Effect of Potting Medium Temperatures
on Release Curves of Slow-Release Fertilizers
in the Presence of Ficus benjamina

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C. A. Conover and R. T. Poole* University of Florida, IFAS,
Central Florida Research and Education Center
CFREC-A Research Report RH-90-15

Osmocote is a slow-release fertilizer of choice for many foliage growers in Florida. Osmocote's release rate varies with temperature, and soil temperatures in containers in Central Florida were found to range from a low of 40°F in winter, to a high of 105°F in summer. None of the Osmocote sources designed to provide nutrients for 12 months performed well in full-sun in central Florida, but several provided adequate nutrients under 63% shade.

The following two experiments were designed to determine influences of soil temperatures and Osmocote sources with different release durations on plant growth as well as to follow the amount of nutrients released by these Osmocote sources over time as determined by measuring soluble salt levels. Both experiments compared the performance of 4 fertilizer sources when used to grow Ficus benjamina at 4 soil temperature levels. Since the two experiments were very similar, experiment 1 is described in detail, while only the differences are listed for experiment 2.

On June 7, six inch tall liners of Ficus benjamina (weeping fig) in 3 inch pots were repotted into 6 inch plastic pots containing a soil mix made up of 3 sedge peat moss:1 mason sand (v/v) amended with 1 lb Micromax and 5 lbs dolomite per yd³. Micromax is a micronutrient blend manufactured by Sierra Chemical Co., Milpitas, CA. Pots were then placed in zoned forced-air chambers where soil temperatures were maintained at 68, 77, 86, and 95°F. Chambers were located in glass greenhouses where light levels reached a maximum of 1000 ft-c and air temperatures ranged from 70 to 90°F. Fertilizer was surface applied at time of placement of plants in the greenhouse using four Osmocote sources with different ratios and release times. The treatments were as follows: (1) 24-5-12, an 8 to 9 months, (2) 24-6-12, a 3 to 4 months, (3) 19-6-12, a 3 to 4 months, and (4) 18-6-12, with an 8 to 9 months release time.

Fertilizer application rates were as follows: (1) 24-5-12, 12 grams per pot every 9 months, (2) 24-6-12, 4 grams per pot every 3 months, (3) 19-6-12, 5 grams per pot every 3 months, and (4) 18-6-12, 15 grams per pot every 9 months. Application amounts were based on % N and release time of the fertilizer, so that all pots theoretically received the same amount of nitrogen. Soluble salt levels were determined every 2 weeks starting on June 17, using the pour-through nutrient extraction method. Data recorded at the conclusion of experiment 1, on October 21, included plant height, top and root fresh weights, plant grade (1 = poor, unsalable to 5 = excellent quality) and root grade (1 = no visible roots to 5 = entire root ball covered by roots). Plant tissue was analyzed to determine percentage of nitrogen, phosphorus, potassium, calcium and magnesium and also parts per million (ppm) copper, iron, manganese and zinc.

Experiment 2 used rooted cuttings of Ficus benjamina potted directly into 6 inch pots on April 15. Plants were grown on a greenhouse bench on a maintenance fertilizer program of 0.15 grams per pot 20-20-20 received weekly. On May 27 plants were pruned to a height of 6 inches and placed in temperature-controlled chambers where soil temperature and fertilizer treatments were started. Two slow-release fertilizer sources tested, (1) 19-6-12, a 3 to 4 months and (2) 18-6-12, an 8 to 9 months, were retained from experiment 1, but 24-6-12 and 24-5-12 were replaced with (3) 24-5-9, an 8 to 9 months, and (4) 24-5-8, with a 12 to 14 month release duration. Fertilizer application rates were as follows: (1) 19-6-12, 5 grams per pot every 3 months, (2) 18-6-12, 15 grams per pot every 9 months, (3) 24-5-9, 12 grams per pot every 9 months and (4) 24-5-8, 16 grams per pot every 12 months. Soluble salts levels were measured every 3 weeks starting June 29. Experiment 2 was terminated on Nov. 2 when plant height, top and fresh weights, plant grade and root grade, as in experiment 1, were recorded.

Plant height was not affected by soil temperature in experiment 1 but in experiment 2 plant height decreased as soil temperature increased. In both experiments plant and root grades decreased by almost a whole grade as soil temperatures increased from 68 to 95°F (Table 1). Data collected from experiments 1 and 2 show root fresh weight decreasing with increasing temperatures and top fresh weight decreasing with increases in soil temperature. Data from both experiments also show suppression of root and top growth at much lower temperatures than previously observed, but this was probably due to plants being exposed to the same soil temperature over a 24 hour day. Tissue samples analyzed for percent of 5 major elements and parts per million (ppm) of 4 minor elements cannot be used to explain growth suppression observed at 95°F (Table 2, soil temperature). Previous research shows summer soil temperatures taken in pots of plants growing in shaded structures in central Florida ranged from a minimum of 73°F to a maximum of 86°F. This means some growth suppression might occur on Ficus benjamina even in commercial shadehouse growing environments. Growth suppression is even more likely to occur where Ficus is grown in full sun, where soil in containers may reach 105°F or higher.

