. As the droplets fall, they solidify into small spheres. The size of the prill depends on the height of the tower, the temperature of the air, as well as the rate of airflow. After they are collected from the bottom of the tower, they are coated using rotary drums. The first prill coatings were made of sulfur urea, but this material would frequently crack or coat the prill unevenly, causing the fertilizer inside to dissolve all at once instead of slowly over time. Their coatings have changed quite a bit since fertilizer prills first became widely available in the 1940s.
It is no secret that the long-term, widespread use of synthetic fertilizers has had a negative impact in many corners of the environment. Excess fertilizer may be good for a container plant, but as it leaches from the pot, those extra nutrients can lead to harmful algae blooms, hypoxic water environments, and even nitrogen-based forms of greenhouse gas. With a slower release of nutrients, prills have the advantage of increasing the efficiency of one fertilizer application, which reduces nutrient losses into the environment. Often, prills are used as a supplement to water-soluble fertilizer to reduce a grower’s reliance on fertigating, as well as the loss of fertilizer to run off.
If prills are handled carefully, they can be used effectively to grow beautiful container plants. Especially in home applications, the ability to set and forget a plant’s fertilizer is advantageous. Even in nursery settings, some studies confirm that when used properly, plants produced with controlled-release fertilizers were of equal or higher quality to those grown with water soluble fertilizers. To use prills properly, it is important to follow the instructions that are always included on the fertilizer packaging. It also helps to remember that hotter weather can cause prills to dissolve more quickly, so the user may need to apply extra during the summer months.
Helping a container plant grow into its full potential can be rewarding and easy. Especially during the growing season when plants get large quickly, fertilizer can be used to prevent symptoms of nutrient deficiencies. But remember, too much fertilizer can be an even bigger problem. Not only will it cause damage to the plant, but the runoff has detrimental environmental effect. Prills are an effective way to supply a container plant with fertilizer, giving it the boost it needs to stay happy and healthy
