Fertilizer math for nursery containers

Photos: Goris B. Passchier and Matt McClellan

Have you ever wondered how fertilizer rates for container nurseries are determined? When you read fertilizer labels, some include rates while others do not. But how exactly are those rates calculated? There are generally two methods:

• Copying rates from others.
• Using a mathematical formula to provide basic guidelines.

In the mid-1980s, Bert Swanson, professor emeritus of the University of Minnesota, suggested to me that container nursery formulations need approximately 1/3 pound of nitrogen (N) per growing month per cubic yard of substrate media.

CRF incorporation math

To calculate the amount of controlled release fertilizer (CRF) needed for a given period, the formula is straightforward:

• For a six-month growing period when you are using a 19% nitrogen CRF product.0.33 pounds of N per cubic yard × six months =1.98 pounds of nitrogen per cubic yard. When the nitrogen content of the fertilizer is 19%, then:1.98 pounds of N per cubic yard/ 19% Nitrogen ​= 10.42 pounds of fertilizer per cubic yard.
• For a nine-month growing period when using a 19% nitrogen CRF product.0.33 pounds of N per cubic yard × nine months = 2.97 pounds of nitrogen per cubic yard. When the nitrogen content of the fertilizer is 19%, then:2.97 pounds of N per cubic yard / 19% nitrogen = 15.63 pounds of fertilizer per cubic yard.

General comments on longevity of your CRF formulations

• Be sure to use a formula with a CRF that is designed to release over an intended period at your geographic location with your operation’s cultural practices.
• Not all six-month fertilizers provide six months of release at the desired nutrient levels and release is dependent on the location of the plants, temperature, humidity and cultural practices.
• Putting more pounds of fertilizer per cubic yard could result in too much fertilizer in the first month(s) release. Does your company representative know the formulation ingredients and how they are stacked to get the release in your geographical area?

Note: This nine-month calculation assumes a true nine-month growing period. If the grower’s region only supports a six-month growing season, they may opt for the six-month rate, which is 10.42 pounds per cubic yard. However, some fertilizer suppliers label nine-month-rated fertilizers with six-month application rates, which can be confusing.

General guideline for a six- and nine-month fertilizer

• A six-month fertilizer of 19% nitrogen typically requires 10 pounds per cubic yard
• A nine-month fertilizer of 19% nitrogen needs 15 pounds per cubic yard, assuming a true nine-month growing period.It’s essential for growers to adjust these rates based on their region’s growing conditions.

Determining low, medium and high rates

Rates of application are determined based on three key factors:

1. Plant species: Application rates for low, medium and high nutrient needs are often derived from resources such as AmericanHort or your local university research and extension. Certain plant species require more or less fertilizer depending on their specific needs.
2. Leachate fraction: Leachate fraction refers to the loss of water and water-soluble minerals from the soil due to irrigation. It is calculated by dividing the amount of leached water by the amount of irrigation water applied. Factors affecting leachate include:
• Soil composition: Different nurseries use different soil mixes, which influence how much fertilizer is retained or leached.
• Watering practices: The volume of water applied daily can increase or decrease nutrient leaching.
• Container height: Taller containers are more prone to higher leachate fractions due to gravity.
3. Growing fertilizer rates based on location of the plants. Generally the rates on container labels for growing plants are based on outside open air growing. When growing in a controlled environment such as a greenhouse, rates in general can be 33% lower.

Example of a rate with certain media mixes of a 19-20% nitrogen six-month fertilizer at 70°F near the prill. Grower observations and trials determine the final rate.

• 100% pine bark: High rate (8-10) pounds/cubic yard) with a 20% leachate fraction.
• 75% pine bark: Medium rate (6-7 pounds/cubic yard) with a 10-15% leachate fraction. Suitable for salt-sensitive plants.
• 30% or more peat: Low rate (4-5 pounds/cubic yard) with a 4-7% leachate fraction. Ideal for highly salt-sensitive species.

CRF top dress math

When using a six-month fertilizer with 19% nitrogen and an incorporation rate of 1.98 pounds of nitrogen per cubic yard, the math for top-dressing individual containers is as follows:

For a one-gallon container (6 3/8 inches in diameter, 7 ¼ inches in height), there are approximately 270 containers per cubic yard. If the rate is 10.42 pounds per cubic yard, divide by 270 containers:10.42 pounds of fertilizer/by 270 containers per cubic yard = 0.038 pounds per container. Convert to grams:0.038 (pound of fertilizer per cubic yard) × 454 (grams per pound) = 17.52 grams per container.

Application rates for #1 container size (270 containers per cubic yard):

• Low rate: 12 grams
• Medium rate: 18 grams
• High rate: 22 grams

Some fertilizer suppliers use a six-month rate for a nine-month fertilizer, particularly in regions with shorter growing seasons (e.g., the northern Midwest). However, a true top-dress rate for a nine-month growing area would be:

#1 container size (270 containers per cubic yard):

• Low rate: 18 grams
• Medium rate: 26 grams
• High rate: 33 grams

Conclusion

Understanding the recommended rates is crucial to optimizing plant growth and achieving the best return on investment. The best application depends on the temperature of geographic area or greenhouse, the specific needs of the plants grown and the grower’s experience.

Goris B. Passchier founded Positive Soil and Plant Health LLC after spending 40 years in the fertilizer business. Reach him at goris.psph@gmail.com.