Growth suppression of Ficus benjamina was not expected at these temperatures but these data help explain the large growth variation that occurs with Ficus benjamina in shaded versus full-sun locations. Fertilizer source had no effect on plant growth and tissue analysis revealed only slight differences in element content of Ficus benjamina tissue grown in experiment 1 using 4 different fertilizers Table 2, fertilizer ratio). This means the method of compensating for variable release terms was satisfactory for these experiments.

Soluble salts levels of the leachate were much higher at the beginning of experiment 1 than experiment 2 (Table 3). This was probably due to the small root systems of the recently repotted liners used in experiment 1 being compared to the established plants used in experiment 2. Decreases in soluble salts levels of the leachate over time were caused by plant usage and irrigation, as well as the designed release time of the fertilizer source.

Direct comparison of the Osmocote sources in both experiments is only possible with 19-6-12 and 18-6-12 (Table 4). In general the long-term materials released nutrients at the same level as the short-term materials in the beginning and continued at higher levels as the short-term release sources were depleted.

The effect of temperature on fertilizer release durations appears to be more complicated than previously suspected. The release rates developed by the manufacturer of Osmocote and those from research done in water show decreasing release time with increasing temperatures. However, these tests did not consider effects plants and soil could have on fertilizer source. Results from these two experiments show nutrient release increased with increasing temperatures up to 86°F and then decreased as temperature rose to 95°F.

In these experiments plants grew and used nutrients as they were released. This could have skewed the data in a way that would indicate low availability of nutrients based on soluble salts levels even though plants were growing rapidly. However, plants had poorer growth measurements when grown at 95°F than at 86°F even though soluble salt levels were lower at 95°F. These data show that rapid release of nutrients and possible plant damage may not be as temperature dependent as is believed and may be strongly influenced by plant size at time of fertilizer application.

*Center Director and Professor and Professor of Plant Physiology, respectively, Central Florida Research and Education Center, 2807 Binion Road, Apopka, Florida 32703-8504.

Additional Reading

1. Conover, C. A. and R. T. Poole. 1985. Influence of fertilizer source, rate and application method on growth of Brassaia actinophylla and Viburnum odoratissimum. Proc. Fla. State Hort. Soc. 98:82-85.

2. Harbaugh, B. K. and G. J. Wilfret. 1982. Correct temperature is the key to successful use of Osmocote. Florists Rev. 170(4403):21-23.

3. Ingram, D. L., C. Ramcharan, and T. A. Nell. 1986. Response of container-grown banana, ixora, citrus and dracaena to elevated root temperatures. HortScience 21:254-255.

4. Joiner, J. N., C. A. Conover, and R. T. Poole. 1981. Nutrition and Fertilization, p. 229-268. In: J. N. Joiner (ed.).Foliage Plant Production. Prentice-Hall Inc., Englewood Cliffs, N. J.

5. Koller, D. C., K. L. Hiller, and R. W. VanDenburgh. 1980. A forced-air system for controlling soil temperature in plastic pots. HortScience 15:189-190.

6. Poole, R. T. and C. A. Conover. 1982. Influence of leaching, fertilizer source and rate, and potting media on foliage plant growth, quality and water utilization. J. Amer. Soc. Hort. Sci. 107:793-797.

7. Waters, W. E. and W. Llewellyn. 1968. Effects of coated slow-release fertilizer on growth responses, chemical composition and soil salinity levels for foliage plants. Proc. Fla. State Hort. Soc. 81:380-388.

8. Wright, R. D. 1986. The pour-through nutrient extraction procedure. HortScience 21:27-229.

1. ^ZPlant grade was rated on a scale from 1 = poor, unsalable to 5 = excellent, highly salable.
   ^YRoot grade was rated on a scale from 1 = no visible roots to 5 = entire root ball covered by roots.

1. ^ZNitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), and Magnesium (Mg) are expressed as percent dry weight in tissue sampled.

   Copper (Cu), Iron (Fe), Manganese (Mn), and Zinc (Zn) are expressed in parts per million (ppm) in tissue sampled.

1. ^Zslow-release fertilizers manufactured by Sierra Chemical Co., Milpitis, CA.

1. ^Zslow-release fertilizers manufactured by Sierra Chemical Co., Milpitis, CA.