Fertilizing Containers

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Fertilizing Containers

After water management, fertilization is the next most common problem with keeping large containerized plants alive through the summer. Most consumers do not fertilize their plants, or if they do, they don’t apply a sufficient amount to maintain the plants at good quality. In this article, we give specific recommendations about fertilizing of 10-inch baskets. For larger size pots and baskets, use these recommendations as a starting point for suggesting fertilizer rates.

How much fertilizer does a 10-inch basket need? Assuming you start with a growing medium that contains preplant fertilizers, an additional 1.5 to 2 grams of nitrogen from a balanced fertilizer is sufficient to produce a 10-inch basket in 12 weeks with no leaching. That is equivalent to the application of 7 to 10 quarts of fertilizer with a concentration of about 200 ppm N.

If you leach, you will need to use more fertilizer. Researchers at Michigan State University found that leaching fractions as low as 25 percent can remove as much as 50 percent of the fertilizer applied to the crop. In other words, if your leaching fraction is 25 percent, you will have to either increase the concentration or the frequency of fertilizer applications in order to maintain similar nutrient levels in the media compared with growing the crop with no leaching.

After production, a minimum of 5 to 6 grams of nitrogen from a balanced fertilizer is required to maintain growth and flowering of plants in hanging baskets for about 20 weeks outside. This amount of fertilizer corresponds to 2 quarts of fertilizer applied at 300 ppm N every two weeks with some leaching. Assuming that 1 level teaspoon of 20-20-20 fertilizer (a common consumer fertilizer) weighs about 6 grams, 1 teaspoon dissolved in 1 gallon of water will supply about 300 ppm N. Since most media in a 10-inch basket will absorb about 64 fl.oz. of water at each irrigation, it is sufficient to apply the fertilizer as a normal irrigation once every two weeks.

We were able to grow and maintain commercially acceptable plants using the above mentioned fertilizer program with several different species used in hanging basket production, including geraniums, ivy geraniums, fuchsia, impatiens, New Guinea impatiens and Non-Stop begonia.

It is often stated that some plants (like geranium) have a high nutrient requirement, while other plants (like New Guinea impatiens) have a low nutrient requirement. We concluded that none of the plant species tested required higher levels of fertilizer. Instead, plants that are listed as having a high nutrient requirement can tolerate higher levels of fertilizer salts in the root media without a reduction in growth. 

Understanding Slow-Release Fertilizer

Another way to supply nutrients to plants is with slow-release fertilizer. As its name implies, slow-release fertilizer is not available to the plant all at once, but rather the nutrients are released slowly to the plant over time. While there are a number of technologies for making fertilizer slow release, the most precise materials use plastic or resin to coat prills of fertilizer salts.

Unlike water-soluble fertilizers where you can measure the exact concentration for each application, the amount of nutrients released by the plastic- or resin-coated fertilizer depends on soil conditions. Once the medium has been moistened and the prill has absorbed water, soil temperature is the only factor that affects the long-term release of nutrients.

Depending on the product, the ideal release temperature can vary from 68ËšF to 80ËšF. For example, one type of resin-coated fertilizer has an ideal release temperature of 70ËšF. For a fertilizer with a 90-day release rate, 90 percent of the nitrogen fertilizer contained in the prills will be released over 90 days if the temperature of the medium is maintained at an average of 70ËšF. The higher the average soil temperature above the ideal release temperature, the faster the salts are released and the shorter the release duration. In contrast, lowering the average soil temperature below the ideal release temperature will slow the salt release and increase the release duration. 

Figure 1. Regular applications of water-soluble fertilizer (left) can maintain the quality of impatiens hanging baskets.

The liquid fertilizer treatment (left picture) received 2 liters of a fertilizer solution (20-10-20 at 300 ppm N) once every two weeks for the 57 days the baskets were outside, whereas the no fertilizer treatment (right) received only clear water.

Consumer Use

It may be unrealistic to educate all our customers to become expert in fertilization and watering. Assuming that you want to supply all the nutrients to the plant for the consumer with the slow-release fertilizer, then the rates should be equal to the amount calculated for water-soluble fertilizer, i.e. 1.5 to 2 grams of nitrogen applied during production (about three months) and 5 to 6 grams of nitrogen applied after production (three to four months).

There are two methods for applying slow-release fertilizer to a crop. The first is to incorporate the fertilizer into the root medium prior to planting. If you base your incorporation rate on the values given above for water-soluble fertilizer, then you need to add enough slow-release fertilizer to supply 6 to 8 grams of nitrogen fertilizer to each 10-inch basket over seven to eight months.

Always read fertilizer labels and do not apply more than the recommended rate. To calculate grams of nitrogen, you need to know the percentage of nitrogen contained in the slow-release fertilizer and the number of baskets that are filled per cubic yard of media. For example, to get an incorporation rate of 7 grams of nitrogen per 10-inch basket using a 17 percent nitrogen fertilizer and assuming a filling rate of 160 baskets per cubic yard of media, then you need to add about 14.5 pounds of fertilizer per cubic yard. If you used a fertilizer containing 13 percent nitrogen, then the incorporation rate would be higher, about 19 pounds per cubic yard (See Calculating Incorporation Rates). If you are using a fertilizer with a release duration of less than seven months, then the incorporation rate needs to be reduced accordingly. If the release duration is less than four months, then there is little benefit to the consumer.

The biggest problem with high incorporation rates is the potential for salt buildup in the root medium during production, especially in salt-sensitive crops, when you apply the high rate on the fertilizer label. Many greenhouses grow multiple levels of crops in the same greenhouse, so leaching a basket crop to remove high fertilizer salts may not be an option. If you choose to use these high rates of fertilizer, make sure the crop can be heavily leached if salt levels in the media reach damaging levels. Decreasing the amount of fertilizer incorporated into the medium at planting will reduce the risk of salt buildup during production, but there may not be enough fertilizer to keep the plant at its highest quality through the summer.

As with any fertility program, when using slow-release fertilizer, you need to soil test on a regular basis so an informed decision can be made whether to leach, apply additional amounts of fertilizer or apply irrigation water during production of the basket crop.

The second method of applying slow-release fertilizer is for the wholesale grower or retailer to apply the material to the root media surface (top-dress) just prior to sale. The advantage of this method is that it does not affect the nutrition management of the crop during production and a shorter-term release material can be used. The disadvantage is that it can be difficult to find the time or the people required to top dress baskets or pots during a busy shipping season and fertilizer prills sitting against the stem can damage the plant.

With top dressing, 5 to 6 grams of nitrogen should be applied if you want the plant to look similar to using water-soluble fertilizers through the summer. One tablespoon of a slow-release fertilizer weighs about 18 grams. If the fertilizer contains nitrogen at 17 percent, then 18 grams will supply about 3 grams of nitrogen (2 tablespoons would supply 6 grams). If the material contained 13 percent nitrogen, then 1 tablespoon will supply about 2.3 grams of nitrogen and 2 tablespoons will supply about 4.6 grams of nitrogen.

Better consumer education about fertilization and the use of lower rates of slow-release fertilizer by the grower may give customers their best success. Next month, we will turn our focus to nutrient tolerances of different plant species in baskets. 

Bill Argo is technical manager of Blackmore Co. You can eMail him at bargo@blackmoreco.com.

Paul Fisher is an associate professor and Extension specialist in the Environmental Horticulture Department at the University of Florida. You can eMail him at pfisher@ufl.edu.

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